Probing the Electronic Structure and Bond Dissociation of SO 3 and SO 3 - Using High-Resolution Cryogenic Photoelectron Imaging.

The SO 3 molecule and its radical anion SO 3 - 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 SO 3 - and determined accurately the electron affinity (EA) of SO 3 and the bond dissociation energy of SO 3 - → SO 2 + O - for the first time. Because of the large geometry changes from the C 3v SO 3 - to the D 3h SO 3 , there is a negligible Franck-Condon factor (FCF) for the 0-0 detachment transition, that defines the EA of SO 3 . By fitting the high-resolution photoelectron spectra with computed FCFs using structures from high-level ab initio calculations, we have determined the EA of SO 3 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 SO 3 - (X 2 A 1 ) → SO 2 (X 1 A 1 ) + O - ( 2 P) to be 4.259 ± 0.006 eV, which also allow us to derive the dissociation energy for the spin-forbidden SO 3 (X 1 A 1 ') → SO 2 (X 1 A 1 ) + O( 3 P) to be 3.594(6) eV. The excited states of SO 3 - 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 SO 3 , as well as the radical anion SO 3 - , which is known in redox reactions involving SO 3 2- and may also play a role in the chemistry of SO 2 in the atmosphere.