DYNAMIC SOLVENT EFFECTS ON THE VIBRATIONAL OVERTONE DEPHASING IN MOLECULAR LIQUIDS - SUBQUADRATIC QUANTUM NUMBER DEPENDENCE

A theoretical study of vibrational dephasing of molecular vibrations i n Liquids is presented with an aim to understand the experimentally ob served sub-quadratic quantum number (n) dependence bf the vibrational dephasing rate, in systems like CH3I and CHCl3 and their deuterated an alogues. The analysis is based on Oxtoby's theory of vibrational depha sing but with a detailed microscopic description of the frequency depe ndent frictional forces on the vibrational mode. The friction on the n ormal coordinate in liquids is found to have a pronounced biphasic beh avior with a dominant Gaussian initial component followed by a slow ex ponential-like relaxation. While the exponential relaxation usually as sumed in Kubo's stochastic theory leads to a quadratic n dependence of the dephasing rate, the biphasic friction is shown to give rise to th e sub-quadratic n dependence. As the biphasic frictional response is e xpected to be a generic feature of the friction on any vibrational coo rdinate in dense liquids, the sub-quadratic quantum number dependence is predicted to be common to most ultrafast overtone dephasing. In add ition, the calculated rates (without any adjustable parameter), are fo und to be in good agreement with the experimental results for the C-I stretching mode in liquid CH3I and for the C-H stretching in liquid CH Cl3. (C) 1997 American Institute of Physics.