Franck-Condon analysis of photoelectron and electronic spectra of small molecules

Currently available methods for the simulation of Franck-Condon envelopes o f electronic and photoelectron spectra of small to medium-sized molecules a re reviewed and discussed. The advantages of combining ab initio molecular orbital calculations with Franck-Condon calculations to simulate vibrationa lly resolved electronic and photoelectron spectra are outlined and the abil ity of such calculations to assign the vibrational components observed in t hese spectra and to derive excited or ionic state geometries is emphasized. Simulation of the emission spectrum of CF2 is taken as an example where a harmonic oscillator model has been used for each electronic state to derive vibrational eigenvalues and eigenvectors and hence calculate Franck-Condon factors. The possibility of improving this model by the use of anharmonic potentials is identified as a likely future development. (C) 1998 Elsevier Science B.V. All rights reserved.