Raman excitation profiles of polyatomic molecules in condensed phases. A stochastic theory

Abstract

The absorption and the Raman excitation profiles of polyatomic molecules in condensed phases are analyzed using a stochastic model of line broadening. The electronic energy gap is assumed to be stochastically modulated by the random force resulting from the interaction with the solvent. The model interpolates all the way from homogeneous broadening (fast modulation) to inhomogeneous broadening (slow modulation). We analyze the conditions whereby the Raman excitation profiles can be expressed as the square of an ensemble-averaged amplitude. An efficient procedure for modeling both absorption and excitation profiles and extracting the relevant structural and dynamical information for large polyatomic molecules is proposed. Application is made to the Raman profiles of azulene in CS2 and in methanol, and the magnitude and the inverse correlation time of the random force are extracted from the experimental data in both cases.