LAUSR

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 C3v SO3- to the D3h 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 SO32- and may also play a role in the chemistry of SO2 in the atmosphere.