Stochastic theory of resonance Raman line shapes of polyatomic molecules in condensed phases

Abstract

A stochastic model is proposed for the calculation of resonance Raman line shapes of polyatomic molecules in condensed phases. The model assumes that the random force exerted on the molecule by the solvent causes a stochastic modulation of the electronic energy gap. Explicit expressions are derived for the absorption, the excitation spectra, and the dispersed Raman line shapes. The model interpolates all the way from homogeneous broadening (fast modulation) to inhomogeneous broadening (slow modulation). The effects of line broadening cannot be incorporated into the Kramers–Heisenberg formula, commonly used to interpret these spectra, since dephasing-induced terms appear which dominate the line shapes for large dephasing rates. A simple explanation is provided for the nature of the homogenous linewidth and for the broadening of the dispersed Raman line shapes, without having to assume an unrealistically short lifetime.