The origin of vibrational dephasing of polyatomic molecules in condensed phases

The vibrational dephasing of polyatomic molecules in condensed phases by intermolecular vibrational energy exchange is treated theoretically. In the exchange model, dephasing arises from random modulation of the vibrational frequency caused by intramolecular anharmonic coupling to low frequency modes which are undergoing intermolecular energy exchange with the bath. The exchange rates are temperature dependent and as a consequence manifest themselves experimentally as a temperature dependent broadening and shift of the Raman spectral line shape. Using a reduced density matrix technique within the constraints of a Markoff approximation, the theory allows the time dependence introduced by the exchange process to be properly accounted for, and allows explicit expressions for the vibrational correlation function and corresponding spectral line shape functions to be derived and related to molecular parameters. Application of the theory can have important consequences experimentally, since an analysis of the temperature dependence of the line shape function allows the determination of the parameters which characterize the dephasing. These include the intermolecular energy exchange rates, the amplitude of the modulation which is in turn determined by the strength of intramolecular anharmonic coupling, and the effective activation energy for exchange which is related to the specific low frequency modes which participate in the dephasing process.