ATR-IR spectroscopic studies of the formation of sulfuric acid and sulfuric acid monohydrate films

The heterogeneous reaction of H2O with SO3 on a Ge surface was studied using infrared absorption spectroscopy in an internal reflection geometry. A concentrated sulfuric acid film was formed and subsequently diluted with H2O to produce a stable hydrate film at low temperature (190 K). Such films are important as potential substrates for spectroscopic probes of the surface chemistry of atmospheric sulfate aerosol particles. Co-deposition of H2O and SO3 at 250 K results in the formation of a thin film of liquid sulfuric acid which, on cooling to 190 K, crystallises to produce a stable solid molecular H2SO4 film. This is followed in detail through absorption band shifts and splitting in the IR spectra. Exposure to water vapour at 190 K dilutes this film to a simple hydrate which shows IR absorption bands which can be assigned to H3O+ and HSO4- ions, indicating the formation of sulfuric acid monohydrate. This is in agreement with predictions from the sulfuric acid phase diagram. It is proposed that further dilution using higher water partial pressures will enable stable hydrates of sulfuric acid to be produced in a reproducible and well characterised manner for reaction studies.