多原子分子激发态非绝热动力学的研究

Chemistry, the breaking and forming of bonds, involves the rearrangement of atoms and the redistribution of valence electrons. At the excited electronic states, the degrees of freedom of atoms and valence electrons are often strongly coupled together, resulting into the so-called non-adiabatic effects. The non-adiabatic effects underlie all of photochemistry of polyatomic molecules, and are often the center of biologic processes. The study of excited state non-adiabatic dynamics have been critical to the understanding of atmospheric chemistry, combustion and biochemistry, etc. In this project, the decay dynamics and the non-adiabatic effects of the excited states of heteroaromatic molecules will be studied, using the time-resolved photoelectron spectroscopy and time-resolved photofragment kinetic energy distribution methods. The intention of this project is twofold: (1) by comparing the excited state dynamics of different heteroaromatic molecules, the general nonradiative decay mechanism underlain will be investigated, and its relationship to the photostability of DNA/RNA bases and aromatic amino acids will be discussed; (2) by progressively modify the excited state potential surfaces, the location and shape of conical intersections and the frequencies of certain critical vibrational modes using chemical substitution, the general rules governing the excited state non-adiabatic dynamics will be investigated and discussed.

化学反应：化学键的断裂和形成，涉及原子核的重新组合和电子云的重新分布。在电子激发态，原子核和电子的运动往往强烈的偶合在一起，导致所谓的非绝热效应。激发态非绝热过程是几乎所有的多原子分子激发态光化学的核心, 也是很多光生物化学过程的关键步骤。激发态非绝热过程的研究是深入理解大气化学、燃烧化学、复杂生物化学过程的基础。本项目利用时间分辨光电子能谱和时间分辨产物平动能谱的方法，研究多原子分子激发态的非绝热动力学。项目以杂环芳香烃分子为例，研究它们激发态的弛豫机理以及非绝热效应，分析不同分子之间的共性和差别，探索其光化学的一般规律以及与生物分子光稳定性的关系；研究杂环芳香烃分子激发态非绝热动力学的化学取代效应，探索势能面的性质、锥形交叉的形状、关键振动模式的频率等对非绝热动力学的影响，探讨多原子分子激发态非绝热动力学的一般规律。

在本项目中，我们研究了2,4-二甲基吡咯和2,5-二甲基吡咯激发态的非绝热动力学，并结合之前对吡咯和N-甲基吡咯的研究，详细讨论了其激发态非绝热动力学的甲基取代效应，深入探讨了吡咯分子激发态非绝热动力学的机理。该研究有助于我们理解一般杂环芳香烃分子激发态的非绝热动力学，从而理解DNA/RNA碱基和芳香烃氨基酸分子的光化学行为，例如DNA/RNA碱基的光稳定性。利用双光子激发和时间分辨的光电子能谱，系统研究了水分子C、D’、D’’、和两个F态的超快衰变动力学，观测到了C态中由于偶然共振引起的新的衰变通道，第一次探索了D’’态和两个F态的衰变机理。初步探索和总结了多原子分子激发态动力学的一般规律，有助于我们建立模型，从而去探索更有实际意义的体系、例如生物分子等中的光化学行为。