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

Intervalence electron transfer spectra in mixed-valence molecules are frequ ently modeled by an interacting pair of adiabatic potential energy surfaces . The presence or absence of a double minimum in the lower surface is corre lated with trapped or delocalized charges, respectively. The coordinate inv olved in this interpretation is the asymmetric normal coordinate representi ng the nuclear motions taking the molecule from one extreme to the other. I n this paper, a model is developed involving both a symmetric and an asymme tric coordinate on an equal footing. The time dependent theory of electroni c spectroscopy is used to calculate both absorption and resonance Raman spe ctra. The model uses physically meaningful interactions in the mixed-valenc e molecule including the electronic coupling, vibrational coupling, vibrati onal force constants, and bond length changes as a result of the electron t ransfer. The effect of these interactions on the relative intensities of sy mmetric and asymmetric modes in both the absorption and resonance Raman spe ctra are examined. The quantitative calculations are discussed in parallel with the physical meaning. The calculations show how the spectra can smooth ly 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.