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

Highlights: • The threshold photoelectron spectrum of SO₂ has been recorded over the entire valence ionization region 12–50 eV for the first time. • The spectrum reveals the presence of numerous configuration interaction features over the whole ionization range. • The threshold photoelectron spectrum of SO₂ closely resembles similar conventional photoelectron spectra of SO₂. - Abstract: The threshold photoelectron spectra of SO₂ 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 ($\tilde{\text{X}}$ ²A₁), ($\tilde{\text{A}}$ ²B₂), ($\tilde{B}$ ²A₂), ($\tilde{C}$ ²B₁), ($\tilde{D}$ ²A₁) and ($\tilde{E}$ ²B₂) being derived from the molecular orbitals: (8a₁), (5b₂), (1a₂), (2b₁), (7a₁) and (4b₂), respectively, but there are also some features that can be associated with the two weak satellite structures (1 ²B₁) and ($3{}_2{B}_2$). 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 ($\tilde{F}$ ²A₁) state of SO₂⁺ with a (6a₁) molecular orbital origin, with the preponderance of the remaining structure due to broad satellite features involving ionization from the inner-valence (3b₂) and (5a₁) molecular orbitals.