Negative‐ion mass spectrometric study of ion‐pair formation in the vacuum ultraviolet. VII—SO 2 → O − + SO + , O − + S + + O

Ion‐pair formation from photoexcitation of SO 2 was studied by negative‐ion mass spectrometry using synchrotron radiation in the 15–35 eV photon energy range. Negative ions O − from SO 2 were observed. The appearance energy in the photodissociation efficiency curve for O − is in good agreement with the thermochemical threshold of 14.49 eV for the formation of O − ( 2 P u ) + SO + (X̃ 2 Π). Three vibrational progressions in the efficiency curves are assigned as resulting from predissociation of the s‐type Rydberg states converging to SO 2 + (D̃ 2 A 1 ). The O − efficiency rises markedly at wavelengths of 622 and 570 Å. These onsets are attributed to formation of triplets of fragments: O − ( 2 P u ) + S + ( 4 S u ) + O( 3 P g ) and O − ( 2 P u ) + S + ( 2 D u ) + O( 3 P g ).