A systematic study is initiated of resonance Raman scattering in molecular systems with overlapping electronic absorption bands. Resonance Raman 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 properties characteristic of both totally symmetric and nontotally symmetric modes. The profiles and polarization dispersion 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 characteristic resonance–antiresonance coincidences. It is shown that interference effects of this sort are a general feature of systems exhibiting strong vibronic coupling.

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Articles you may be interested inTheoretical modeling of UV-Vis absorption and emission spectra in liquid state systems including vibrational and conformational effects: Explicit treatment of the vibronic transitions It is shown that progressions of totally symmetric vibrations in vibronically induced spectra do not show the same intensity distribution as comparable progressions in allowed spectra. This is due to cross terms. involving both totally symmetric and inducing mode coordinates, in the Herzberg-Teller expansion. These cross terms express the modulation of the vibronic coupling resulting from differences in interatomic distances between the borrowing and lending states. The effect is universal, as opposed to effects based on normal-coordinate rotation or totally symmetric inducing modes. and enters even in the absence of anharmonic coupling. It leads to anomalous intensity distributions which, moreover, are different for emission and absorption spectra. Quantitative estimates are presented which indicate that it should be readily observable in many molecules.(3)