Photodissociation of SO3 at 193 nm investigated by photofragment translational spectroscopy

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.

Photochemistry of HOSO2 and SO3 and Implications for the Production of Sulfuric Acid

Sulfur trioxide (SO₃) and the hydroxysulfonyl radical (HOSO₂) are two key intermediates in the production of sulfuric acid (H₂SO₄) on Earth's atmosphere, one of the major components of acid rain. Here, the photochemical properties of these species are determined by means of high-level quantum chemical methodologies, and the potential impact of their light-induced reactivity is assessed within the context of the conventional acid rain generation mechanism. Results reveal that the photodissociation of HOSO₂ occurs primarily in the stratosphere through the ejection of hydroxyl radicals (^•OH) and sulfur dioxide (SO₂). This may decrease the production rate of H₂SO₄ in atmospheric regions with low O₂ concentration. In contrast, the photostability of SO₃ under stratospheric conditions suggests that its removal efficiency, still poorly understood, is key to assess the H₂SO₄ formation in the upper atmosphere.