Theoretical Studies of Reaction Dynamics

Stuart Rice教授获得了理论与计算化学计划的资助，研究化学动力学中的基本过程，目的是：(1)改进化学反应中激光控制产物选择性的理论，(2)改进单分子反应速率的理论。现有的激光控制量子系统演化的理论将扩展到非绝热反应的情况，并且该理论将通过结合反馈机制来改进，该机制可以在不精确了解系统哈密顿量中所有分子参数的情况下形成最佳激光场。基于现代非线性力学的单分子反应速率的描述将通过映射方法的扩展得到改进，映射方法提供了二自由度系统动力学的精确表示到多自由度系统。该结果将用于研究几种类型的单分子裂解和异构化反应。一个量子版本的经典理论的单分子反应速率也将发展。在化学的整个进化过程中，对反应中产物形成的选择性控制一直是人们孜孜不业的追求。迄今为止，这种控制只能在宏观层面上完成，通过操纵外部因素，如温度、压力、溶剂性质等。随着可调谐激光的出现，人们希望在微观水平上控制产物的选择性，并且可以选择性地破坏分子内的单个键。人们很快发现，被分子吸收的能量不容易局限在单个键上，而是倾向于在整个分子中重新分布。然而，最近，激光选择化学的概念有了重生，由赖斯和其他人发展的理论表明，能量可以通过仔细选择规定形状的复杂激光脉冲序列来定位。如果能将这一理论应用于实验，仍有可能实现化学反应微观选择性控制的目标。

Professor Stuart Rice is supported by a grant from the Theoretical and Computational Chemistry Program to perform studies of elementary processes in chemical dynamics for the purpose of (i) improving the theory of laser control of product selectivity in a chemical reaction, and (ii) improving the theory of unimolecular reaction rates. Existing theories of laser control of the evolution of a quantum system will be extended to the case of nonadiabatic reactions, and the theory will be improved by incorporation of a feedback mechanism that shapes the optimal laser field without precise knowledge of all of the molecular parameters in the system Hamiltonian. The description of unimolecular reaction rates based on modern nonlinear mechanics will be improved by extension of the mapping methods which provide an exact representation of the dynamics of systems with two degrees of freedom to systems with many degrees of freedom. The results will be used to study several types of unimolecular fragmentation and isomerization reactions. A quantum version of the classical theory of unimolecular reaction rates will also be developed. %%% The achievement of selective control of product formation in a reaction has been sought persistently throughout the evolution of chemistry. To date, such control has only been accomplished at the macroscopic level, by the manipulation of external factors such as temperature, pressure, solvent character, etc. With the advent of the tunable laser, it was hoped that product selectivity could be controlled at the microscopic level, and that individual bonds within a molecule could be selectively broken. It was very quickly found that energy absorbed by a molecule was not easily localized in individual bonds, but preferred to redistribute throughout the molecule. More recently, however, the concept of laser selective chemistry has had a rebirth, and theories developed by Rice and others have shown that energy could be localized by carefully choosing a complex sequence of laser pulses of prescribed shapes. If this theory can be put to practice experimentally, it may still be possible to realize the goal of microscopic selective control of chemical reactions.