A model for calculating the vibrational frequency shifts from the S0 to S1 and T1 states:: application to the Raman data of pyrene in the excited states

A modification of F-matrix in the force field due to electronic excitation (from the S-0 to S-1 and T-1 states) is modeled on the basis of the Huckel approximation and simplified form including the first order configuration interaction (CI) introduced by Pople, in order to estimate the vibrational shifts by the electronic excitations. The calculated Raman frequencies by applying the model to pyrene are compared with the data on the spontaneous Raman (S-0) and the transient resonance CARS (coherent anti-Stokes Raman spectroscopy; S-1, T-1). By using the same force fields, the vibrational intensity patterns over the absorption and fluorescence spectra are calculated to demonstrate a dramatic loss of mirror symmetry between both the observed spectra as reported in the literature. (C) 1998 Elsevier Science B.V. All rights reserved.