Effects of the mixing of charge transfer and molecular excitations on the resonance Raman properties of symmetric radical dimers

The mixing of charge transfer (CT) and molecular excitation (ME) states in a molecular radical dimer is investigated based on a model Hamiltonian for a dimer with two orbitals per site filled with two electrons. The CT/ME mixing brings about an intensity enhancement of intramolecular modes in the Raman spectra resonant with the CT absorption. The relevant parameter for this effect is the transfer integral tab for the hopping of one electron from orbital a in one molecule to orbital b in the other. This theoretical prediction is confirmed by Raman excitation profiles measured for the cation radical salt (TTF)2(W6O19).