Mechanisms for Ultrafast Vibrational Energy Relaxation of Polyatomic Molecules

Abstract

In recent years ultra fast vibrational relaxation processes have been observed for molecules in a liquid[1]. The rate of energy depopulation of CH-stretching modes can vary by two orders of magnitude for different molecules like benzene(T₁ =1ps) on the one side and acetylene (T₁ =200ps)[1]on the other side. In order to understand the determining factors that influence the deactivation process one may argue on the basis of harmonic model calculations that the number of quanta exchanged in the transfer process should be small and the energy mismatch between the initial and the final state should also be small. These criteria, however, have to be specified further. For instance the medium or collision induced energy transfer between two CH-stretching modes, which may be localized predominantly at different sites R_n and R _mwill depend upon the local amplitude squared the mode localized at R_m has at the site R_n and vice versa. Only if both modes have essentially the same local amplitudes and only differ in the phases the energy relaxation is fast as the pure dephasing process of these modes. If the initial and the final state differ by many vibrational quanta the rate may still be fast provided these two states mix due to anharmonic interaction. If both states are actually in Fermi resonance the medium or collision induced transfer is again of the same magnitude as the corresponding dephasing process. Therefore, it is very essential to know the anharmonic coupling constants.