Franck–Condon effects in resonance Raman spectra and excitation profiles

Resonance Raman spectra and excitation profiles are calculated for totally symmetric vibrations not involved in vibronic coupling and thus deriving all their intensity from Franck–Condon effects. Theoretical results are derived for harmonic oscillators undergoing displacements and frequency shifts upon electronic excitation of the molecule. The model does not only consider the effect of interference between several totally symmetric modes but also includes a number of basic refinements absent from most previous treatments such as inhomogeneous line broadening, perturbation of a weak resonance transition by the preresonance spectrum of a strong neighboring transition, and normal coordinate rotation (Dushinsky effect). The model calculations are used to interpret observed Raman spectra and excitation profiles of totally symmetric modes in the chromate ion and the β-carotene molecule.