Theory of resonance Raman scattering beyond the Condon approximation for a system with two modes of different symmetry

Abstract

A theoretical study is presented of resonance Raman excitation profiles and depolarization dispersion curves for a molecule with two Raman-active normal modes of vibration and three electronic states. The modes are taken to be of different symmetry, one totally and the other non-totally symmetric. The three electronic states are the ground state and a pair of excited states coupled through the non-totally symmetric mode. The energy gap separating the pair is allowed to vary from zero (Jahn–Teller limit, strong coupling) to large values (Herzberg–Teller case, weak coupling). The totally symmetric coordinate is allowed to assume different equilibrium values not only between the ground state and the resonant state, but also between the two coupled excited states. As a result, the effective coupling depends indirectly on the totally symmetric coordinate, causing breakdown of the Condon approximation. The effect of this breakdown is demonstrated by calculating excitation profiles for all one and two quantum transitions involving the two modes. The profiles show progression of the totally symmetric mode with or without superimposed non-totally symmetric quanta. If the Condon approximation breaks down, the intensity distribution in these progressions generally depends on the presence of such quanta. For weak coupling, the effect should be most readily observable in combination bands. For strong coupling, it should also be observable in totally symmetric fundamentals and overtones. Special attention is given to systems with resonance–preresonance interference and to intermediate-coupling systems.