Threshold photoelectron spectrocopy of SO2 over the photon energy range 12–50 eV

Abstract The threshold photoelectron spectra of SO 2 were recorded over the photon energy range (12–50 eV) under high-resolution conditions using synchrotron radiation. Extensive vibrational ‘filling-in’ is found in the Franck-Condon gap regions between the three lowest ionic-state band systems that is attributed to autoionization of Rydberg states lying in these regions. The outer-valence ionization region (12–18 eV) consists mainly of the vibrationally well-defined electronic states of ( X ˜ 2 A 1 ), ( A ˜ 2 B 2 ), ( B ˜ 2 A 2 ), ( C ˜ 2 B 1 ), ( D ˜ 2 A 1 ) and ( E ˜ 2 B 2 ) being derived from the molecular orbitals: (8a 1 ), (5b 2 ), (1a 2 ), (2b 1 ), (7a 1 ) and (4b 2 ), respectively, but there are also some features that can be associated with the two weak satellite structures (1 2 B 1 ) and ( 3 B 2 2 ). On the other hand, in the inner-valence ionization region (18–50 eV) the opposite is true, i.e., a well-defined electronic state structure is limited to the ( F ˜ 2 A 1 ) state of SO 2 + with a (6a 1 ) molecular orbital origin, with the preponderance of the remaining structure due to broad satellite features involving ionization from the inner-valence (3b 2 ) and (5a 1 ) molecular orbitals.