Solvent-induced dynamic polarization and vibrational dephasing of electronically excited molecules

In order to account for the extra vibrational Raman bandwidths observed specifically for electronically excited molecules in solution, a new model, the dynamic polarization model, has been introduced. In this model, the vibrational frequency of a highly polarizable electronically excited molecule in solution is stochastically modulated by the solvent-induced polarization. An olefinic C=C double bond is considered as the simplest prototype, for which the carbon-carbon stretch frequency can take values between the unpolarized (C=C) and the completely polarized (C+-C-) frequencies. The two-frequency exchange model is first introduced, followed by more general models. A focus is placed on the asymmetric exchange limit in which the solvent-induced polarization can be treated as a perturbation, so that the effect of the dynamic polarization manifests as a shift of the peak position and an increase in the bandwidth of the original unperturbed C=C stretch vibrational band. The experimental results on S-1 trans-stilbene are discussed, based on the formulae derived.