A MODEL FOR CALCULATING THE VIBRATIONAL FREQUENCY-SHIFTS FROM THE S-0TO S-1 AND T-1 STATES - APPLICATION TO THE RAMAN DATA OF PYRENE IN THE EXCITED-STATES

A modification of F-matrix in the force field due to electronic excita tion (from the S-0 to S-1 and T-1 states) is modeled on the basis of t he Huckel approximation and simplified form including the first order configuration interaction (CI) introduced by Pople, in order to estima te the vibrational shifts by the electronic excitations. The calculate d Raman frequencies by applying the model to pyrene are compared with the data on the spontaneous Raman (S-0) and the transient resonance CA RS (coherent anti-Stokes Raman spectroscopy; S-1, T-1). By using the s ame force fields, the vibrational intensity patterns over the absorpti on and fluorescence spectra are calculated to demonstrate a dramatic l oss of mirror symmetry between both the observed spectra as reported i n the literature. (C) 1998 Elsevier Science B.V. All rights reserved.