UNEXPECTEDLY RICH VIBRONIC STRUCTURE IN SUPERSONIC JET SPECTRA OF SULFUR-DIOXIDE BETWEEN 360 AND 308 NM

An extensive vibrational resolution study of SO, between 27780 and 325 00 cm(-1), spanning absorption to the B-3(1), (1)A(2), and B-1(1), ele ctronic states, has been carried out via fluorescence excitation in a molecular beam, achieving substantial improvements in sensitivity and rotational cooling over previous studies. Characterization of the depe ndence of transition intensities on molecular beam conditions has been used to identify and separate the contributions of cold and hot trans itions in the spectra. Vibrational-mode-selective, carrier-gas-depende nt cooling is observed and is instrumental in the identification of a group of vibrational levels exhibiting novel Franck-Condon patterns an d little or no cold absorption. At least 135 vibronic levels are ident ified in the energy range studied, adding 71 to the previous total rep orted from our own and other laboratories, Eight levels below 29000 cm (-1) are assigned to the B-3(1) electronic state, supported by a set o f anharmonic constants to approximate the observed vibrational structu re. In the higher energy region, the accepted two-singlet-electronic-s tate model is compared to the experimental results and shown to be ina dequate to account for the large number of observed levels.