Exploring Ultrafast H-Atom Elimination versus Photofragmentation Pathways in Pyrazole Following 200 nm Excitation

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• 作者：Craig A. Williams ; Gareth M. Roberts ; Hui Yu ; Nicholas L. Evans ; Susanne Ullrich ; Vasilios G. Stavros
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：March 22, 2012
• 年：2012
• 卷：116
• 期：11
• 页码：2600-2609
• 全文大小：915K
• 年卷期：v.116,no.11(March 22, 2012)
• ISSN：1520-5215

文摘

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative ¹蟺蟽* states localized along X鈥揌 (X 鈺?N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the ¹蟺蟽* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy ¹蟺蟽* state (1¹A鈥?鈫?X¹A鈥?. These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to ¹蟺蟽* mediated N鈥揌 fission. The time constants extracted for the low and high energy features are 120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright ¹蟺蟺* state (2¹A鈥?鈫?X¹A鈥?, by performing time-resolved ion yield measurements. These results are modeled using a ¹蟺蟺* 鈫?ring deformation 鈫?photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that ¹蟺蟽* driven N鈥揌 fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role ¹蟺蟽* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.