NEGATIVE-ION MASS-SPECTROMETRIC STUDY OF ION-PAIR FORMATION IN THE VACUUM ULTRAVIOLET .7. SO2-]O--+S++O(SO+,O)

Ion-pair formation from photoexcitation of SO2 was studied by negative -ion mass spectrometry using synchrotron radiation in the 15-35 eV pho ton energy range. Negative ions O- from SO2 were observed. The appeara nce energy in the photodissociation efficiency curve for O- is in good agreement with the thermochemical threshold of 14.49 eV for the forma tion of O-(2P(u)) + SO+(X2PI). Three vibrational progressions in the e fficiency curves are assigned as resulting from predissociation of the s-type Rydberg states converging to SO2 +(D2A1). The O- efficiency ri ses markedly at wavelengths of 622 and 570 angstrom. These onsets are attributed to formation of triplets of fragments: O-(2P(u)) + S+(4S(u) ) + O(3P(g)) and O-(2P(u)) + S+(2D(u)) + O(3P(g)).