Resonance Raman scattering in molecular dimers

Abstract

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.

Exciton–vibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

“…[42] the theory for calculating hyper-Rayleigh and resonance Raman scattering signals of excitonic dimers was formulated (for earlier work, see Refs. [43,44]). This approach was used by Leng et al to calculate resonance Raman spectra for merocyanine dyes, as monomers in dichloromethane solution and as H-dimers in dioxane solution [45].…”

Depolarization dispersion of a fundamental line of a totally symmetric vibration in resonance Raman scattering

“…We see that one can get any value for pI in preresonance Raman scattering in the range f to 4 when R approaches the nondegenerate electronic state that is vibronically coupled to one of different symmetry. [12]. It is worth noting that this effect does not depend on how many totally symmetric modes are active in absorption and fluorescence.…”

“…Figure 1 shows the excitation profiles and depolarization dispersion curves for such a case. [12]). We have chosen the same value of the damping constant r for all rip.…”

“…During the last few years there has been a growing activity, both experimental and theoretical, in the field of resonance Raman scattering (RRS) from molecules [1][2][3][4][5][6][7][8][9][10][11][12]. It has been shown that RRS is a very useful tool for the investigation of molecular properties in excited states.…”

“…It has been shown that RRS is a very useful tool for the investigation of molecular properties in excited states. A recent paper [12] that investigates theoretically RRS from molecular dimers, being the first of a series of papers dealing with RRS from overlapping electronic states, also shows depolarization dispersion for a mode of mixed symmetry character: one line in the RRS spectrum belongs to the symmetric and antisymmetric mode simultaneously. This has been demonstrated for a fundamental line of a non-totally symmetric mode [9,101.…”