Resonance Raman scattering in molecular dimers

Abstract

A systematic study is initiated of resonanceRaman scattering in molecular systems with overlapping electronic absorption bands. ResonanceRaman spectra, excitation profiles, and depolarization ratios are calculated for a simple molecular dimer in which the two overlapping Franck–Condon progressions are related through the permutational symmetry of the dimer. The model used is highly idealized, but can be solved exactly for a complete range of intermolecular coupling strengths. In the weak and strong intermolecular coupling regions, the exact results are compared with the physically more transparent results obtained from a low‐order perturbation treatment. The spectra and profiles show propertiescharacteristic of both totally symmetric and nontotally symmetric modes. The profiles and polarizationdispersion curves are subject to both electronic and vibronic interference effects. Anomalous extrema of the depolarization ratio are often found to coincide with minima rather than maxima in the excitation profile. A pairwise comparison of the excitation profiles of Rayleigh lines, Raman fundamentals, and Raman overtones shows characteristicresonance–antiresonance coincidences. It is shown that interference effects of this sort are a general feature of systems exhibiting strong vibronic coupling.