Symmetry and local mode coupling in absorption and Raman spectroscopy of intervalence electronic transitions

Intervalence electron transfer spectra in mixed-valence molecules are frequently modeled by an interacting pair of adiabatic potential energy surfaces. The presence or absence of a double minimum in the lower surface is correlated with trapped or delocalized charges, respectively. The coordinate involved in this interpretation is the asymmetric normal coordinate representing the nuclear motions taking the molecule from one extreme to the other. In this paper, a model is developed involving both a symmetric and an asymmetric coordinate on an equal footing. The time dependent theory of electronic spectroscopy is used to calculate both absorption and resonance Raman spectra. The model uses physically meaningful interactions in the mixed-valence molecule including the electronic coupling, vibrational coupling, vibrational force constants, and bond length changes as a result of the electron transfer. The effect of these interactions on the relative intensities of symmetric and asymmetric modes in both the absorption and resonance Raman spectra are examined. The quantitative calculations are discussed in parallel with the physical meaning. The calculations show how the spectra can smoothly go from domination by one type of mode to the other. The most important effects are caused by the bond length changes, the electronic coupling, and the force constant changes.