The vibronically-resolved emission spectrum of disulfur monoxide (S2O): An algebraic calculation and quantitative interpretation of Franck–Condon transition intensities

Abstract

Emission spectra obtained from jet-cooled disulfur monoxide (S2O) molecules have been interpreted by means of a novel Lie algebraic formalism that makes possible the facile evaluation of multidimensional Franck–Condon factors. Fluorescence accompanying selective excitation of isolated vibronic bands in the S2O C̃ 1A′←X̃ 1A′(π*←π) absorption system has been dispersed under moderate spectral resolution, allowing assignment of ground state levels possessing up to 20 quanta of vibration in the ν2 S–S stretching mode [Evib(X̃)⩽13 900 cm−1]. Aside from providing a rigorous and economical description for the inherently anharmonic nature of highly-excited polyatomic species, our algebraic approach enables quantitative information on molecular wavefunctions to be extracted directly from spectroscopic data. The emerging picture of S2O vibrational dynamics suggests that the X̃ 1A′ potential surface is substantially more “local” in character than the C̃ 1A′ manifold. While the observed pattern of X̃ 1A′ vibrational energies could be reproduced well through use of model Hamiltonians that include only diagonal anharmonicities in the local algebraic basis, successful treatment of the C̃ 1A′ state necessitated explicit incorporation of off-diagonal anharmonicities that lead to pervasive mixing of local vibrational character. This disparate behavior is manifest strongly in measured C̃–X̃ transition strengths, thereby allowing detailed investigations of Franck–Condon intensities to discern the underlying dynamics. Structural parameters deduced from algebraic analyses are in good accord with previous predictions of the change in S2O geometry accompanying π*←π excitation.