Unraveling of vibronically scrambled electronic spectra: The S₂-S₀ transition in azulene

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.