Fluorescence excitation spectra, absorption spectra, and dispersed fluorescence spectra following excitation of a variety of absorption features associated with the S2(1A1)-S0(1A 1) electronic transition of azulene have been measured. Spectra have been obtained for low-pressure azulene vapor at 300 K and for a free-jet expansion of azulene seeded in argon. A theory involving strong vibronic coupling between the S2(1A1) and S4(1A1) electronic states, mediated by totally symmetric vibrational modes, is developed and implemented in an analysis of the strongly perturbed S2-S0 absorption system. The analysis shows that S2-S4 vibronic coupling in azulene involves significant participation by at least 6 of the set of 12 a1 modes (excluding C-H stretches) in an interaction network that causes substantial scrambling of the identities of vibrational levels in the S2 manifold. The spectral evidence in combination with the theoretical analysis also indicates that there is further scrambling of vibrational identity in the S2 state as a result of weaker interactions (coupling constants of ∼10-20 cm-1) between two-quantum a1 combinations that stem from second-order vibronic effects. The analysis serves to illustrate how vibronically scrambled spectra may be unraveled by using dispersed fluorescence spectroscopy to probe the vibrational identity of the mixed states.
|Number of pages||19|
|Journal||Journal of Physical Chemistry|
|Publication status||Published - 1 Feb 1990|