Spectroscopic and computational evidence for SO2 ionization on 128 K ice surface

Abstract

The experimental part of this study focuses on FTIR spectroscopy of SO₂ adsorbate on the surface of ice nanoparticles at 128 K, in the 0.5–1 monolayer coverage range. In addition to the infrared spectroscopic features due to molecular SO₂, a structured band is observed at ∼1030 cm⁻¹. A similar band was observed in past spectroscopic studies of SO₂ aqueous solutions, and assigned to anionic products of SO₂ ionization. Ab initio normal mode analysis of HSO₃⁻ yielded intense SO stretch bands in the vicinity of the observed “ionic” feature. The relative intensities of the molecular and the anionic bands indicate that 0.3∼0.5 of the adsorbate is ionized. These results are consistent with the previously published data on isotopic exchange in SO₂-covered ice nanoparticles (Devlin and Buch, J. Chem. Phys., 2007, 127, 091101) which pointed towards substantial SO₂ ionization at low temperatures. Density functional theory modeling of molecular and ionized adsorbate on a crystal ice slab suggests that anion solvation by molecular SO₂ adsorbate facilitates the SO₂ ionization process at the ice surface.