Dispersed Emission from the Lower Vibronic Levels of the A~ 1 A 2 State of Sulfur Dioxide

Author： Rassias G. Metha G.F. McGilvery D.C. Morrison R.J.S. O'Dwyer M.F.

ISSN： 0022-2852

Source： Journal of Molecular Spectroscopy, Vol.181, Iss.1, 1997-01, pp. : 78-90

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Abstract

Single vibronic level fluorescence has been recorded for 25 levels in the low energy region of the lowest singlet absorption system of sulfur dioxide. Excitation to higher bands results in much more complicated emission, so we have concentrated on trying to interpret the proposed vibronic coupling between the theoretically predicted forbidden 1 A 2 and allowed 1 B 1 states in this lower energy region. The vibrational content of the emission is surprising in that the dominant progression is in nu 1 , the symmetric stretching vibration, with very little activity in nu 2 , the bending vibration. This is unexpected, since rotational analyses of a number of the bands show a large angle change with only a slight lengthening of the bonds. In some of the spectra the nu 1 progression is built onto 2nu 3 , the antisymmetric stretching vibration. For a few of the higher energy levels the nu 1 progression built onto one quantum of nu 2 becomes the dominant progression. Two of the excited states of sulfur dioxide ( 1 A 2 and 1 B 2 ) show a large change in bond angle from the ground state. The 1 B 2 state behaves normally and has very long progressions in both nu 1 and the bending vibration (nu 2 ) with a Franck-Condon maximum up to 14 000 cm -1 from the origin, while for the 1 A 2 state the progressions are much shorter (showing at most one or two quanta of nu 2 ) with a Franck-Condon maximum at 2000-3000 cm -1 . This unexpected behavior is shown to be due to the different vibrational overlaps involved when Born-Oppenheimer interaction with a state with very different geometry is involved. The emission pattern is determined by the overlap of the vibrational wavefunctions of the perturbing state with the ground state rather than those of the excited state with the ground state. Attempts to fit this to a model of vibronic coupling result in there seeming to be too many levels.