Molecular Structure and Dynamics Beyond the Born-Oppenheimer Approximation

在普渡大学化学系物理化学项目的这个项目中，E.R. Grant教授将研究高激发小分子中分子内弛驰和电子弹射的状态对状态动力学。要研究的具体体系是碳氢化合物和金属中心自由基分子。多共振激光方法将用于分离使分子接近其电离阈值的单个跃迁。实验还将获得电离势、自由基的光电离截面和相应阳离子的旋转-振动结构的高分辨率信息，以及位于绝热电离阈值以下的激发态的振动结构。关于多原子分子性质的传统观点是基于Born-Oppenheimer （B-O）近似，根据该近似，它的总能量是与平移、旋转、振动和电子自由度相关的独立能量的总和。在第一次近似中，假设它们之间的实际不消失耦合很小，可以忽略不计。然而，化学反应通常涉及高度激发的电子和振动自由度，因此B-O近似的有效性可能值得怀疑。涉及接近或高于第一电离阈值的能量的过程，其中分子内能量弛豫的动力学导致特别是电子弹射，不适于用B-O近似处理。这项研究关注的是在电离阈值附近的激发态中发生的过程的基本问题，并将为化学反应性理论模型的发展提供合适的信息。

In this project of the Physical Chemistry Program in the Chemistry Division Prof. E.R. Grant of Purdue University will pursue a program of research on the state-to-state-dynamics of intramolecular relaxation and electron ejection in highly excited small molecules. Specific systems to be investigated are hydrocarbon and metal-centered free radical molecules. Multiple-resonance laser methods will be used to isolate individual transitions that bring the molecules close to their ionization thresholds. Experiments will also be performed to obtain high-resolution information on ionization potentials, photoionization cross sections of free radicals and the rotational-vibrational structure of the corresponding cation, as well as on the vibrational structure of excited electronic states that lie below the adiabatic ionization thresholds. %%% The conventional view of the properties of a polyatomic molecule is based on the Born-Oppenheimer (B-O) approximation according to which its total energy is the sum of independent energies associated with the translational, rotational, vibrational, and electronic degrees of freedom, the actual non-vanishing couplings between these being assumed to be small and negligible in the first approximation. Chemical reactions, however, often involve highly excited electronic and vibrational degrees of freedom and the B-O approximation may be of questionable validity. Processes that involve energies near or above the first ionization threshold, where the dynamics of intramolecular energy relaxation leads in particular to electron ejection, are not amenable to be treated in the B-O approximation. This research concerns fundamental questions about the processes that occur in excited molecular states near the ionization threshold and will provide information suitable for the development of theoretical models of chemical reactivity.