Quantum Localization and Manifestations of Chaos in Model Polyatomic Systems.

A theoretical understanding of chemical reaction mechanisms on the basis of detailed quantum-mechanical calculations is necessary for advancement in the field of laser chemistry. The current lack of theoretical understanding of the processes by which complex molecules absorb laser energy is a major obstacle to further development of selective laser chemistry. This work addresses a first-principles approach to understanding the relationship of quantum localization and mode-selective excitation. An effective quantum number approach, based on simple perturbation theory, is applied to conservative Hamiltonian systems having C_ {3nu} and octahedral symmetry. This method is used to pick out the quantum states that are localized and are related by semiclassical quantization to the classical localized quasiperiodic motion. The work clearly shows that intra-molecular vibrational dynamics in systems having symmetry similar to real molecules, such as SF_6, demonstrate behavior that should facilitate selective laser chemistry. Although the existence of quantum chaos is not claimed, in the system with C_{3nu } symmetry, a relationship between the quantum energy states and their respective classical Lyapunov exponents is observed.