Quantum-chemical analysis of vibrational intensity distributions in the S 0-S 1 absorption spectra and raman excitation profiles of azulene

Abstract

The vibronic, Franck—Condon and Dushinsky activity of the totally modes of azulene in S 0-S 1 transition is investigated by quantum-chemical methods. It is found that the 900 and 1400 cm −1 mode mixing proposed earlier to explain the relative intensities of Raman fundamentals in the region of the S 1 state is not supported by the calculations. The alternative mechanism arising from vibronic activity of the a 1 modes produces signs of the non-Condon parameters in agreement with those found from fitting the Raman excitation profiles; however, the absolute value of this parameter for the 900 cm −1 mode is almost one order of magnitude too small. Thus, although quantum-chemical calculations favour non-Condon effects over Dushinsky effects in shaping the relative intensities of Raman fundamentals, they do not give a conclusive answer. It is argued that such an answer can be obtained from measurements of the 900 + 1400 cm −1 combination band Raman excitation profiles.