Resonance Raman excitation profiles and depolarization dispersion curves, and their use in the analysis of vibronically coupled excited states

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Abstract
The relationship between vibronic coupling and resonance Raman scattering is studied on a model system consisting of two excited electronic states coupled by a non-totally-symmetric mode of vibration. Scattering cross sections and depolarization ratios are calculated as a function of the wavelengths of the incident and scattered light, giving rise to Raman excitation profiles and Raman spectra, respectively. The model system allows exact solution for arbitrary values of the electronic energy gap between the coupled states, of their vibrational frequencies, and of linear and nonlinear adiabatic coupling parameters. The results are reported in the form of three-dimensional graphs for combinations of parameter values typical for weak, strong, and intermediate vibronic coupling. They are compared with previous results, based on simpler models and/or more approximate solution methods. In addition, they are related to the results obtained for the molecular dimer [J. Chem. Phys. 63, 5475 (1975)], which is the prototype of an exactly solvable vibronic coupling model. It is shown that resonance Raman spectroscopy generally allows one to probe more deeply into vibronically coupled states than conventional absorption–emission spectroscopy and thus provides a more powerful tool for their spectroscopic analysis. This is particularly significant in the region intermediate between weak and strong vibronic coupling where interference between Franck–Condon and vibronic coupling effects or between adiabatic and nonadiabatic coupling (in the adiabatic Born–Oppenheimer representation) may produce highly irregular spectra. The basic advantage of resonance Raman scattering over absorption spectroscopy is that one can obtain a higher effective resolution of excited state spectra, namely, by studying each vibrational mode separately, by comparing the excitation profiles of fundamentals and overtones, and by investigating the dispersion of the depolarization ratio.