Intramolecular vibrational energy redistribution from a high frequency mode in the presence of an internal rotor: Classical thick-layer diffusion and quantum localization

TL;DR

This study investigates how an internal rotor influences vibrational energy redistribution in molecules, revealing classical diffusion mechanisms and quantum suppression effects due to localization and selection rules.

Contribution

It demonstrates the classical thick-layer diffusion process in IVR and shows quantum localization suppresses this diffusion, highlighting differences between classical and quantum dynamics.

Findings

Classical diffusion occurs via a thick layer of chaos.

Quantum dynamics show suppressed energy redistribution.

Rotor selection rules and localization effects inhibit diffusion.

Abstract

We study the effect of an internal rotor on the classical and quantum intramolecular vibrational energy redistribution (IVR) dynamics of a model system with three degrees of freedom. The system is based on a Hamiltonian proposed by Martens and Reinhardt (J. Chem. Phys. {\bf 93}, 5621 (1990).) to study IVR in the excited electronic state of para-fluorotoluene. We explicitly construct the state space and show, confirming the mechanism proposed by Martens and Reinhardt, that an excited high frequency mode relaxes via diffusion along a thick layer of chaos created by the low frequency-rotor interactions. However, the corresponding quantum dynamics exhibits no appreciable relaxation of the high frequency mode. We attribute the quantum suppression of the classical thick-layer diffusion to the rotor selection rules and, possibly, dynamical localization effects.