Relaxation Phenomena in Raman Spectroscopy

Abstract

AbstractThe line shape of Raman bands and its relation to rotational and vibrational relaxation is the topic of the present contribution. First, rotational and vibrational correlation functions are introduced. Depending on the vibrational mode under study the rotational correlation functions contain information on rotations about different molecular axes. The discussion will be restricted to linear and symmetric top molecules, for the first of which “tumbling” and the latter “tumbling” and “spinning” reorientational motions are of importance. Experimental correlation functions for CO2, CH3J, and CH3CN of totally symmetric and degenerate bands are discussed. A comparison of the experimental correlation functions with those obtained from the rotational J‐diffusion model will be given. — Next, vibrational relaxation (dephasing) is considered. Several mechanisms may contribute to vibrational dephasing. The popular Kubo‐model for vibrational relaxation is introduced and the relaxation of the v2‐band of CH3J is discussed in terms of this model. — The paper closes with a brief discussion of interaction induced scattering. Interaction induced scattering contributes to the wings of depolarised Rayleigh (pure rotational Raman) and Raman scattering and therefore is a severe source of error for the short time behaviour of rotational and vibrational correlation functions. Different approaches to correct experimental spectra are considered.