Probing the Electronic Structure and Bond Dissociation of SO₃ and SO₃^– Using High-Resolution Cryogenic Photoelectron Imaging

The SO₃ molecule and its radical anion SO₃^– are important chemical species atmospherically. However, their thermodynamic properties and electronic structures are not well known experimentally. Using cryogenically cooled anions, we have obtained high-resolution photoelectron images of SO₃^– and determined accurately the electron affinity (EA) of SO₃ and the bond dissociation energy of SO₃^– → SO₂ + O^– for the first time. Because of the large geometry changes from the C_3v SO₃^– to the D_3h SO₃, there is a negligible Franck–Condon factor (FCF) for the 0–0 detachment transition, that defines the EA of SO₃. By fitting the high-resolution photoelectron spectra with computed FCFs using structures from high-level ab initio calculations, we have determined the EA of SO₃ to be 2.126(6) eV. By monitoring the appearance of the O^– signal in the photoelectron images at different photon energies, we are able to measure directly the bond dissociation energy of SO₃^–(X²A₁) → SO₂(X¹A₁) + O^–(²P) to be 4.259 ± 0.006 eV, which also allow us to derive the dissociation energy for the spin-forbidden SO₃(X¹A₁′) → SO₂(X¹A₁) + O­(³P) to be 3.594(6) eV. The excited states of SO₃^– are calculated using high-level ab initio calculations, which are valuable in aiding the interpretation of autodetachment processes observed at various photon energies. The current study provides valuable information about the fundamental molecular properties of SO₃, as well as the radical anion SO₃^–, which is known in redox reactions involving SO₃^2– and may also play a role in the chemistry of SO₂ in the atmosphere.