Резонансная ИК-фотохимия адсорбированных молекул тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Песцов Олег Сергеевич

• Песцов Олег Сергеевич
• кандидат наук
• 2023

Оглавление диссертации кандидат наук Песцов Олег Сергеевич

Введение

Глава 1. Представления о резонансной ИК- фотохимии адсорбированных молекул. Литературный обзор

1.1. Резонансные лазерно-индуцированные процессы

1.2. Применение резонансного ИК возбуждения для разделения изотопов

1.3. ИК спектроскопия адсорбированного озона

1.4. Диссипация колебательной энергии

1.5. Заключение литературного обзора

Глава 2. Техника эксперимента

2.1. Низкотемпературная вакуумная кювета

2.2. Синтез озона

2.3. Вакуумная установка

2.4. Лазерная система

Глава 3. Экспериментальные данные

3.1. Адсорбция молекул озона на оксидных адсорбентах

3.2. Инициирование процесса разложения адсорбированного озона лазерным излучением

3.3. Озонолиз адсорбированных молекул дихлорэтилена, стимулированный ИК излучением

3.3.1. Озонолиз дихлорэтилена на SiO2

3.3.2. Озонолиз дихлорэтилена на ТЮ2

Основные результаты

Выводы

Благодарности

Список литературы:

Введение диссертации (часть автореферата) на тему «Резонансная ИК-фотохимия адсорбированных молекул»

Введение

Идея использования резонансного лазерного возбуждения для селективного инициирования различных процессов в молекулярных системах привлекает внимание специалистов вот уже более полувека [1]. Большинство исследований резонансного взаимодействия лазерного излучения с веществом было выполнено для газов. Однако энергии активации гомогенных химических реакций обычно превышают энергию квантов инфракрасного лазера, а ангармонизм молекулярных колебаний препятствует многофотонному резонансному возбуждению молекул до высоких энергий [2].

Гетерогенные процессы характеризуются низкой энергией активации по сравнению с гомогенными, и отличаются высокой чувствительностью к небольшим изменениям энергии возбуждения участвующих в них молекул. Поглощение одного инфракрасного кванта молекулой может существенно изменить ее поведение, что позволяет использовать относительно низкие мощности лазерного излучения [3].

Карлов и Прохоров [3] сообщили о первых результатах экспериментов, когда колебательное возбуждение определенных изотопсодержащих молекул СО2 приводило к различию сорбционных свойств на охлаждаемой поверхности по сравнению с невозбужденными молекулами. Совсем недавно [4] было показано, что инфракрасное лазерное излучение влияет на процесс кластеризации молекул СБ^Бг в их смеси с аргоном в зоне газодинамического расширения. Было показано, что резонансное колебательное возбуждение молекул непрерывным излучением CO2-лазера ведет к селективному по изотопам брома подавлению кластеризации.

Более перспективным представляется применение лазеров для инициирования поверхностных процессов, индуцированных прямым возбуждением колебательных состояний адсорбированных частиц. Были предприняты многочисленные попытки осуществить резонансную лазерную

десорбцию, когда возбуждение инфракрасным лазерным излучением способствует десорбции тех адсорбированных молекул, частота колебаний которых совпадает с частотой колебаний возбуждающего излучения. Важнейшей прикладной задачей этих исследований была оценка возможности использования резонансного возбуждения колебательных состояний поверхностных соединений для разделения изотопов. Резонансное возбуждение колебаний адсорбированных молекул осуществлялось в экспериментах по лазерной десорбции СО [5]-[7], CH3F [8] и аммиака [9], [10]. Однако было показано, что облучение смеси на частоте колебаний одного изотополога не приводит к его ожидаемой селективной десорбции, по-видимому, по этой причине интерес к подобным исследованиям в последующие годы несколько ослаб. Можно отметить лишь статьи по резонансной десорбции N2O с поверхности хлорида натрия [11] и CD3F с поверхности NaCl [12], SF6 с поверхности кремния и пиридина с поверхности KCl [13] и спиртов с поверхности меди [14].

Redlich и др. [15] изучали индуцированную ИК-лазером десорбцию из полимолекулярных слоев различных изотопологов молекул метана, конденсированных на монокристаллической поверхности NaCl (100) с использованием времяпролетной масс-спектрометрии. Десорбция из слоев чистого CH4, CD4 или CD3H, а также из слоя смеси двух разных изотопных соединений исследовалась в зависимости от длины волны воздействующего инфракрасного излучения перестраиваемого лазера на свободных электронах. Десорбция метана происходила только тогда, когда возбуждающий свет находился в резонансе с внутренними колебательными модами молекул. Для слоев смешанных изотопных соединений десорбция наблюдалась только на резонансных частотах, и не зависела от плотности энергии возбуждающего лазера. Было показано, что возбуждение колебаний одного изотополога вызывает десорбцию другого.

Очевидно, что в слое адсорбированных молекул происходит достаточно быстрый обмен энергией колебательного возбуждения. Чтобы понять причины

отсутствия изотопной селективности, необходимо проанализировать данные о механизме и эффективности этого обмена, о времени жизни и скорости диссипации колебательного возбуждения и, насколько возможно, о вероятности стимулирования десорбции или реакции колебательно возбужденных поверхностных соединений. Такой анализ был произведен автором данной диссертации в [16], и в результате него были сформулированы требования к системам, где можно было бы ожидать изотопной селективности.

Целью настоящего исследования является исследование возможности осуществления процессов, индуцированных резонансным лазерным возбуждением колебательных состояний поверхностных соединений. Это процессы десорбции, изомеризации, разложение нестабильных соединений, реакции между совместно адсорбированными молекулами. Применение изотопозамещенных молекул должно помочь установить резонансный характер стимулируемых процессов и способствовать решению практически важной задачи по развитию методов разделения изотопов.

Для достижения данной цели были поставлены следующие задачи:

• Разработка и создание вакуумной системы для её использования с низкотемпературной кюветой.

• Создание системы облучения для обеспечения оптимального попадания луча лазера на образец.

• Исследование адсорбированного озона на ряде оксидных адсорбентов методами ИК спектроскопии.

• Проведение экспериментов по резонансному воздействию ИК лазерным излучением на изотопную смесь озона, адсорбированного на поверхности 8Ю2, Се02 и ТЮ2.

• Изучение стимулирования реакции озонолиза дихлорэтилена на поверхности БЮ2 и ТЮ2.

Новизна проведенного исследования заключается в том, что для выбранных систем впервые были проведены эксперименты по изучению процессов стимулированных резонансным возбуждением колебательных состояний.

Положения, выносимые на защиту:

• Повышение частоты составного колебания озона у1+у3 для большинства оксидных адсорбентов связано с электроноакцепторной способностью (льюисовской кислотностью) катионов этих адсорбентов, а при хемосорбции на ТЮ2 и Се02 является следствием уменьшения ангармоничности.

• Расщепление спектральных полос озона, хемосорбированного на ТЮ2 для фундаментального у1, и составного у1+у3 колебаний указывает на потерю симметрии молекулы в результате адсорбции.

• Воздействие резонансного ИК лазерного излучения приводит к преимущественному разложению изотопологов озона, на частотах которых производится облучение.

• Облучение ИК лазером ускоряет реакцию озонолиза дихлорэтилена, адсорбированного на ТЮ2, причем ее скорость увеличивается с повышением мощности лазера.

• Реакция озонолиза цис-С2Н2С12 адсорбированного на ТЮ2 идет спонтанно уже при 77К, тогда как на SiO2 для инициирования реакции требуется повышение температуры до 168 К.

Личный вклад автора. Автор совместно с научным руководителем участвовал в постановке целей и задач. Автором проведён самостоятельный анализ научной литературы. Все эксперименты были проведены лично автором или совместно с коллегами. Автор совместно с коллегами создавал вакуумную установку необходимую для работы. Эксперименты с применением лазерного излучения были проведены при техническом сопровождении сотрудниками ресурсного центра "Оптические и лазерные методы исследования вещества"

Научного Парка СПбГУ. Разработка и дальнейшая юстировка установки для использования лазерного возбуждения выполнялась лично автором. Полученные результаты и выводы обсуждались с научным руководителем. Автор совместно с соавторами и научным руководителем готовил статьи для публикаций и самостоятельно представлял результаты на научных конференциях.

Достоверность результатов работы основывается на высоком методическом уровне применения современных физико-химических методов исследования, воспроизводимости и согласованности полученных результатов с литературными источниками. Результаты диссертационной работы докладывались научных семинарах кафедры Фотоники СПбГУ, а также представлены на четырёх международных конференциях: EUROPACAT 2017, EUCMOS 2018, Science and Progress 2020, 5th International Symposium on Molecular Photonics 2021. По результатам проведенных исследований опубликовано 5 статей в рецензируемых журналах, из них 3 статьи по теме диссертационной работы: [16]—[18].

Глава 1. Представления о резонансной ИК- фотохимии адсорбированных молекул. Литературный обзор.

1.1. Резонансные лазерно-индуцированные процессы

Одно из первых наблюдений эффекта резонансной ИК-фотохимии адсорбированных молекул является сообщение от авторов Umstead и Lin [19], которые заметили, что лазерное излучение на длине волны поглощения муравьиной кислоты влияет на скорость её разложения на платиновой проволоке. Хотя авторы не смогли однозначно различить, был это результат возбуждения газообразных или адсорбированных молекул, их результаты указывают на возможность управления каталитическими реакциями с помощью инфракрасного лазерного излучения.

Проявления лазерно-индуцированной изомеризации в молекулах и комплексах исследованы в ряде работ. Под действием резонансного поглощения ИК-излучения на частоте колебаний наблюдались изомерные превращения димеров муравьиной кислоты в аргоновой матрице [20].

Эксперименты по резонансной инфракрасной лазерной абляции полиэтиленгликоля на двух разных длинах волн резонансного возбуждения, которые соответствовали разным валентным колебаниям полимера и имели совершенно разные коэффициенты поглощения, показали, что абляция за счет возбуждения более слабой поглощающей валентной моды O-H задерживается во времени на несколько микросекунд по сравнению с более сильно поглощающей валентной C-H модой [21]. Расчеты показали, что такая чувствительность к коэффициентам поглощения обусловлена другим механизмом абляции, фазовым взрывом для C-H или нормальным кипением при возбуждении для O-H.

Livingston и др. [22] использовали импульсный Er:YAG (в качестве активной среды используется иттрий-алюминиевый гранат, легированный эрбием) лазер с длиной волны 2,94 мкм для резонансного возбуждения ОН валентных колебаний H2O. В итоге с каждым импульсом происходил

резонансный нагрев и дальнейшая десорбция молекул H2O с поверхности льда. Этот метод был применён для контролируемой по глубине диффузии HCl или Na в лед.

Focsa и др. [23] сообщили о ИК-резонансной десорбции молекул H2O c поверхности льда при облучении импульсным перестраиваемым параметрическим генератором среднего инфракрасного диапазона в области полосы валентных колебаний ОН при 100К. Десорбированные молекулы детектировались методами УФ многофотонной ионизации и времяпролетной масс-спектрометрии. Было обнаружено, что скорость десорбции пропорциональна энергии импульса и величине поглощения. Распределение продуктов десорбции по скоростям позволило разделить их на две части, соответствующие различным механизмам процесса. Первый быстрый процесс - "взрывная" десорбция, т. е. лазерный импульс очень быстро нагревает поверхность образца до температуры близкой к критической температуре воды, что приводит к высвобождению всего объема взаимодействия в газовую фазу за очень короткое время. Второй механизм представляет собой последующую термическую десорбцию более глубоких кластеров молекул, которые регистрируются масс-спектрометром с ощутимой задержкой. Позже авторы применили этот метод для пленок аммиака и некоторых ароматических углеводородов [24].

Uckert и др. [25] продемонстрировали возможность резонансно-усиленной десорбции для обнаружения органических веществ во внеземных телах на примере льда, допированного триптофаном. Их метод включал ИК-лазерную десорбцию поверхностных частиц с последующей ионизацией десорбированных частиц УФ-лазером для времяпролетного масс-спектрометрического анализа. Эксперименты показали, что при облучении на частоте поглощения резко усиливался сигнал от всех десорбирующихся частиц.

Hassel и др. [26] изучали адсорбцию водорода на гранях кристаллов меди с помощью спектроскопии характеристических потерь энергии электронов и пришли к выводу, что десорбция H2 и D2, физически адсорбированных на

поверхности Cu (510), вызывается тепловым излучением стенок кюветы. Расчеты дипольно-возбужденных колебательных переходов на высоколежащие уровни в физсорбционной яме свидетельствуют о возможности прямой инфракрасной фотодесорбции. Для молекул, адсорбированных на грани (100), расчеты согласуются с экспериментальными результатами. Для HD теория предсказывает возможность вызвать резонансную фотодесорбцию с помощью инфракрасного лазера со скоростью, намного большей, чем при возбуждении фононов подложки [27].

Важнейшей прикладной задачей этих исследований была оценка возможности использования резонансного возбуждения колебательных состояний поверхностных соединений для разделения изотопов. Некоторый прогресс в этой области был достигнут с помощью перестраиваемых CO2 лазеров, используемых для многофотонного возбуждения молекул в газовой или конденсированной фазе. Например, была продемонстрирована возможность селективной многофотонной диссоциации молекул 2,3-дигидропирана с выделением кислорода определенного изотопного состава из смеси молекул, содержащих 16O и 18O, при лазерном возбуждении [28]. Для оптимальной длины волны облучения, давления газа и плотности энергии лазерного излучения был достигнут коэффициент обогащения 18O равный 751. Метод получения изотопа серы 33S путем облучения молекул SF6 резонансным излучением перестраиваемого С02-лазера при температуре ниже точки кристаллизации (195K), сопровождающейся диссоциацией молекул гексафторида серы, был предложен авторами Mathiet и др. [29]. Другой пример применения такой техники - работа Miyamoto и др. [30], где описан процесс обогащения продуктов изотопом 13С в результате селективной диссоциации молекул бета-пропиолактона с образованием диоксида углерода и этилена.

1.2. Применение резонансного ИК возбуждения для разделения изотопов

Первым кто смог выделить чистые металлы из водного раствора оксалатных комплексов Fe и Co с помощью селективного лазерного возбуждения оказался автор работы [31]. При облучении лазером на частоте поглощения хлорида

палладия, из его водного раствора, извлекался металлический палладий [32]. В работе [33] проводилось селективная диссоциация молекул с помощью воздействия одновременно двух CO2 лазеров. Авторами экспериментально исследовались зависимости скорости диссоциации газообразного SF6 при совместном облучении кюветы двумя лазерами с различными частотами. При двухфазной диссоциации повышается селективность процесса, что существенно для разделения изотопов тяжелых элементов с малым изотопическим сдвигом.

Heidberg и соавторы [5] изучали ИК спектры смесей изотопологов монооксида углерода (12С16О/13С16О), адсорбированных на пленке NaCl и поверхности монокристалла NaCl (100). Обнаружено, что при лазерном возбуждении фундаментального колебания конкретного изотополога десорбция происходит без изотопной селективности. Возможным объяснением этого феномена может быть энергетический обмен между молекулами различного изотопного состава друг с другом и между молекулами и подложкой.

Hussla и др. [9] изучали десорбцию молекул NH3 и ND3 при возбуждении импульсами ИК-лазера в области N-H колебаний. В ходе эксперимента изучаемые молекулы адсорбировались на поверхностях монокристаллических пленок Cu (100) и Ag в условиях высокого вакуума и низких температур. Наблюдать значительную изотопную селективность при фотодесорбции не удалось. Было высказано предположение, что это связано с процессами переноса энергии в адсорбированных молекулярных слоях. Для решения этой проблемы было предложено использовать низкие покрытия поверхности, чтобы уменьшить латеральные межмолекулярные взаимодействия. Также авторы предполагают, что эксперименты с молекулярными системами с относительно слабыми межмолекулярными взаимодействиями и большими различиями в частотах колебаний между разными изотопологами могли бы позволить наблюдать изотопную селективность при ИК-фотодесорбции.

Амбарцумян и соавторы в своей работе [34] исследовали селективную диссоциацию молекулы SF6 инфракрасным лазером в Ar и CO матрицах при

температурах 8-10К. При этом было исключен тепловой эффект нагрева матрицы и обычное испарение молекул. Было показано, что при облучении на частоте легкого изотополога 328Бб соотношение интенсивности его полосы к полосе более тяжелого 348Бб уменьшалось. Таким образом, приведенные экспериментальные результаты свидетельствуют о наблюдении диссоциации изолированной молекулы под действием ИК излучения.

Было проведено множество работ по использованию лазерного излучения для селективного воздействия на молекулы определённого изотопного состава. В работе [35] изучалось поведение молекул ВС13 в ксеноновой матрице в отношении 1:100 при облучении СО2 лазером в непрерывном режиме. Автором обнаружено, что молекулы изотополога 11ВС13 образуют мелкие кристаллы на охлаждаемой пластине-подложке, в то время как молекулы 10ВС13 высвобождаются вместе с возгонкой Хе. Этот эффект объясняется миграцией 11ВС13 при лазерном облучении вглубь матрицы непосредственно к подложке. Позднее такой механизм был подтверждён в работах [36], [37]. В работе [38] исследовалось влияние селективного лазерного возбуждения на молекулярные кристаллы тетразина для изотопного разделения и наблюдения фотохимического выжигания провалов. Представленные результаты показали, что тетразин при низких температурах способен к последовательному поглощению двух фотонов.

Десорбция молекул N0 с поверхности толстой пленки С60 была изучена в работе [39]. Авторы использовали импульсный УФ-лазер с длительностью импульса в 7 нс. При анализе с помощью время-пролётной масс спектрометрии было предложено три различных канала десорбции. Первый канал, который характеризуется сильной вращательно-поступательной связью можно объяснить процессом десорбции, вызванной электронными переходами. Скорость десорбции для быстрого канала линейно возрастает с энергией импульса. Второй канал был отнесен к лазерно-индуцированной термической десорбции. Третий канал совмещен со вторым, но характеризуется очень поздним временем регистрации детектором масс-спектрометра, что предположительно связано с задержкой

десорбции. Такую задержку могли вызвать диффузионные процессы или долгоживущие возбужденные состояния.

Авторами работы [40] продемонстрирован процесс десорбции нейтральных атомов водорода с поверхности графита при облучении импульсами фемтосекундного УФ-лазера. Десорбированные атомы были распределены на несколько групп по скоростям. Такое распределение по скоростям оказалось результатом десорбции с различных адсорбционных центров. С помощью теории функционала плотности было показано, что отличия в энергии адсорбции определяются латеральными взаимодействиями водорода на поверхности.

В работе [12] авторы десорбировали CDзF, конденсированный на поверхности монокристалла №С1 (100). Температура подложки (Т=40К) была выбрана достаточно низкой, чтобы гарантировать, что за время эксперимента не произойдет существенной термодесорбции. Десорбция проводилась с помощью возбуждения внутримолекулярных колебании CD3F перестраиваемым ИК лазером. Оказалось, что молекулы десорбируются только в случае, когда частота возбуждающего излучения совпадает с частотой колебания молекулы. В продолжении авторы исследовали десорбцию различных изотопологов метана (СН4, СЭ4 и CD3H) с поверхности грани (100) монокристалла №С1 [41]. Десорбция наблюдалась при облучении на длинах волн, соответствующим нормальным колебаниям молекул, например, у4 (7.69 и 10.11 мкм) для молекул СН4, СЭ4, и на длине волны у2 (7.79, 9.75 мкм) и у4 (9.98 мкм) молекулы CD3H. В смешанных системах (при коадсорбции двух изотопологов одновременно) наблюдалась десорбция сразу двух изотопологов в слое, поэтому невозможно говорить о только процессе прямой десорбции. Необходимо учитывать резонансное нагревание и взаимодействие возбужденных молекул с невозбужденными.

1.3. ИК спектроскопия адсорбированного озона

В ряде предшествующих работ были исследованы ИК-спектры молекул озона, адсорбированных на оксидных адсорбентах [42]-[45]. Подробно изучались ИК спектры 16О3, 18О3 и смешанных изотопных модификаций озона адсорбированных на поверхности БЮ2, ТЮ2 и СеО2 при температуре жидкого азота в работах [42]-[44]. В ходе исследований обнаружено, что на сильных Льюисовских центрах оксидов титана и алюминия молекулярной адсорбции не происходит, а идет реакция разложения озона с образованием атомарного кислорода, который участвует в реакциях окисления или становится катализатором реакции разложения озона, сопровождаемая рекомбинационной люминесценцией [46].

В работе [43] исследовались ИК спектры различных изотопных модификаций озона, адсорбированного на поверхности БЮ2. Показано, что озон адсорбируется на поверхностных гидроксильных группах, связываясь водородной связью одним из концевых атомов кислорода.

В работе [47] проведено исследование ИК спектров растворенного озона в жидком кислороде при температуре 77К в области 3200-650 см-1. Представлена полная интерпретация полученных спектров и измерены относительные силы переходов для наблюдаемых полос.

Мсо1аБ и др. [45] исследовалось разложение озона на поверхностях, твердого ТЮ2 в матрице Si02. Авторы изучали влияние концентрации ТЮ2 на взаимодействие с образца с молекулой озона. Кроме того обнаружено, что при облучении ближним УФ светом количество поглощенного озона увеличивается с одновременным уменьшением концентрации озона.

Авторы [48] исследовали фотокаталитическое разложение озона на поверхности тонкой пленки ТЮ2 в проточном реакторе. Показано, что кинетику разложения озона можно описать механизмом Ленгмюра-Хиншельвуда. Изучалось влияние интенсивности света на скорость реакции, где показана

степенная зависимость скорости реакции от интенсивности излучения. Полученные в ходе эксперимента параметры применялись для расчета распределения концентрации озона в проточном реакторе.

В работе [49] представлен расчет энергии и различных структур озона, адсорбированного на гидратированной поверхности нанокристаллического TiO2 с помощью теории функционала плотности. Обнаружено, что образуются несколько типов комплексов озона с катионами титана оксида, а именно триплетные монодентатные, синглетные бидентатные, синглетные монодентатные и триплетные бидентатные. Отсутствие в эксперименте триплетных монодентатных комплексов объясняется их легким распадом на синглетные нанокластеры TigOn и молекулярный кислород. Обнаружено совпадение между экспериментальным значением разницы частоты фундаментальных колебаний (v1-v3) и значением наиболее стабильного синглетного бидентатного комплекса.

Thomas и др. [50] изучали адсорбцию озона методами ИК спектроскопии при низких температурах. В качестве адсорбента была выбрана поверхность оксида алюминия (AI2O3). В ходе исследования обнаружено, что разложение озона происходит на поверхности оксида алюминия даже при низких температурах.

В работе [51] изучалась адсорбция озона на поверхность CaO в температурном диапазоне от 77-300К методами ИК спектроскопии. Обнаружены сдвиги полос, соответствующих OH- и OD-группам, расположенным на поверхности. Наблюдаемые при этом небольшие сдвиги фундаментальных полос озона могут означать образование связи между кислородом гидроксильной группы и центральным атомом кислорода молекулы озона, который имеет положительный заряд. Показано, что озон не диссоциирует на поверхности CaO при низких температурах (77К). Для появления реакции разложения озона на поверхности необходима термическое воздействие на образец.

В работе [44] изучалась система адсорбированного озона на поверхности Се02, с предварительно адсорбированными различными молекулами СО, пиридином, ацетонитрилом и метанолом, в температурном диапазоне от 7 до 300К. Обнаружено, что помимо физической адсорбции и образования слабой водородной связи с наиболее кислотными центрами ОН-групп, появляются комплексы озона с двумя типами кислотных центров. Из ИК спектров обнаружено образование озонидов 03- (792, 772 см-1) и супероксидов 02-(1128 см-1). Показано, что адсорбция молекулярного озона на поверхности СеО имеет взрывной характер, чего можно избежать, проводя адсорбцию из раствора жидкого кислорода.

Исследование адсорбированного озона на поверхности поликристаллического М^О было проведено в работе [52]. В спектрах обнаружены два типа полос в областях 1105-1024 см-1 и 1140-1038 см-1, соответствующие валентным колебаниям различных атомов кислорода адсорбированной молекулы озона. Высокочастотный сдвиг полос появляется из-за латерального взаимодействия между адсорбированными молекулами. Третий тип полос, расположенных в интервале 1730-1170 см-1, относится к колебаниям продуктов окисления, адсорбированных на поверхности М§0.

Диссоциацию озона в газовой фазе при интенсивном многофотонном возбуждении С02-лазером наблюдали Рш^ и БсЬгоеёег [53]. Было получено, что диссоциация происходит только когда частота излучения лазера совпадает с частотой поглощения молекул.

1.4. Диссипация колебательной энергии

Для успешного результата резонансного возбуждения адсорбированных молекул необходимо учитывать соотношения между характерными временами диссипации или передачи энергии другим частицам и желаемым процессом, таким как десорбция или реакция.

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SAINT PETERSBURG STATE UNIVERSITY

Manuscript copyright

Pestsov Oleg

RESONANT IR PHOTOCHEMISTRY OF ADSORBED MOLECULES

Scientific specialization 1.3.8. Condensed matter physics

DISSERTATION

submitted for the degree of candidate of physical and mathematical sciences

Translation from Russian

Scientific supervisor D. Sc. in Physics and Mathematics,

prof. Tsyganenko Alexey A.

Saint Petersburg 2022

Content

Introduction.....................................................................................................................87

Chapter 1. Concepts of resonant IR photochemistry of adsorbed molecules. Literature review..............................................................................................................................91

1.1. Resonant laser-induced processes........................................................................91

1.2. Application of resonant IR excitation for isotope separation..............................93

1.3. IR spectroscopy of adsorbed ozone......................................................................96

1.4. Vibrational energy dissipation.............................................................................98

1.5. Literature review conclusion..............................................................................103

Chapter 2. Experimental technique...............................................................................106

2.1. Low temperature vacuum cell............................................................................106

2.2. Ozone synthesis..................................................................................................108

2.3. Vacuum setup.....................................................................................................109

2.4. Laser system.......................................................................................................110

Chapter 3. Experimental data........................................................................................112

3.1. Adsorption of ozone molecules on oxide adsorbents........................................112

3.2. Initiation of the decomposition of adsorbed ozone by laser radiation...............127

3.3. Ozonolysis of adsorbed molecules stimulated by IR radiation..........................130

3.3.1. Ozonolysis of dichloroethylene on SiO2.....................................................131

3.3.2. Ozonolysis of dichloroethylene on TiO2.....................................................138

Main results...................................................................................................................149

Conclusions...................................................................................................................150

Acknowledgments.........................................................................................................151

Literature.......................................................................................................................152

Introduction

The idea of using resonant laser excitation for the selective initiation of various processes in molecular systems has attracted the attention of specialists for more than half a century [1]. Most studies of the resonant interaction of laser radiation with matter have been carried out for gases. However, the activation energies of homogeneous chemical reactions usually exceed the energy of infrared laser quanta, and the anharmonicity of molecular vibrations prevents multiphoton resonant excitation of molecules to high energies [2].

Heterogeneous processes are characterized by low activation energy compared to homogeneous processes and are highly sensitive to small changes in the excitation energy of the molecules involved. The absorption of one infrared photon by a molecule can significantly change its behavior, which makes it possible to use relatively low laser radiation powers [3].

Karlov and Prokhorov [3] reported the first experimental results where vibrational excitation of certain isotope-containing CO2 molecules led to a difference in sorption properties on a cooled surface compared to unexcited molecules. Quite recently [4], it was shown that infrared laser radiation affects the process of clustering of CF3Br molecules in their mixture with argon in the gas dynamic expansion zone. It was shown that resonant vibrational excitation of molecules by CO2 laser radiation leads to selective suppression of clustering with respect to bromine isotopes.

More potential application is the use of lasers to initiate surface processes induced by direct excitation of vibrational states of adsorbed particles. Numerous attempts have been made to carry out resonant laser desorption, when excitation by infrared laser radiation promotes the desorption of those adsorbed molecules whose vibration frequency coincides with the exciting radiation frequency. The most important application task of these investigations was to evaluate the possibility of using resonant excitation of vibrational states for the isotopic separation of surface species. Resonant excitation of adsorbed molecule vibrations was carried out in experiments on laser

desorption of CO [5]-[7], CH3F [8], and ammonia [9], [10]. However, it was shown that irradiation of the mixture at the vibrational frequency of one isotopologue does not lead to selective desorption as it was expected. Apparently, for this reason, interest in such studies somewhat weakened in subsequent years. We can only mention articles on resonant desorption of N2O from the surface of sodium chloride [11] and CD3F from the surface of NaCl [12], SF6 from the surface of silicon, pyridine from the surface of KCl [13] and alcohols from the surface of copper [14].

Redlich et al. [15] studied the IR laser-induced desorption from polymolecular layers of various methane isotopologue molecules condensed on a NaCl (100) single-crystal surface using time-of-flight mass spectrometry. Desorption from layers of pure CH4, CD4, or CD3H, as well as from a layer of a mixture of two different isotopic compounds, was studied depending on the wavelength of the infrared radiation of a tunable free electron laser. Methane desorption occurred only when the exciting light was in resonance with the internal vibrational modes of the molecules. For layers of mixed isotopic species, desorption was observed only at resonant frequencies and did not depend on the exciting laser fluence. It was shown that excitation of vibrations of one isotopologue causes desorption of another.

It is obvious that in the layer of adsorbed molecules a rather fast exchange of vibrational excitation energy takes place. To understand the reasons for the lack of isotope selectivity, it is necessary to analyze data on the mechanism and efficiency of this exchange, on the lifetime and dissipation rate of vibrational excitation, and, as far as possible, on the probability of stimulating desorption or reaction of vibrationally excited surface species. Such an analysis was carried out by the author of this dissertation in [16], and as a result, requirements were formulated for systems where one could expect isotopic selectivity.

The aim of this research is to study the possibility of implementing processes induced by resonant laser excitation of vibrational states of surface species. These are the processes of desorption, isomerization, decomposition of unstable molecules, reactions between co-adsorbed molecules. The use of isotopically substituted molecules

should help to establish the resonant nature of the stimulated processes and contribute to the solution of the practically important problem of developing methods for isotope separation.

To achieve this aim, the following tasks were set:

• Development and construction of a vacuum system for use with a low-temperature cell.

• Creation of an irradiation system to ensure optimal exposure of the laser beam to the sample.

• Study of adsorbed ozone on oxide adsorbents using IR spectroscopy.

• Carrying out experiments on the resonant effect of IR laser radiation on the isotopic mixture of ozone adsorbed on the surface of SiO2, CeO2 and TiO2.

• Study of stimulation of dichloroethylene ozonolysis reaction on the surface of SiO2 and TiO2.

The novelty of the research lies in the fact that for the first time for the selected systems, experiments were carried out to study the processes of excitation of vibrational states during resonant excitation.

Thesis statements to be defended:

• Increase in the frequency of the v1+v3 combinational ozone vibration for most of the oxide adsorbents is associated with the electron-accepting ability (Lewis acidity) of the cations of these adsorbents, and in the case of chemisorption on TiO2 and CeO2 is due to the decrease of anharmonicity.

• The splitting of the spectral bands of ozone chemisorbed on TiO2 for fundamental V1 and combinational V1+V3 vibrations indicates the loss of molecular symmetry due to adsorption.

• Exposure to resonant IR laser radiation leads to the predominant decomposition of ozone isotopologues, at the frequencies of which irradiation is performed.

• Irradiation with IR laser accelerates the reaction of ozonolysis of dichloroethylene adsorbed on TiO2, and its rate increases with the increase of laser power.

• The reaction of ozonolysis of cis C2H2Cl2 adsorbed on TiO2 proceeds spontaneously already at 77K, while on SiO2 the temperature rise to 168K is required to initiate the reaction.

The author's personal contribution. The author, together with the supervisor, participated in setting goals and objectives. The author carried out an independent analysis of scientific literature. All experiments were carried out personally by the author or jointly with colleagues. The author, together with colleagues, created a vacuum installation necessary for work. Experiments with the use of laser radiation were carried out with technical support from the staff of the resource center "Optical and laser methods for studying matter" of the Science Park of St. Petersburg State University. The development and further adjustment of the installation for the use of laser excitation was carried out personally by the author. The results and conclusions were discussed with the supervisor. The author, together with co-authors and supervisor, prepared articles for publications and independently presented the results at scientific conferences.

The reliability of the research results is based on a high methodological level of application of modern physicochemical research methods, reproducibility and consistency of the results obtained with literature sources. The results of the work were reported at scientific seminars of the Department of Photonics of St. Petersburg State University and presented at four international conferences: EUROPACAT 2017, EUCMOS 2018, Science and Progress 2020, 5th International Symposium on Molecular Photonics 2021. Based on the results of the research, 5 articles were published in peer-reviewed journals, of which 3 articles were on the topic of the dissertation work [16]-[18].

Chapter 1. Concepts of resonant IR photochemistry of adsorbed molecules. Literature review

1.1. Resonant laser-induced processes

One of the first observations of the effect of resonant IR photochemistry of adsorbed molecules is a report by Umstead and Lin [19], who noticed that laser radiation at the absorption wavelength of formic acid affects the rate of its decomposition on a platinum wire. Although the authors could not clearly distinguish whether this was the result of excitation of gaseous or adsorbed molecules, their results indicate the possibility of controlling catalytic reactions using infrared laser radiation.

Manifestations of laser-induced isomerization in molecules and complexes have been studied in several works. Under the resonant absorption of IR radiation at the oscillation frequency, isomeric transformations of formic acid dimers in an argon matrix were observed [20].

Experiments on resonant infrared laser ablation of polyethylene glycol at two different wavelengths of resonant excitation, which corresponded to different stretching vibrations of the polymer and had completely different absorption coefficients, were carried out. They showed that ablation due to excitation of the weaker absorbing O-H stretching mode is delayed in time by several microseconds in time compared with the more strongly absorbing stretching C-H mode [21]. Calculations showed that such sensitivity to absorption coefficients is due to another ablation mechanism, phase explosion at the C-H mode or normal boiling for excitation at the O-H mode.

Livingston et al. [22] used a pulsed Er:YAG (Erbium-doped yttrium aluminum garnet as the active medium) laser with a wavelength of 2.94 цт to resonantly excite OH stretching vibrations of H2O. As a result, resonant heating, and further desorption of H2O molecules from the ice surface occurred with each pulse. This method has been applied to depth-controlled diffusion of HCl or Na into ice.

Focsa et al. [23] reported on the IR resonant desorption of H2O molecules from the ice surface under irradiation with a pulsed tunable mid-infrared parametric oscillator in the region of the OH stretching vibration band at 100 K. Desorbed molecules were detected by UV multiphoton ionization and time-of-flight mass spectrometry. It was found that the rate of desorption is proportional to the pulse energy and the absorption amount. The velocity distribution of the desorption products made it possible to divide them into at least two parts, corresponding to different mechanisms of the process. The first fast process is "explosive" desorption, i.e., the laser pulse very quickly heats the sample surface to a temperature close to the critical temperature of water, which leads to the release of the entire interaction volume into the gas phase in a very short time. The second mechanism is the subsequent thermal desorption of deeper molecular clusters, which are recorded by a mass spectrometer with a noticeable delay. Later, the authors applied this method to films of ammonia and some aromatic hydrocarbons [24].

Uckert et al. [25] demonstrated the possibility of resonant enhanced desorption to detect organic substances in extraterrestrial bodies using tryptophan-doped ice as an example. Their method included IR laser desorption of surface particles followed by ionization of the desorbed particles with a UV laser for time-of-flight mass spectrometric analysis. Experiments have shown that irradiation at the absorption frequency sharply intensifies the signal from all desorbing particles.

Hassel et al. [26] studied the adsorption of hydrogen on the copper crystalline faces using characteristic electron energy loss spectroscopy. They concluded that the desorption of H2 and D2 physically adsorbed on the Cu (510) surface is caused by thermal radiation from the cell walls. Calculations of dipole-excited vibrational transitions to high-lying levels in the physisorption well indicate the possibility of direct infrared photodesorption. For molecules adsorbed on the (100) face, the calculations agree with the experimental results. For HD, the theory predicts the possibility of inducing resonant photodesorption using an infrared laser at a rate much higher than when excitation of substrate phonons [27].

The main application of these investigations was to evaluate the possibility of using resonant excitation of vibrational states of surface species for isotope separation. Some progress in this area has been made with tunable CO2 lasers used for multiphoton excitation of molecules in gaseous or condensed phase. For example, the possibility of selective multiphoton dissociation of 2,3-dihydropyran molecules with the release of oxygen of a certain isotopic composition from a mixture of molecules containing 16O and 18O under laser excitation was demonstrated [28]. For the optimal irradiation wavelength, gas pressure, and laser energy density, an 18O enrichment factor of 751 was achieved. A method for producing the 33S sulfur isotope by irradiating SF6 molecules with resonant radiation from a tunable CO2 laser at a temperature below the crystallization point (195 K), accompanied by the dissociation of sulfur hexafluoride molecules, was proposed by Mathiet et al. [29]. Another example of the application of this technique was the work of Miyamoto et al. [30], who described the enrichment of products with the 13C isotope due to selective dissociation of beta-propiolactone molecules with the formation of carbon dioxide and ethylene.

1.2. Application of resonant IR excitation for isotope separation

The author of [31] was the first to be able to isolate pure metals from an aqueous solution of Fe and Co oxalate complexes using selective laser excitation. When irradiated with a laser at the absorption frequency of palladium chloride, metal palladium was extracted from its aqueous solution [32]. In [33], the selective dissociation of molecules was carried out using the simultaneous action of two CO2 lasers. The authors experimentally studied the dependences of the dissociation rate of gaseous SF6 upon joint irradiation of a cell by two lasers with different frequencies. With two-phase dissociation, the selectivity of the process increases, which is essential for the separation of isotopes of heavy elements with a small isotopic shift.

Heidberg et al. [5] studied the IR spectra of mixtures of carbon monoxide isotopologues (12C160/13C160) adsorbed on a NaCl film and on the surface of a NaCl (100) single crystal. It was found that upon laser excitation of the fundamental vibration of a specific isotopologue, desorption occurs without isotope selectivity. A possible

explanation for this phenomenon may be the energy exchange between molecules of different isotopic composition with each other and between molecules and the substrate.

Hussla et al. [9] studied the desorption of NH3 and ND3 molecules upon excitation by IR laser pulses in the region of N-H vibrations. During the experiment, the studied molecules were adsorbed on the surfaces of single-crystal Cu(100) and Ag films under conditions of high vacuum and low temperatures. It was not possible to observe significant isotopic selectivity during photodesorption. It was suggested that this is due to the energy transfer processes in the adsorbed molecular layers. To solve this problem, it has been proposed to use low surface coatings to reduce lateral intermolecular interactions. The authors also suggest that experiments with molecular systems with relatively weak intermolecular interactions and large differences in vibrational frequencies between different isotopologues could make it possible to observe isotope selectivity in IR photodesorption.

Ambartsumyan et al. [34] studied the selective dissociation of the SF6 molecule by an infrared laser in Ar and CO matrices at temperatures of 8-10 K. In this case, the thermal effect of heating the matrix and the usual evaporation of molecules were excluded. It was shown that, upon irradiation at the frequency of the light 32SF6 isotopologue, the intensity ratio of its band to the band of the heavier 34SF6 decreased. Thus, the above experimental results indicate the observation of the dissociation of an isolated molecule under the action of IR radiation.

Many works have been carried out on the use of laser radiation for the selective effect on molecules of a certain isotopic composition. In [35], the behavior of BCl3 molecules in a xenon matrix in a ratio of 1:100 under continuous CO2 laser irradiation was studied. The author found that 11BCl3 isotopologue molecules form small crystals on a cooled substrate plate, while 10BCl3 molecules are released along with Xe sublimation. This effect is explained by the migration of 11BCl3 during laser irradiation deep into the matrix directly to the substrate. Later, such a mechanism was confirmed in [36], [37]. In [38], the effect of selective laser excitation on tetrazine molecular crystals for isotopic separation and observation of photochemical hole burning was studied. The

presented results showed that tetrazine at low temperatures is capable of consecutive absorption of two photons.

The desorption of NO molecules from the surface of a thick C60 film was studied in [39]. The authors used a pulsed UV laser with a pulse duration of 7 ns. When analyzed using time-of-flight mass spectrometry, three different desorption channels were found. The first channel, which is characterized by a strong rotational-translational coupling, can be explained by the process of desorption caused by electronic transitions. The desorption rate for the fast channel increases linearly with the pulse energy. The second channel was assigned to laser-induced thermal desorption. The third channel is aligned with the second but is characterized by a very late registration time by the mass spectrometer detector, which is presumably due to the desorption delay. Such a delay could be caused by diffusion processes or long-lived excited states.

The authors of [40] demonstrated the process of desorption of neutral hydrogen atoms from the surface of graphite under irradiation with femtosecond UV laser pulses. The desorbed atoms were divided into several groups according to their velocities. This velocity distribution turned out to be the result of desorption from various adsorption sites. Using the density functional theory, it was shown that the differences in the adsorption energy are determined by the lateral interactions of hydrogen on the surface.

In [12], the authors desorbed CD3F condensed on the surface of a NaCl (100) single crystal. The substrate temperature (T=40K) was chosen low enough to ensure that no significant thermal desorption would occur during the experiment. Desorption was carried out by excitation of intramolecular vibrations of CD3F with a tunable IR laser. It turned out that the molecules are desorbed only when the frequency of the exciting radiation coincides with the vibrational frequency of the molecule. Later the authors studied the desorption of various methane isotopologues (CH4, CD4, and CD3H) from the surface of the (100) face of a NaCl single crystal [41]. Desorption was observed upon irradiation at wavelengths corresponding to the normal vibrations of molecules, for example, v4 (7.69 and 10.11 ^m) for CH4 and CD4 molecules, and at wavelengths v2 (7.79, 9.75 ^m) and v4 (9.98 ^m) for CD3H molecules. In mixed systems (with

coadsorption of two isotopologues simultaneously), desorption of two isotopologues in a layer was observed at once, so it is impossible to speak about only the process of direct desorption. It is necessary to consider resonant heating and the interaction of excited molecules with unexcited ones.

1.3. IR spectroscopy of adsorbed ozone

In a number of previous works, the IR spectra of ozone molecules adsorbed on oxide adsorbents were studied [42]-[45]. The IR spectra of 16Û3, 18O3 and mixed isotope modifications of ozone adsorbed on the surface of SiO2, TiO2, and CeO2 at liquid nitrogen temperature were studied in detail in [42]-[44]. During the research, it was found that molecular adsorption does not occur on strong Lewis sites of titanium and aluminum oxides, but the ozone decomposition reaction occurs with the formation of atomic oxygen, which participates in oxidation reactions or becomes a catalyst for the ozone decomposition reaction, accompanied by recombination luminescence [46].

In [43], the IR spectra of various isotopic modifications of ozone adsorbed on the SiO2 surface were studied. It has been shown that ozone is adsorbed on the surface hydroxyl groups by hydrogen bonding with one of the terminal oxygen atoms.

The authors of [47] studied the IR spectra of dissolved ozone in liquid oxygen at a temperature of 77K in the range 3200-650 cm-1. A complete interpretation of the obtained spectra is presented and the relative transition strengths for the observed bands are measured.

Nicolas et al. [45] studied the decomposition of ozone on the surfaces of solid TiO2 in a SiO2 matrix. The authors studied the influence of the TiO2 concentration on the interaction between a sample and an ozone molecule. In addition, it was found that when irradiated with near UV light, the absorption value increases with a simultaneous decrease in the ozone concentration.

The authors of [48] studied the photocatalytic decomposition of ozone on the surface of a thin TiO2 film in a flow reactor. It is shown that the kinetics of ozone decomposition can be described by the Langmuir Hinshelwood mechanism. The

influence of light intensity on the reaction rate was studied, where the power dependence of the reaction rate on the radiation intensity was shown. The parameters obtained during the experiment were used to calculate the distribution of the ozone concentration in the flow reactor.

In [49], the energy and various structures of ozone adsorbed on the hydrated surface of nanocrystalline TiO2 are calculated using the density functional theory. It was found that several types of ozone complexes with titanium oxide cations are formed, namely triplet monodentate, singlet bidentate, singlet monodentate and triplet bidentate. The absence of triplet monodentate complexes in the experiment is explained by their easy decomposition into singlet Ti8Oi7 nanoclusters and molecular oxygen. A coincidence was found between the experimental value of the difference in the frequency of fundamental vibrations (V1-V3) and the value of the most stable singlet bidentate complex.

Thomas et al. [50] studied ozone adsorption by IR spectroscopy at low temperatures. The surface of aluminum oxide (A^O3) was chosen as the adsorbent. The study found that ozone decomposition occurs on the surface of aluminum oxide even at low temperatures.

In [51], the adsorption of ozone on the surface of CaO in the temperature range from 77-300 K was studied by means of IR spectroscopy. Shifts of the bands corresponding to OH and OD groups located on the surface were found. Small shifts of the fundamental ozone bands observed in this case may indicate the formation of a bond between the oxygen of the hydroxyl group and the central oxygen atom of the ozone molecule, which has a positive charge. It has been shown that ozone does not dissociate on the surface of CaO at low temperatures (77K). For the occurrence of the ozone decomposition reaction on the surface, a thermal effect on the sample is necessary.

In [44] the systems of adsorbed ozone on the surface of CeO2, with pre-adsorbed various CO molecules, pyridine, acetonitrile, and methanol, in the temperature range from 7 to 300K were studied. It was found that, in addition to physisorption and the

formation of a weak hydrogen bond with the most acidic sites of OH groups, ozone complexes with two types of acidic sites appear. The formation of ozonides O3-(792, 772 cm-1) and superoxides O2-(1128 cm 1) was detected from the IR spectra. It has been shown that the adsorption of molecular ozone on the surface of CeO has an explosive character, which can be avoided by adsorption from a solution of liquid oxygen.

The study of adsorbed ozone on the surface of polycrystalline MgO was carried out in [52]. The spectra showed two types of bands in the regions 1105-1024 cm-1 and 1140-1038 cm-1 corresponding to stretching vibrations of different oxygen atoms of the adsorbed ozone molecule. The high-frequency shift of the bands appears due to the lateral interaction between the adsorbed molecules. The third type of bands located in the range 1730-1170 cm-1 refers to the vibrations of the oxidation products adsorbed on the MgO surface.

Proch and Schroeder [53] observed the dissociation of ozone in the gas phase upon intense multiphoton CO2 excitation by a laser. It was found that dissociation occurs only when the laser emission frequency coincides with the absorption frequency of the molecules.

1.4. Vibrational energy dissipation

For a successful result of resonant excitation of adsorbed molecules, it is necessary to consider the relationship between the characteristic dissipation times or energy transfer to other particles and the desired process, such as desorption or reaction.

Information about the decrease in the excitation energy of surface particles can be obtained from direct spectral measurements with time resolution. The first attempts in this direction were made by Casassa et al. [54]-[56]. The technique of picosecond laser spectroscopy allowed the authors to measure the energy relaxation time of the vibrational states of the surface OH groups of silicon oxide. It was found that the relaxation time of vibrational excitation of free Si-OH groups at room temperature is about 204 ps and decreases to 159 ps after adsorption of CCl4 [55]. For the same groups in the volume of fused quartz, the time turned out to be shorter and drops from 109 to

15 ps with an increase in temperature from 100 to 1450 K [56]. Such significant lifetimes are in no way consistent with the observed width of the spectral bands. From this, it was concluded that the effect of inhomogeneous broadening or phase relaxation leads to an additional broadening of the spectral bands.

For the acid bridged hydroxyl groups of the simple and deuterated zeolites of the mordenite structure, the excitation lifetimes were measured using the picosecond pump probe technique [57], and the times obtained were 58 and 21 ps for the OH and OD vibration bands, respectively. It is noteworthy that, within the experimental error, the decay time of the absorption bands from the first vibrational state to the second turned out to be the same as the decay time of the transition signals from the ground state to the first excited one for the OH and OD vibration bands, which indicates that the measured decay times and recovery were equal to the lifetime of the first vibrationally excited state. The spectral widths of the pump-induced signals for both groups were broadened by 4 cm-1, which, according to the authors, is due to the dephasing of the pump pulses and the IR probe pulse. For the OD band of the stretching vibration of the zeolite, it was found that the widths of the transition bands turned out to be 11.3 and 14.4 cm-1, while for the absorption band of the unexcited state it was 23 cm-1, a similar feature was observed for the OH vibration band of the zeolite. This means that the inhomogeneous broadening explains the width of the absorption bands of the unexcited state. When the OH and OD groups were perturbed by adsorbed Xe, the frequencies of the hydroxyl bands shifted towards lower wavenumbers. The band width for hydroxyl groups perturbed by adsorbed Xe turned out to be six times larger than in an isolated system, which contradicts the statement that the broadening usually observed for the OH band of systems with hydrogen bonds is inhomogeneous.

Calculations within the framework of the density functional theory considering the Xe atom bound to the Si-OH-Al group allowed the authors of [57] to explain the broadening of the absorption bands. The Xe-H bond, which determines the frequency of the OH stretching vibration band, oscillates many times during the time of IR pulses,

and the authors conclude that the anharmonicity of the potential function of OH stretching vibrations is responsible for the broadening.

An analysis of the experimental contour of the stretching vibration band of free O -D groups of silicon oxide by the method of autocorrelation functions gave the relaxation times of 5.4 and 3.9 ps for the spectra recorded at 80 and 293 K, respectively [58]. However, it was shown that the time of energy relaxation measured directly by time-resolved spectroscopy for O-H and O-D groups of silicon oxide [54]-[56] is two orders of magnitude more. From this, it was concluded that the main reason for the broadening of the band is the process of phase relaxation due to interaction with low-frequency oscillations. The times obtained, estimated from the autocorrelation functions for OH groups, are of the same order of magnitude as those measured in experiments with pumping and probing, which indicates the adequacy of the contour analysis method for studying relaxation processes.

The rapid development of tunable ultrashort pulse lasers for the mid-infrared region, in particular femtosecond laser sources, allows new experiments in time-resolved vibrational spectroscopy. By monitoring the lime-line evolution of vibrations of surface molecules using vibrational frequency summation spectroscopy (SFG), it has become possible to understand the dynamics of surface processes such as desorption, reaction, diffusion, and energy transfer through vibrational modes. The SFG spectroscopy method was used to study the dynamics of the stretching vibration of CO adsorbed on Si(100) [59]. The 12CO and 13CO isotopologues have intense absorption bands centered at 2081 and 2036 cm-1, respectively, with a width of 11-13 cm-1. For a saturated surface, a decay time of vibrational excitation of 2.3 ps was obtained for both 12CO and 13CO. For a surface with mixed isotopologues, the decay time was shorter, about 0.9 ps, on a surface coated with a mixture of equal contents of 12CO and 13CO.

The measured vibrational (relaxation) lifetimes of molecules adsorbed on metal surfaces are in the range of a few picoseconds, which is too short for surface reactions or transformations, with the exception of reactions occurring through physisorbed intermediates. Shirhatti et al. [60] directly recorded the adsorption and subsequent

desorption of vibrationally excited CO molecules from the Au(111) surface. It was shown that vibrationally excited CO molecules stay on the surface for an unexpectedly long time, about 100 ps. This means that vibrational stimulation of metal surface chemistry cannot be completely excluded.

The possibility of determining the time scale of elementary stages of chemical reactions on a surface using femtosecond laser technologies was demonstrated by Gerhard Ertl (later a Nobel Prize winner) and co-authors [61]-[64]. Frequency summation spectroscopy has been used to monitor adsorbates and reaction intermediates directly on the surface. The dynamics of stretching vibrations of CO adsorbed on Ru(001) after optical excitation leading to CO desorption is studied at various temperatures and laser radiation powers. The authors explain the dependence of the frequency and bandwidth of CO vibrations on temperature by the anharmonic interaction with hindered translational mode or other low-frequency vibrations, especially with hindered rotation strongly excited during desorption [63], [64]. The excitation mechanism of CO desorption induced by high-intensity femtosecond laser pulses, studied using subpicosecond dynamics measurements, was described by an empirical model. The results allowed the authors to choose between electron or phonon processes. While heating the surface of ruthenium with co-adsorbed atomic oxygen and CO leads exclusively to the desorption of carbon monoxide, excitation with femtosecond infrared laser pulses leads to the formation of carbon dioxide. Desorption is caused by the interaction of the adsorbate with metal phonons, while the oxidation reaction is initiated by hot electrons, as follows from the observed reaction dynamics, confirmed by calculations using the density functional theory [62].

More recent developments in the field of ultrafast dynamics of surface vibrations, studied using ultrashort tunable laser pulses in the mid-infrared range, were reviewed by Arnolds and Bonn [65]. New ideas on the dynamics of surface processes, including desorption, reaction, diffusion, and energy transfer through vibrational modes in adsorbed layers, have been considered, mainly for metals, but also for more complex systems such as liquid-solid interfaces.

Matsumoto and Watanabe in their review [66] described recent progress in understanding excitation damping and dephasing of oscillatory motions on surfaces and interfaces irradiated with femtosecond laser pulses, focusing on the results of nonlinear time-resolved spectroscopic studies and the theoretical foundations of these processes. Two main channels of energy scattering of a vibrationally excited molecule on the surface are considered: phonon emission and electron-hole excitation. While for adsorbates on insulators or semiconductors with a large energy gap, the population decay due to multiphonon generation is the main deactivation pathway, excitation of an electron-hole pair plays on the main role in deactivation metal surfaces.

The measured full-width at half-height (FWHM) of the infrared absorption band of the CO stretching mode on Cu(100) was 4.6 cm-1 frequency shows a longer lifetime of 2.0 ps, presumably due to inhomogeneous broadening. The measured lifetimes are in good agreement with theoretical predictions, in which the calculated lifetime of the nonadiabatic energy transfer to the excitation of an electron-hole pair was 1-3 ps. Since deactivation by excitation of an electron-hole pair is practically independent of temperature, the strong temperature dependence found for CO on Ni, Ru, and Pt has been attributed to pure dephasing due to anharmonic coupling between the V1 mode and low frequency modes such as hindered rotation. The bandwidth extrapolated to zero temperature in some cases corresponds to a lifetime of up to 8 ps, which is much longer than expected. This discrepancy has been attributed to lateral adsorbate-adsorbate interactions, which shorten the lifetime at high coverages.

Krishna and Tully [67] reported density functional theory calculations of electron-hole pair-induced vibrational lifetimes of diatomic molecules adsorbed on metal surfaces. For CO on certain Cu, Ni, and Pt crystalline faces, they found that the C-O stretching vibration and bending modes have lifetimes in the range of 1 -6 ps. Surface oscillations of CO and modes of hindered rotation relax more slowly, with a lifetime of more than 10 ps. This strong mode selectivity, confirmed by earlier calculations, should be expected for some other metal adsorbents, but not for all. For example, it was not found for NO and CN on platinum (111), where two stretching vibrations have

approximately the same lifetime of about 15 ps, which is in reasonable agreement with the available experimental data.

In the case of oxide adsorbents, the lifetime of vibrational excitation, as noted above, is much longer and for isolated OH groups on the SiO2 surface reaches 200 ps [55]. The resonant dipole-dipole interaction (RDD) in the layer of adsorbed molecules manifests itself in the spectra as the so-called dynamic shift of the absorption band maximum, which, for dipole moment oscillations perpendicular to the surface, is the result of the splitting of the collective oscillation of the layer into two modes, active and inactive in IR spectrum [68]. The magnitude of such a splitting can be interpreted as the frequency of vibration energy exchange between molecules. For CO molecules adsorbed on ZnO, the dynamic shift is 6 cm-1, which corresponds to a frequency of 3.6 1011 Hz or a vibration localization time of 2.8 ps. For the very intense vibration v3 of SF6 or CF4 molecules during adsorption on ZnO [69], the splitting reaches 70 cm-1, and the localization time of the vibration turns out to be an order of magnitude shorter, about 0.3 ps. In the joint adsorption of different molecules with close vibrational frequencies, vibrational energy can be exchanged between them, as was shown by us in the example of the joint adsorption of 34SF6 and NF3 on zinc oxide [70]. This means that the energy exchange of vibrational excitation between molecules of different isotopic modifications of the same compound during adsorption on the surface of oxides can occur faster than the dissipation of energy due to its transfer to the adsorbent lattice.

1.5. Literature review conclusion

The resonant excitation of vibrational states attracts much attention of specialists, mainly because of the hope for a selective action to develop a method for isotope separation. Despite the low efficiency of multiphoton dissociation of free molecules in the gas phase, some success has been achieved. The expected progress in heterogeneous laser-induced processes has faced serious challenges. Attempts to separate isotopes by direct laser desorption processes have not been successful. Studies of various surface processes induced by IR laser radiation demonstrate the real resonant nature of the action, which occurs only upon irradiation in the absorption bands of the species under

study, but the expected isotope selectivity has never been observed, apparently due to a fast energy exchange. Knowledge of the considered processes allows us to formulate some recommendations on the choice of systems to achieve isotope selectivity.

The dissipation time of the vibrational energy of the absorbing molecule should be sufficiently long, i.e., these should not be metals, in which the excitation of an electron-hole pair leads to a rapid damping of the vibrational excitation, but rather insulators or wide-gap semiconductors. To minimize the phonon generation, the molecules should be weakly coupled to the surface, and the frequency of the excited molecular vibrations should be as far as possible from the frequencies of the phonon modes. Considering that, for most oxide adsorbents, lattice vibrations are close to 1000 cm-1 transitions in systems with binding isomerism or to stimulate reactions of intermediate surface species stabilized at 77K. It is significantly higher than the average energy of thermal motion, which excludes spontaneous excitation of such vibrational states even at room temperature.

The vibrational energy exchange in the layer of adsorbed molecules occurs according to the mechanism of resonant dipole-dipole interaction (RDD) [68]. The influence of the RDD interaction depends on the absorption coefficient of the corresponding vibration of the molecule, and on the distance between the interacting molecules. Energy exchange can be minimized by choosing molecules with not too high intensity of vibrational bands and by using relatively small surface coatings.

To avoid an inverse reaction, the process should be as irreversible as possible. It is not possible to combine all the listed requirements in one system. Giving primary importance to irreversibility, ozone was chosen as the object of study. Ozone adsorbed on some oxide catalysts readily decomposes at room temperature, while on active CeO2 this reaction can be explosive even at 77 K [44]. IR studies have shown that on the surface of titanium and cerium oxides, ozone molecules are deformed, losing their symmetry [44], [50], and their absorption bands are noticeably shifted with respect to the frequencies of gaseous O3. It was assumed that this form of adsorption is an intermediate in the reaction of ozone decomposition. The band of the combinational

v1+v3 mode in the region of 2100 cm-1 of the spectrum of ozone of mixed isotopic composition chemisorbed on TiO2 is split into eight separate peaks. Each peak was unambiguously interpreted [17], which allows selectively exciting molecules of a specific ozone isotopologue, in a certain way associated with the surface atom of titanium. Attempts to initiate the selective decomposition of adsorbed ozone using resonant excitation by an IR laser have not been made before, at least we have not found such works.

In connection with the above mentioned, the aim of this work was to study the possibility of initiating the vibrational states of ozone adsorbed on titanium dioxide, its decomposition processes, and the reaction of ozonolysis of co-adsorbed cis-dichloroethylene by resonant excitation. The use of isotopic substitution should provide information on the selectivity of the processes initiated by the excitation of a particular ozone isotopologue.

Chapter 2. Experimental technique

The IR spectra of adsorbed molecules were recorded using Nicolet 510 and IR Prestige 21 IR Fourier spectrometers in the wavenumber range from 5000 to 400 cm-1 with a resolution of 1-4 cm-1. During the experiments, the internal volume of the instruments was purged with dry nitrogen to reduce the absorption bands of atmospheric water and carbon dioxide. The obtained spectra were processed using the OMNIC, IR Solution, and MagicPlot programs. All spectra of adsorbed molecules illustrated in this work are presented after subtracting the spectrum of the substrate.

To prepare the samples, industrial powders were used: TiO2, Hombifine N with a specific surface area of 340 m2/g; SiO2, Aerosil 380 m2/g; CeO2 99.5%, Alfa Aesar, 3050 m2/g; ZnO, Paris, 50 m2/g; BeO "for luminophores"; zeolite NaY; NH4ZSM-5; homemade silicalite-1, MgY with a SiO2/Al2O3 ratio of 60; and Cu-mordenite obtained from Na-mordenite with a Si/Al ratio of 6.5, Zeolyst Int., CBV 10A, exchanged with an aqueous solution of CuSO4. The samples were pressed into tablets and pre-treated by heating at 723 K in vacuum for 1 hour. Then TiO2 and ZnO were cooled in oxygen to avoid reduction.

2.1. Low temperature vacuum cell

The sample was treated and the spectra were recorded using a low-temperature vacuum cell (Fig. 2.1.1) made of stainless steel, the construction of which is described in detail in [71], [72]. The cell consists of two parts. The outer body of the cell performs a thermally insulating function for the internal volume and has a pair of windows made of crystalline KBr. The internal volume is separated from the external one by a pair of internal windows made of crystalline ZnSe. To ensure the tightness of the cell, each window is sealed with indium gaskets.

Fig. 2.1.1 Low temperature vacuum cell construction. Sample (1), sample holder in the inner volume (2), armature (3), quartz tube (4), hook (5), oven (6), Viton gaskets (7), outer body of the call (8), volume for liquid nitrogen (9), internal windows ZnSe (10), external windows KBr (11), indium gaskets (12), valve (13), fluoroplastic gasket (14).

The cell allows measurements at low temperatures when cooled with liquid nitrogen, while it is possible to achieve temperatures up to 55 K by pumping liquid nitrogen vapors with a fore vacuum pump. To measure the temperature, a thermocouple sensor is used, which was placed in a volume for pouring liquid nitrogen.

The pressed sample was placed in a special metal holder, which can be moved using a magnet inside a quartz tube connected to the internal volume. If the sample is moved to the upper part of the tube, then it becomes possible to heat treat it using a heater made of nichrome wire wound on a ceramic tube, in which a chromel-alumel thermocouple is placed opposite the sample.

To record the spectrum, the sample is lowered into the lower part of the cell, perpendicular to the course of the IR radiation of the spectrometer. The lowered sample is cooled by liquid nitrogen being poured into the volume surrounding the interior of the vacuum cell. To ensure thermal contact between the sample and the walls of the cell cooled by liquid nitrogen, 0.2-1 Torr of helium was added to its internal volume.

The pressure was measured using Edwards Barocel absolute capacitance membrane sensors with measurement limits up to 1000 Torr in the external volume located before the cell and up to 10 Torr in the internal volume of the cell.

2.2. Ozone synthesis

For the synthesis of ozone, oxygen obtained by heating potassium permanganate was used. For the preparation of isotopically substituted ozone, 18O2 was used with an enrichment of 76%. The resulting oxygen was filled into a curved glass tube ozonator (Fig. 2.2.1) at a pressure of 10-20 Torr. Ozone was obtained in a high-frequency discharge created using a Tesla transformer during freezing with liquid nitrogen.

Fig. 2.2.1. Ozonator tube scheme. U-shaped trap (1), sealed end of the tube (2), copper wire coil to increase the volume of ionization by the Tesla generator (3).

This was followed by purification of the resulting ozone from oxygen residues and by-products resulting from ionization. The main by-product is CO2 formed by the oxidation of various organic contaminants on the walls of the tubes of the vacuum setup. First, a short-term pumping out of the cooled volume of the ozonator from oxygen residues was carried out. Then the Dewar vessel with liquid nitrogen was

moved so that only the sealed end of the ozonator tube remained submerged, where ozone and all by-products condensed. Then only the U-shaped trap was cooled and the evaporation of ozone from the sealed end of the tube was visually controlled, and immediately after evaporation the entire volume of the ozonator was cooled. Since the temperature at which ozone can evaporate is lower than that of CO2 molecules, after these manipulations, ozone turned out to be a U-shaped trap, and CO2 at the end of the tube. To inject ozone into the internal volume of the call, the ozonator was taken out of liquid nitrogen and, by controlling the pressure increase using a manometric sensor, the first portions of ozone were let into the volume with the sample. With this inflow method the CO2 molecules, to reach the cell, it is necessary to overcome a cooled U-shaped trap, and since the temperature at which ozone can evaporate is lower than that of CO2, it was possible to almost completely clean ozone from impurities and only a small part of CO2 got into internal volume of the cell.

2.3. Vacuum setup

Experiments requiring laser action on samples were carried out on the basis of the resource center of the scientific park of St. Petersburg State University "Optical and laser methods for studying matter". For this, another vacuum installation was built (Fig. 2.3.1), which completely duplicates the previously described installation. Thus, it became possible to conduct experiments without using the first installation necessary for other studies that do not include laser irradiation and without transporting the cell to another building.

Fig. 2.3.1. New vacuum setup: Edwards XD10 vacuum pump (1), Alcatel Adixen ATP 100 turbomolecular pump (2), Barocel vacuum sensor (0.001-10 Torr) (3), Adsorption pump (4), Flexible interface for connecting cell (5).

The main components of the setup are two pumps connected in series: an Edwards XD10 vacuum pump and a turbomolecular Alcatel Adixen ATP 100 pump. The vacuum installation makes it possible to create a vacuum sufficient for the required measurements, on the order of 10-5 Torr.

2.4. Laser system

The samples were irradiated with a MIRcat Tunable Mid IR External Cavity Laser System laser from Daylight Solution. The laser beam was introduced into the optical spectrometer scheme after the device's interferometer using a motorized rotary mirror and focused on the central part of the sample. Since the laser beam area was about 1 mm2 and the sample area was about 5 mm2, a parabolic mirror was used to increase the laser beam area to the size of the sample.

It was possible to tune the laser radiation frequency in the range from 1989 to 2330 cm-1, due to two built-in modules, the power dependence of which on frequency is shown in Figure 2.4.1. The laser was used in a pulsed mode with a pulse frequency of 1 MHz and a pulse duration of 100 ns.

22S0 2230 21S0 2130 ?080 2030 wavenumber (cnvl)

Fig. 2.4.1. Diagram of laser power versus radiation frequency, the diagrams correspond to two laser modules when used in continuous radiation mode.

The average laser power for frequencies of 2100 and 2200 cm-1 is about 45 mW and differed by no more than 30% (Fig. 2.4.1), and at a frequency of 2022 cm-1 it was approximately 10% of the power at a frequency of 2200 cm-1. For some experiments, the "sweep mode" mode was used, in which the frequency changes within the indicated limits during the entire irradiation time at a rate of 5 cm-1/s.

All experiments using laser radiation were carried out on the basis of the resource center of the scientific park of St. Petersburg State University "Optical and laser methods for studying matter" https://researchpark.spbu.ru/laser-rus.

Chapter 3. Experimental data

3.1. Adsorption of ozone molecules on oxide adsorbents

As a preparation for experiments with laser radiation, the IR spectra of ozone molecules adsorbed at 77K on various substrates (TiO2, CeO2, Al2O3, ZnO, BeO, Cu mordenite) were experimentally obtained. Ozone was synthesized from pure 16O2 oxygen or a mixture of 16O2 and 18O2. In the spectra of adsorbed ozone, special attention was paid to the position and isotopic splitting of the v1+v3 vibration band.

The obtained spectra of ozone adsorbed on A^O3 are shown in Figs. 3.1.1. Only one adsorption type of molecules with fundamental frequency bands of 1110 and 1036 cm-1, as well as their combinational vibration of 2114 cm-1 and an overtone of the v3 band of 2046 cm-1 (Fig. 3.1.1, spectrum 1) was found. It is consistent with the previously obtained data [50]. An irreversible increase in pressure with increasing temperature was observed, apparently caused by the ozone decomposition.

The spectrum of ozone adsorbed on ZnO (Fig. 3.1.1, spectrum 2) also shows bands of only one adsorption type, although in some experiments a couple of additional bands at 825 and 768 cm-1 and a weak band around 1019 cm-1 were also observed. As in the case of adsorption on alumina, an increase in pressure was observed, indicative of ozone decomposition.

m

o

Fig. 3.1.1. IR spectra of adsorbed ozone on the surface of AI2O3 (1) and ZnO (2) at 77 K.

The region of fundamental ozone oscillations in the spectra of zeolites and beryllium oxide overlaps with the strong absorption of the adsorbate. Therefore, the spectra of adsorbed ozone were studied only in the region of the combinational vibration V1+V3. The results are presented in Fig. 3.1.2 where the band corresponding to the ozone vibration adsorbed on silicalite is at a frequency of 2098 cm-1. For the HZSM-5 zeolite, the position of the band is somewhat higher than 2101 cm-1.

Fig. 3.1.2. R spectra of adsorbed ozone on the surface of zeolites: (1) silicalite-1, (2) HZSM-5, (3) NaY, (4) MgY, and (5) BeO at 77 K. The spectra are normalized by the intensity at the maximum of the band, which for different adsorbents was in the range of 0.01-0.6.

The spectra of NaY, Cu-mordenite, MgY, and BeO, in addition to the 2114-2107 cm-1 band, contain bands at much higher frequencies: 2126, 2138, and 2146 cm-1 for NaY, MgY, and BeO, respectively. Pumping out excess ozone leads to a decrease in the low-frequency band, and the bands at higher wavenumbers shift a little higher, reaching 2140 cm-1 for MgY. For Cu-mordenite, the removal of weakly bound ozone with a band at 2111 cm-1 (Fig. 3.1.3) is accompanied by an increase in the bands at 2163, 2144, and 2124 cm-1, which then also disappear simultaneously with the appearance of a new strong single maximum at 2156 cm-1. After that, this band decreases and finally disappears after pumping out at a temperature of 300 K.

_i_i_i_i_i_i_i_i_i_i_i_i_

2180 2160 2140 2120 2100 cm"1

Fig. 3.1.3. IR spectra of adsorbed ozone on the Cu-mordenite surface at 77 K (1), 1 hour after adsorption (2), after evacuation at 200 (3) and 300 K (4).

The absence of chemisorbed ozone molecules on AI2O3 is explained by the low resistance of such complexes to dissociation, which leads to desorption of oxygen, while the resulting atomic oxygen remains attached to the cationic site [50]. Apparently, the same reason prevents the molecular chemisorption of ozone on ZnO, where, in addition to the ozone decomposition, which manifests itself in the emission of O2, the formation of ozonide or peroxide particles is also possible, which explain the bands at 825 and 768 cm-1.

In the region of v1+v3 vibrations, the spectra of metal-substituted zeolites and beryllium oxide exhibit bands of adsorbed ozone shifted to the high-frequency region. For silicalite or HZSM-5, such bands are absent, which indicates the ability of ozone to form complexes with the cations of these adsorbents. An increase in the frequency of the v1+v3 combinational vibration from NaY to MgY reflects the increasing Lewis acidity of the cations. The band at 2126 cm-1 in the spectrum of ozone adsorbed on Cu-mordenite, which coincides in frequency with the same band in the spectrum of NaY zeolite, can be explained by the presence of residual Na+ cations that were not replaced during synthesis. The growth of a new band during desorption with a decrease in the

intensity of other bands is characteristic of the lateral interaction between adsorbed molecules [68]. Thus, the two bands at 2163 and 2144 cm-1, which disappear together with the growth of the new band at 2156 cm-1, can be attributed to a complex of two ozone molecules attached to the same Cu+ cation and strongly interacting with each other. Such an effect is well known for copper carbonyl complexes in zeolites, when bands of di- and monocarbonyl particles can be successively observed during CO desorption [73].

In Fig. 3.1.4 the spectra of the isotopic mixture of ozone adsorbed on TiO2 at a temperature of 77K are shown. After the removal of physisorbed molecules under vacuum pumping at a temperature of about 100 K, the bands of chemisorbed molecules remain; such a spectrum was subtracted from the initial spectrum before pumping out to obtain a pure spectrum of weakly bound ozone mixtures (spectra 1-3). Six prominent bands in the region of the v1+v3 combinational vibration practically coincide in position with those observed in the spectrum of ozone mixtures on silicon oxide [43]. In the range of v1 oscillations, several very weak bands can be distinguished at a frequency of 1111 cm-1 for 16O3 and at frequencies of 1092 and 1062 cm-1 for the isotope mixture of ozone. The v1+v3 band contour of chemisorbed molecules (spectra 4-6), obtained after the removal of physisorbed ozone, has a structure of 8 well-resolved maxima of almost the same intensity for a 50% mixture. For a mixture with the predominant 18O isotope, the most intense low-frequency band at 2019 cm-1 is accompanied by less intense satellites at 2084, 2051, and 2038 cm-1. In the range of v1 vibration, only three maxima are observed at 1145, 1118-1115, and 1084 cm-1, with the central band twice as intense as the others for a 50% mixture. The central band maximum shifts somewhat towards lower wavenumbers with an increase in the 18O content, when the low-frequency component becomes more intense.

10,62

—I-1-1-1-1-1" " ' T " "' -1-1-1—Illi-

2150 2100 2050 2000 1150 1100 cm"1

Fig. 3.1.4. IR spectra of adsorbed ozone on the TiO2 surface at 77 K. Physisorbed ozone with 76% (1), 50% (2) and 0% (3) 18O content; chemisorbed ozone with 76% (4), 50% (5) and 0% (6) 18O content. The spectrum of chemisorbed ozone was obtained by pumping out the using cell volume at 100 K. The spectrum of weakly bound ozone was obtained by subtracting the spectrum of chemisorbed ozone from the original spectrum before pumping out.

The study of the ozone adsorption spectra on CeO2, obtained with a better signal-to-noise ratio than in [44], made it possible to observe a combinational band V1+V3, not observed before, both for weakly bound and chemisorbed ozone. From fig. 3.1.5, spectrum 2 one can see that the band positions are decreasing with time and finally disappearing 40 min after the ozone injection, do not differ from those previously presented [44] and are close to the positions of dissolved ozone. The spectrum of this adsorption type, obtained by subtracting spectrum 3 from the initial spectrum of adsorbed ozone (spectrum 1), is also shown in the figure (spectrum 2). The spectrum presented at number 3 was obtained 40 minutes after the addition of ozone, the increase in pressure indicates its partial decomposition. The band shoulder at 1024 cm-1 and the second band at 1019 cm-1 (spectrum 2) are due to chemisorption on weaker sites, which agrees with previous data [44].

2150 2100 1150 1100 1050 1000 cm"1

Fig. 3.1.5. IR spectra of adsorbed ozone on the surface of CeO2 at 77 K, after adsorption (1), spectrum of weakly bound 16O3 (2), and chemisorbed 16O3 (3).

Adsorption of 18O3 with an enrichment of 76% leads to the spectra shown in Fig. 3.1.6. Weakly bound ozone (spectrum 1) was obtained by subtracting the spectrum of chemisorbed ozone (spectrum 2) from the spectrum recorded after O3 adsorption. The V3 bands form two triplets of maxima shifted relative to each other by approximately 35 cm-1. The intensity of the low-frequency components (1013 and 978 cm-1) is approximately two times higher than that of the central bands (1021 and 987 cm-1). The band maximum positions (1035, 1021, 1013, 1002, 987, 978 cm-1) practically coincide with the positions of the ozone isotope mixture in liquid oxygen (1035.0, 1021.3, 1012.3, 1001.7, 987.4, 978 .1 cm-1) [47]. The second spectrum is the spectrum of chemisorbed ozone remaining on the surface one hour after the addition of O3. If we compare the positions of the maxima (1004, 990, 972, and 957 cm-1) with those previously observed for a 50% mixture of ozone isotopes on cerium (1009, 992, 977, 960 cm-1) [44], then we can see that the given values are lower by 3-5 cm-1, and for pure 18O3 (957 cm-1) they are the same. Due to the low intensity, the structure of the band of Raman vibrations of chemisorbed ozone is not clearly resolved; we can only state that the lowest component falls at 1985 cm-1 and is shifted by 115 cm-1 relative to that for pure 16O3 (2100 cm-1).

Fig. 3.1.6. IR spectra of the adsorbed isotope mixture of ozone (76% 18O) on the surface of CeO2 at 77 K, weakly bound (1), chemisorbed (2).

The data on ozone adsorption on TiO2 agree with the previously published results [74] and provide more detailed information about the spectrum in the region of the V1+V3 combinational vibration. The band of weakly bound ozone splits into 6 maxima, consisting of two triplets (Fig. 3.1.7a), as in the spectrum of ozone adsorbed on silicon oxide [43] or in solution [47]. Only six maxima means that the two oxygen atoms in the molecule are still equivalent, as they are for a free molecule. Then the replacement of terminal atoms gives a triplet, the central band of which is twice as intense as the others for a 1:1 mixture, and the replacement of the central atom shifts the entire triplet by an amount A, which is not the same for three normal vibrations of the molecule. In the spectrum of chemisorbed ozone remaining on the surface after desorption of physisorbed molecules, the V1+V3 band splits into 8 distinct maxima (Fig. 3.1.7b). This can happen if the ozone molecule loses its symmetry, then the replacement of each oxygen atom leads to its specific shift. This situation is illustrated in fig. 3.1.7b, and the three shifts in our case are 22, 33, and 63 cm-1, respectively.

1200 U50 nOO 1050 cm"1 -M-

e

1000 980 960 940 cm"1

Ai-1

Fig. 3.1.7. Scheme of isotopic splitting of ozone adsorbed on TiO2. Splitting of the V1+V3 vibration band of physisorbed ozone (a), chemisorbed ozone (b), V1 of chemisorbed ozone. Scheme of the monodentate complex of chemisorbed ozone (d), calculated V3 spectrum of chemisorbed ozone (e). A means the value of the frequency shift due to the replacement of the central atom in the molecule.

In fig. 3.1.8 the results of deconvolution of spectra 4 and 5 into Gaussians from fig. 3.1.4 are shown. Eight components have similar intensities and band widths for 1:1 mixture. If the spectrum with 76% 18O by a set of Gaussians in the same positions and the same width is approximated, then one can see that after the most intense band 2019 cm-1 corresponded to 18O3, the next intensities in descending order are comparable in intensity at 2038, 2051 and 2084 cm-1, which should be attributed to isotopologues with two 180 atoms in different positions. The remaining three less intense bands should belong to the types of ozone adsorption with one 18O atom. The relatively intense band at 2137 cm-1, corresponding to 16O3, may be due to its overlap with the 2v1 band of 18O3, and traces of weakly bound ozone, apparently, explain the additional component below 2019 cm-1.

_i_i_i_i_i_i_i_i_i_i_i_i_i_«_i_i_i_i_

2160 2140 2120 2100 2080 2060 2040 2020 2000 cm-'

Fig. 3.1.8. Deconvolution spectrum of the V1+V3 band contour of ozone with 50% (a) and 76% (b) content of 18O chemisorbed on the surface TiO2 at 77 K.

In this case, the v1 band splits into 3 maxima with a more intense central one. This means that for this vibration, two of the 8 surface ozone isotopologues contribute to the high-frequency maximum, the other two contribute to the low-frequency maximum, and the remaining four have their frequencies near the position of the central maximum (Fig. 3.1.7c).

Table. 3.1.1. The Band position of ozone chemisorbed on TiO2.

isotopologue* V1+3, cm-1 V1, cm-1 V3 (calculated), cm-1

666 2137 1145 992

866 2115 1145 970

668 2104 1118 986

868 2084 1115 969

686 2074 1118 956

886 2051 1115 936

688 2038 1084 954

888 2019 1084 935

* "666" means 16O-16O-16O, "668" refers to 16O-16O-18O, etc.

Knowing the values of the vibration frequencies v1+v3 and v1, it is possible to calculate, without taking into account the anharmonicity, the vibration frequencies v3 for each of the 8 adsorption types. These calculations are presented in Table 3.1.1, where ozone molecules with a certain number of 16O and 18O atoms are designated as isotopologues 666, 668 ... etc., where the sequence of numbers corresponds to the sequence denoting oxygen atoms as shown in fig. 3.1.7d. When completing the table, the spectra of pure 16O3 or 18O3 molecules were considered, where the maximum or minimum value of the v1+v3 band corresponds to the maximum or minimum value of the frequency of the v1 band, respectively.

To obtain the frequency values v3 for each isotopologue, it is necessary to subtract one of the three v1 values, either 1145 cm-1, or others, from the remaining six frequencies v1+v3. If not 1145, but 1115 cm-1 or less is subtracted from 2115 cm-1, then the obtained values of v3 will be higher than for 16O3, which completely contradicts the experiment. Thus, the value of v1 corresponding to the 2115 cm-1 band should be 2115— 1145 = 970 cm-1. Similarly, the 2038 cm-1 band should correspond to 954 cm-1. Now the four structures contributing to 1145 cm-1 and 1084 cm-1 are correlated, and all other four types correspond to the central band of the v1 vibration. Considering that the exact position of this band shifts from 1118 to 1115 cm-1 with increasing enrichment, we can classify them as isotopologues with one and two 18О atoms, respectively. The full spectrum of v3 vibrations is schematically shown in Fig. 3.1.7e. As expected, the spectrum consists of 4 pairs of bands of equal intensity for a 1:1 mixture of isotopologues.

The formation of chemisorbed ozone complexes is accompanied by an increase in the difference between the frequencies v1 and v3 from 61 cm-1 for gas to 158 cm-1 for TiO2. Apparently, this reflects the different force constants of the O-O bonds, which are schematically shown in Fig. 3.1.7d. The splitting of the v1 band into three maxima is consistent with its assignment to a vibration corresponding to a stronger O-O bond. Then it is natural that the frequency v1 is insensitive to the substitution of atom 1 in the scheme. The splitting of the low frequency v3 band into four maxima should be

expected for a vibration localized on a weaker bond between two nonequivalent oxygen atoms marked 1 and 2 in the scheme. Such a vibration should not be sensitive to the substitution of atom 3 and have different frequency shifts upon substitution in positions 1 and 2. A single non-conflicting choice results in the assignment presented in Table 3.1.1. The presented data on isotopic splitting confirm the monodentate type of adsorption, in which the molecule is no longer symmetrical and does not have equivalent oxygen atoms. The performed quantum chemical calculations also confirm this conclusion [18].

Table 3.1.2 summarizes the data published and obtained in the course of the work on the spectra of adsorbed ozone, as well as gaseous ozone, a solution in liquid oxygen, and ozone in an Ar matrix for comparison. Separate rows indicate data on the same adsorbents but taken from different literature sources or obtained under different pretreatment conditions or when recording spectra. In addition to the positions of the observed bands, separate columns give the sum (V1)+(V3) of the observed fundamental frequencies V1 and V3 and the difference between this sum and the frequency of the maximum of the combination v1+v3 band. This difference characterizes the anharmonic interaction of two modes, which denotes as %13 = (V1)+(V3)-(V1+V3). It is noteworthy that for a free O3 molecule and for adsorbed ozone on SiO2 and CaO, where there are no acidic OH groups, and for weakly bound ozone on most oxides, the position of the combinational V1+V3 band is lower than the sum of the corresponding fundamental V1 and v3 vibrations by 30-35 cm-1, which indicates a significant contribution of anharmonicity. However, this fact does not apply to strong adsorption on cerium or zirconium dioxide, where the difference is two times smaller, and for titanium dioxide, both values practically coincide. This means that the high-frequency position of the combinational band of chemisorbed ozone is associated to a greater extent not with an increase in the frequency values of the fundamental modes, but with a decrease in anharmonicity. Therefore, the V1+V3 band position of ozone vibrations in the spectra of those adsorbents, where the region of fundamental vibrations is overlapped by strong

absorption of the adsorbent itself, could help to obtain information about the mechanism of ozone activation in such systems.

Table. 3.1.2. The band positions in the gas phase, in liquid oxygen solution, in the Ar matrix, and in the adsorbed state.

Adsorbent Treatment temperature, K v1, cm-1 v3, cm-1 V1+ V3, cm-1 (V1)+(V3), cm-1 -X13, cm-1 Ref.

Gas phase - 1103.1 1042.1 2110.8 2145.2 34.4 [47]

liquid oxygen solution - 1102.5 1035.0 2102.3 2137.5 35.2 [47]

Ar matrix, 11 K - 1105.1 1039.7 2108.7 2144.8 36.1 [43]

SiO2 923 1104 1037 2106.4 2141 34.6 [52]

MgO 1073 1140 1038 - 2178 - [52]

1110 1024 - 2134 - [52]

- 1125 1022 - 2147 - [42]

CaO 300 1106 1035 2106 2141 35 [51]

973 1109 1034.5 2113 2143.5 30.5 [51]

TiO2 hydrated 1108 1034 - 2142 - [74]

773 1147 989 2136 2136 0 [74]

ZrO2 - 1138 1001 2122 2139 17 [42]

CeO2 - 1126 1004 - 2130 - [42]

hydrated 1116 1035 2105 2141 36 [44]

300 1098 1019 - - - [44]

773 1102 1008 2100 2110 10 [44]

773 1116 1034 2111 2150 39 This work

1104 1009 2099 2113 14

ZnO 773 1105 1034 2107 2139 32 This work

Al2O3 773 1110 1036 2114 2146 32

Al2O3 770 1113 1038 2151 [50]

According to the data from [44], in the spectrum of ozone adsorbed on CeO2, in addition to the bands of physisorbed molecules, chemisorbed particles with reduced vibrational frequencies v3 are found. Two adsorption types can be distinguished with the position of the v3 bands at 1024-1016 cm-1 and 1012-1009 cm-1. The relative intensity of the bands of these two particles depends on the pretreatment temperature. In contrast to titanium oxide, the band corresponding to the v1+v3 combinational vibration is not

shifted towards large wave numbers and, as can be seen from Table 3.1.2 differs from the sum of v1 and v3 by 14 cm-1, demonstrating a smaller anharmonicity contribution than for free or physisorbed ozone molecules.

A group of four bands in the region of the V3 vibration was previously observed in the spectrum of a 50% ozone isotope mixture chemisorbed on cerium oxide [44]. The small discrepancy mentioned above between the positions of the band maxima indicated there and those observed in this paper for 76% ozone isotope mixtures can now be explained. In fact, the origin of the bands is the same as for the TiO2 case shown in Fig. 3.1.7. In each unresolved doublet, the low frequency component has one more 18O atom and thus dominates in the higher enrichment mixture, while the average value is observed for the 50% mixture.

The strong perturbation of chemisorbed ozone during adsorption affects not only the band positions. The intensity of the V1 band with respect to V3 band, which for ozone in the gas phase, dissolved in liquid oxygen, or physisorbed on titanium dioxide, is 4%, 0.7%, and about 2.5% [74], respectively, and increases significantly to 12% on CeO2, and up to 60% for ozone chemisorbed on TiO2.

Thus, the band frequencies for the fundamental v1 and combinational v1+v3 vibrations for all eight mixed ozone isotopologues chemisorbed on TiO2 were obtained. All ozone isotopologues have different distinct bands of the V1+V3 combinational vibration, while the V1 band is divided into three maxima containing 2 pairs of unresolved doublets, and the central maximum consists of four close bands. The splitting structure of the V3 band has four maxima of unresolved doublets, as was previously found for CeO2, where such a structure was clearly observed.

The completely different shifts caused by the isotopic substitution of each oxygen indicate a complete loss of the molecular symmetry and confirm the assumption of a monodentate structure of the surface complex, where one of the terminal oxygen atoms of the molecule is bonded to the surface titanium atom. The formation of such a complex is accompanied by a shift of the V1+V3 combinational band to higher

wavenumbers by about 30 cm-1, despite a decrease in the frequency of the v3 band by about 40 cm-1. The shift of the combinational band is rather due to a decrease in anharmonicity, which explains the difference between the sum of the frequencies of the fundamental vibrations (v1)+(v3) and the observed position of the combinational band v1+v3 by about 35 cm-1. For free, dissolved, or physisorbed molecules, this shift decreases to 14-17 cm-1 for CeO2 or ZrO2 and almost completely disappears for TiO2.

An increase in the frequency of the combinational vibration is not necessary for chemisorption; in the case of CeO2, where the frequency v3 is also lowered, the v1+v3 band occupies the same position at 2099 cm-1 as physisorbed ozone. However, in the spectra of BeO or zeolites, in addition to the band of physisorbed ozone, there is one or even two bands of the v1+v3 combinational vibration, shifting to the high-frequency region up to 2126 cm-1 for NaY, and up to 2168 cm-1 for Cu-mordenite. In the latter case, the disappearance of two bands is accompanied by the appearance of a new one in an intermediate position. This can be explained by the initial adsorption of two interacting O3 molecules on the same cation, followed by desorption of one of them.

Thus, titanium and cerium oxides are the most promising adsorbents for experiments with laser initiation of ozone decomposition and ozonolysis reactions. When adsorbed on these adsorbents, the ozone decomposition reaction occurs spontaneously, but at the same time slowly enough to hopefully notice changes caused by laser radiation. Perhaps, to slow down the processes, it will be necessary to pre-hydrate the surface of the samples. The V1+V3 band has sufficient intensity both for the usual 16O3 ozone isotopologue and for its isotopic mixture enriched in the 18O isotope. Changes in the relative intensity of the components in the spectrum of adsorbed ozone of a mixed isotopic composition under the laser radiation at the frequency of certain isotopic modifications should make it possible to observe changes in their concentration and judge the possibility of selective initiation of the ozone decomposition reaction by IR radiation. A detailed interpretation of the v1+v3 band contour makes it possible to select and control changes in the concentration of each specific ozone isotopologue.

3.2. Initiation of the decomposition of adsorbed ozone by laser radiation

The oxides of cerium CeO2 and titanium TiO2 were chosen as adsorbents for experiments with ozone decomposition under the laser radiation. During the experiments described in the previous chapter, it was shown that the reaction of ozone decomposition on the surface of titanium oxide occurs spontaneously already at 77 K. It is possible to significantly reduce the rate of ozone decomposition by preliminarily infusing water onto the surface of the sample and removing its excess by pumping at 363 K. The resonant excitation of molecular vibrations of specific isotopologues in the region of the v1+v3 transition should presumably lead to the selective decomposition of ozone and the release of oxygen molecules of a certain isotopic composition.

A sample of hydrated CeO2 with adsorbed ozone of mixed isotope composition was irradiated for several hours at vibrational frequencies of various adsorbed ozone isotopologues. The progress of the reaction was monitored by successive recordings of the spectrum after each irradiation. However, no noticeable changes in the spectrum were found. It should be noted that the intensity of the v1+v3 band is an order of magnitude lower (Fig. 3.1.5) than the same band for ozone adsorbed on TiO2, and it was not possible to notice an insignificant effect of a decrease in the band of a particular isotopologue.

The ozone adsorption at 77 K on hydrated TiO2 leads to the appearance and gradual increase in the intensity of the absorption bands of adsorbed molecules. The band positions of ozone adsorbed on hydrated TiO2 slightly differs from the data of previous works [17], [42]. The fig. 3.2.1 shows the spectrum of ozone with a mixed isotopic composition (76% 18O3) in the v1+v3 band region, where the curves are normalized to the low-frequency band 1996 cm-1 corresponding to vibrations of 18O3 molecules. Spectrum 1 was recorded 70 min after the adsorption onset, after which a series of successive sessions of laser irradiation at a frequency of 2030-2020 cm-1 was performed, which lasted a total of 115 min. As it can be seen from fig. 3.2.1, irradiation causes a slight decrease in the intensity of the 2022 cm-1 band relative to 1996 cm-1. At the same time, in the intervals between sessions, over time, the initial ratio of the

intensities of these bands is restored. Subsequent irradiation at a frequency of 2200 cm-1 in the region where there are no ozone absorption bands leads to an accelerated return to the initial intensity ratio. The total integrated intensity of the V1+V3 ozone bands, which increased during irradiation at a frequency of 2030-2020 cm-1, slightly decreases with irradiation at a frequency of 2200 cm-1.

Fig. 3.2.1. Spectrum of ozone (O3 76%) adsorbed on TiO2 in the V1+V3 vibration range after 70 min. after puffing (1), after irradiation at a frequency of 2030-2020 cm-1 for 115 min (2) and subsequent irradiation at a frequency of 2200 cm-1 for 45 min. (3). The spectra are normalized to the 1996 cm-1 band.

The irradiation effect is clearly seen in Fig. 3.2.1, which shows the dependence of the ratio of the integral intensities of the 2022 and 1996 cm-1 bands on the exposure time. The integral intensity of the bands was determined by deconvolution the general contour into components representing Gaussian curves. Already after the first 25-min irradiation session at a frequency of 2030-2020 cm-1, the intensity ratio changed noticeably in favor of the low-frequency band. Further irradiation at this frequency increases this ratio, initially 1.32, up to 1.49. Irradiation at a frequency of 2200 cm-1

shows the reverse dynamics of changes in the ratio of the intensities of the absorption bands. The errors on the graph were obtained for a confidence level of 0.95 with six measurements (each measurement was made with a baseline variation).

Fig. 3.2.1. Dynamics of the ratio of the integrated intensities of the bands 1996 and 2022 cm-1, when irradiated at a frequency of 2030-2020 cm-1 (dots on the graph) for 0 (1), 25 (2), 55 (3) and 115 minutes (4), and at subsequent irradiation at a frequency of 2200 cm-1 (squares on the graph) for 20 (5) and 45 minutes (6).

Unfortunately, the intensity of the bands of adsorbed ozone at 77 K is not constant during the experiment. Initially, the intensity increases due to continued adsorption from the gas phase. After that, the rate of spontaneous ozone decomposition, which increases with increasing coverage, is compared with its adsorption rate, and a gradual decrease in coverage begins due to the consumption of the total amount of ozone in the cell. As a result, it is very difficult to directly observe the decrease in the intensity of the ozone bands under the action of irradiation, that is why only the relative intensity were compared.

The difficulty of observing the relative intensity change of the bands of adsorbed ozone is aggravated by the fact that under the experimental conditions with hydrated

TiO2 as an adsorbent, the spectrum contains the absorption of both physisorbed and chemisorbed ozone, and the band intensities of the latter is extremely low, and it is not possible to track its change. Thus, the vibrations of both chemisorbed 18O3 and physisorbed 161818O3 contribute to the band at 2022 cm-1 [17]. It is not clear which of the modifications is more prone to decomposition under IR irradiation, and the lasing region of the laser used does not allow irradiate into the 1990 cm-1 band of physisorbed 18O3 [17].

Nevertheless, in experiments with mixed isotopic ozone, irradiation at a frequency of 2030-2020 cm-1 leads to a change in the ratio between the intensities of the bands of different isotopologues. In this case, the relative intensity of the band at the frequency of which irradiation is performed decreases. With time without irradiation, the intensity ratio gradually returns to its original value, apparently because of the continued adsorption of ozone from the gas phase. This process is somewhat accelerated by irradiation outside the ozone absorption band at a frequency of 2200 cm-1.

Thus, the influence of laser radiation at the frequency of the combinational V1+V3 vibration of certain ozone isotopologues adsorbed on titanium oxide leads to an insignificant but reliably fixed decrease in the relative intensity of the band at the frequency of which irradiation is performed. The dynamics of the band ratio as a function of irradiation confirms the selective decomposition of a certain ozone isotopologue under the laser radiation.

3.3. Ozonolysis of adsorbed molecules stimulated by IR radiation

Adsorbed ozone easily reacts with some molecules. Thus, ozonolysis of ethylene adsorbed on silicon oxide proceeds actively even at 77 K [76]. With chlorinated ethylene the reaction proceeds more slowly, but the titanium oxide surface catalyzes the ozonolysis of dichloroethylene, the rate of which can be slowed down by hydrating the TiO2 surface. Ozonolysis stimulation of adsorbed molecules by resonant excitation of vibrations of adsorbed ozone is very tempting, since the sensitivity of the method in detecting the appearance of new absorption bands is much higher than in terms of

intensity changes. While it is more difficult to observe changes in the intensity of the absorption bands of ozone itself, especially since at 77 K, having a sufficient vapor pressure (about 102 Torr), it continues to be adsorbed and desorbed. With coadsorption of ozone and chlorinated ethylene, the ozonolysis reaction is possible according to the scheme described in [77] (Fig. 3.3.1.). When the reaction is not spontaneous, there is a chance that the vibrational excitation of ozone can initiate ozonolysis. The possibility of carrying out a selective reaction is not ruled out if radiation at the absorption frequency of a specific ozone isotopologue is used. To verify these statements, a series of experiments was carried out on the ozonolysis of adsorbed cw-dichloroethylene on the surface of SiO2 and TiO2 under the influence of laser irradiation at a temperature of liquid nitrogen or higher.

Fig. 3.3.1. Ozonolysis reaction scheme Criegee (top). Possible products (bottom) of the reaction: phosgene (a), formaldehyde (b), formyl chloride (c).

3.3.1. Ozonolysis of dichloroethylene on SiO2

The SiO2 sample was pressed from Aerosil 380 powder with a specific surface area of 380 m2/g. Sample area 1.8 cm2, weight 12.3 mg. Temperature treatment was carried out under vacuum pumping at 873K for 40 minutes. After treatment, the sample was cooled to 223K and cw-dichloroethylene was injected (Fig. 3.3.2 and 3.3.3). This was followed by cooling the sample to liquid nitrogen temperature and subsequent ozone release.

j_i_i_i_i_i_i_i_l

3800 3750 3700 3650 3600 3550 cm1

Fig. 3.3.2. IR spectra in the region of OH vibrations of SiO2 at 293K(1), after the injection of CZ5-C2H2CI2 molecules at 213K (2), after 5 (3), 15 (4), 20 minutes (5), after cooling to 77K (6)

1610 1600 1590 1580 1570 1560cm1

Fig. 3.3.3. IR spectra in the range of cis-C^HiCh vibrations. Initial SiO2 sample at 293K(1), after dichloroethylene filling at 213K(2), after 5 (3), 15 (4), 20 minutes (5), after cooling to 77K (6)

In the spectrum of SiO2 before adsorption, a band of silanol groups was observed at a frequency of 3750 cm-1. After the release of cis-C2H2Cl2, the intensity of the band of silanol groups decreased, while a broad band of perturbed silanol groups appeared at a frequency of 3650 cm-1, which indicates the adsorption of molecules on these groups. In addition to the perturbation of hydroxyl groups, there was an increase in bands at

3084, 1590, 1296, 703 cm-1, corresponding to the vibrational frequencies of dichloroethylene slightly lower relative to the free molecule due to adsorption.

Figure 3.3.4 shows the difference spectra obtained by subtracting the spectrum obtained before the ozone release from the spectra after the release. After adding ozone (synthesized from oxygen with 76% enrichment in 18O) to the cell volume, an increase in the bands of adsorbed ozone in the region of the combinational V1+V3 vibration was noticeable, with intense peaks at 2019 and 1991 cm-1 for ozone with two and three substituted 18O isotopes, respectively. Over time, the total integrated intensity of the ozone bands increased, apparently because of continued adsorption. Irradiation at a frequency of 1991 cm-1 generally did not affect the nature of the change in the spectrum. Irradiation at 2019 cm-1 had the effect of reducing the intensity of all ozone bands. But at the same time, at a frequency of 2019 cm-1, the integrated intensity decreased more than for 1991 cm-1 by 2%, the method by which the intensity ratio was determined was similar to that described in the previous chapter: the general contour was decomposed into curves described by Gaussian functions (Fig. 3.3.5), after which the area under these curves was compared.

1991

4J CJ

G

cs &

u o

CS

_I_I_I_

2200 2100

2000

1900

Fig. 3.3.4. Spectrum of the O3 isotopic mixture adsorbed on SiO2 at 77 K in the V1+V3 combinational vibration region. 10 minutes after the start of adsorption (1), 20 minutes (2), after 60 minutes, the last 20 of which were

irradiated at a frequency of 1991 cm-1 (3), after 120 minutes, the last 20 of which were irradiated at a frequency of 2019 cm-1 (4). The spectrum of the initial sample with adsorbed C/5-C2H2Q2 was subtracted.

0.01 \

\ /

/ \ / \ / № V

f\A /\ V 1 \! t / \ '1 } ! \ 1 \

■ 1 1

2150 2100 2050 2000 1950cm1

Fig. 3.3.5. Deconvolution spectrum bands in the V1+V3 combinational vibration region. 120 minutes after the start of adsorption, while the last 20 minutes were irradiated at a frequency of 2019 cm-1.

As a result of ozone adsorption, the dichloroethylene band in the region of 1590 cm-1 undergoes some changes (Fig. 3.3.6). The difference spectra show a decrease in intensity at a frequency of 1593 cm-1 and a simultaneous increase at 1587 cm-1. At the same time an increase in the band of perturbed silanol groups and a decrease in the band of unperturbed groups were observed, as was observed during the adsorption of dichloroethylene. This is because ozone adsorption occurs in the vicinity of hydroxyl groups, where the inactive preadsorbed cis-CiHiCh is already located. Changes in the region of 1590 cm-1 occur due to the influence of adsorbed ozone. As a result of irradiation at 2019 cm-1 frequency, the intensity of perturbed hydroxyls decreased. The growth of new bands corresponding to the expected products of ozonolysis at 77 K was not detected.

1610 1600 1590 1580 1570 1560cm1

Fig. 3.3.6. Change in the spectrum of adsorbed dichloroethylene after the addition of an isotopic mixture of O3 at 77 K in the V1+V3 vibration region. 10 minutes after the start of adsorption (1), 20 minutes (2), after 60 minutes, the last 20 of which were irradiated at a frequency of 1991 cm-1 (3), after 120 minutes, the last 20 of which were irradiated at a frequency of 2019 cm-1 (4). The spectrum of the initial sample with adsorbed cis-C2H2O2 was subtracted.

The decrease in the band of adsorbed ozone under laser irradiation at 2019 cm-1 frequency can be associated with its decomposition or desorption. The reaction takes place simultaneously for all isotope modifications of ozone. At the same time, there is a predominance of a decrease in the 2019 cm-1 band by 2% compared to 1991 cm-1. The observed difference is rather small, and the accuracy of the device does not allow one to speak reliably about the process selectivity. The weak selectivity of the reaction can be explained by the rapid exchange of energy between different isotopologues. Also, the reaction can occur due to resonant heating of the sample and energy exchange of molecules with the adsorbent. The effect for irradiation at a frequency of 1991 cm-1 was not noticeable, since the laser power at this frequency is an order of magnitude lower compared to 2019 cm-1. That is, when irradiated with a lower power, the process of continued ozone adsorption prevailed over the process of decomposition and desorption because of irradiation.

When the sample was heated to 158 K, the previously noted dynamics of the spectrum did not change: the intensity of the perturbed silanol groups increased, the

V1+V3 bands of ozone increased in intensity, and the contour changed near the 1590 cm-1 band, hence the only reaction was the continued ozone adsorption.

With a further increase in temperature, serious changes began to occur. The band at 3750 cm-1 began to grow, for the band at 3650 cm-1 the redistribution of intensity to the region of lower frequencies began (Fig. 3.3.8). New bands appeared at frequencies of 1770, 1754, 1730, 1713 cm-1, the growth of which was accompanied by a decrease in the absorption bands of O3 and cis-C2H2Cl2 (Fig. 3.3.9). Bands at 2340, 2324, 2306 cm-1 frequencies appeared (Fig. 3.3.7).

2400 2350 2300 cm1

Fig. 3.3.7. Change in the SiO2 spectrum during heating in the CO2 absorption region, after the adsorption of dichloroethylene and the addition of an isotope mixture of O3, after 120 minutes and after two sessions of irradiation at 77K. The spectra were recorded at temperatures: 158, 168, 183, 193, 198, 203, 213K with a difference of 3 minutes (1-7). The spectrum of the initial sample with adsorbed CW-C2H2CI2 was subtracted.

3800 3700 3600 3500 3400cm1

Fig. 3.3.8. Change in the spectrum of SiO2 upon heating in the region of absorption of silanol groups, after adsorption of dichloroethylene and addition of an isotopic mixture of O3, after 120 minutes and after two sessions of irradiation at 77K. The spectra were recorded at temperatures: 158, 168, 183, 193, 198, 203, 213K with a difference of 3 minutes (1-7). The spectrum of the initial sample with adsorbed cis-C2H2Cl2 was subtracted.

2000 1900 1800 1700 1600cm1

Fig. 3.3.9. Changes in the spectrum of SiO2 upon heating, after the adsorption of dichloroethylene and the addition of an isotope mixture of O3, after 120 minutes and after two sessions of irradiation at 77K. The spectra were recorded at temperatures: 158, 168, 183, 193, 198, 203, 213K with a difference of 3 minutes (1-7). The spectrum of the initial sample with adsorbed cis-CiH-2Cl2 was subtracted.

The decrease in the bands related to ozone and dichloroethylene and the simultaneous growth of other bands can be associated with the ozonolysis reaction. The bands of ozonolysis products at 1754 and 1713 cm-1 are close in position to the vibration frequency of free formyl chloride molecules at 1784 cm-1 (HCOCl) [78], [79]. While formyl chloride is the main product of the incomplete oxidation of dichloroethylene, hence the bands at 1754 and 1713 cm-1 can be attributed to adsorbed formyl chloride with a light oxygen atom 16O and heavy 18O, respectively. The bands at 2340, 2324, and 2306 cm-1 belong to different isotope modifications of CO2, which is also formed during the decomposition of ozone. Considering the fact that two oxygen atoms in a CO2 molecule are equivalent, the band splits into three maxima, the intensity ratio of which is close to that expected for the enrichment of the oxygen used.

3.3.2. Ozonolysis of dichloroethylene on TiO2

A TiO2 sample was pressed from Hombifine N Anatase powder with a specific surface area of 340 m2/g, sample area 1.0 cm2, weight 16.0 mg. After pressing, the sample was subjected to thermal vacuum treatment at 723 K for 30 min, followed by heating at the same temperature in oxygen (10-20 Torr) for 15 minutes and another 15 minutes of evacuation. The sample was cooled to room temperature in the presence of about 1 Torr of oxygen to avoid the reduction of its surface. To reduce its reactivity, several Torr of H2O vapors were let into the volume with the sample at room temperature and thermal vacuum treatment was again carried out at 363 K for 20 min. Then the cell was cooled and the effect of resonant exposure to IR radiation at the v1+v3 vibration frequency of ozone was studied on ozonolysis of adsorbed cis-dichloroethylene. Ozone adsorption and irradiation, as a rule, was carried out at the boiling point of liquid nitrogen (77 K). To study ozonolysis, cis-dichloroethylene was released at 223 K, followed by cooling to liquid nitrogen temperature and ozone adsorption.

After thermal treatment and hydration of the TiO2 surface, cis-C2H2Cl2 was puffed at 223 K (Fig. 3.3.10 and Fig. 3.3.11). The spectrum in the region of hydroxyl groups had undergone some changes. The band intensity of unperturbed hydroxyls at a

frequency of 3675 cm-1 decreased and a band of perturbed hydroxyls appeared at a frequency of 3575 cm-1. Adsorbed dichloroethylene bands were observed at 3084, 1590 and 1296 cm-1 frequencies. Changes in the bands of hydroxyl groups are similar to those observed during the adsorption of dichloroethylene on the surface of SiO2.

3500 3000 2500 cm"1

Fig. 3.3.10. Spectrum of hydrated TiO2 at 223 K (1), after adsorption of cis-C^HiCh (2)

1700 1600 1500 1400 1300 cm1

Fig. 3.3.11. Spectrum of hydrated TiO2 at 223 K (1), after adsorption of cis-C^HiCh (2)

Ozone adsorption at 77 K was noticeable by the growth of bands corresponding to the V1+V3 and V1 vibrations for light isotope modifications. Already a few minutes after the adsorption of ozone, an increase in the bands at 1732, 1694, 1657 cm-1 and a decrease in the dichloroethylene bands at 3084, 1590 and 1296 cm-1 (Fig. 3.4.12) were

noticeable, which are observed in the difference spectrum as negative absorption bands at 1590 cm-1 frequency. An increase in the bands of the CO2 isotopologues at 2340, 2324, 2306 cm-1 was observed (Fig. 3.4.13).

10 9

j_i_1_i_i_i_l

1750 1700 1650 1600 1550 см1