DISPERSED EMISSION FROM THE LOWER VIBRONIC LEVELS OF THE A(1)A(2) STATE OF SULFUR-DIOXIDE

Single vibronic level fluorescence has been recorded for 25 levels in the low energy region of the lowest singlet absorption system of sulfu r 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 B-1(1) states in this lower energy region. The vibratio nal content of the emission is surprising in that the dominant progres sion is in v(1), the symmetric stretching vibration, with very little activity in v(2), the bending vibration. This is unexpected, since rot ational analyses of a number of the bands show a large angle change wi th only a slight lengthening of the bonds. In some of the spectra the v(1) progression is built onto 2v(3), the antisymmetric stretching vib ration. For a few of the higher energy levels the v(1) progression bui lt onto one quantum of v(2) becomes the dominant progression. Two of t he excited states of sulfur dioxide ((1)A(2) and B-1(2)) show a large change in bond angle from the ground state. The B-1(2) state behaves n ormally and has very long progressions in both v(1) and the bending vi bration (v(2)) with a Franck-Condon maximum up to 14 000 cm(-1) from t he origin, while for the (1)A(2) state the progressions are much short er (showing at most one or two quanta of v(2)) with a Franck-Condon ma ximum 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 wave functions of the perturbing state with the ground state rather than th ose of the excited state with the ground state. Attempts to fit this t o a model of vibronic coupling result in there seeming to be too many levels. (C) 1997 Academic Press, Inc.