Infrared spectra of SO 2 and SO 2:H 2O ices at low temperature

Abstract

Fourier Transform infrared reflection spectroscopy (incidence angle of 5°) was used to characterize thin films of sulfur dioxide and thin films containing water and sulfur dioxide in various ratios between 10 and 200 K under a pressure of 10 −7 mbar. Pure solid sulfur dioxide begins to sublimate above 108 K with an sublimation enthalpy of 29±3 kJ mol −1.When sulfur dioxide is deposited on a water ice surface or trapped in the ice bulk, no reaction is observed even after annealing. Gaseous mixtures of sulfur dioxide containing water traces deposited at 30 K leads to condensation of pure sulfur dioxide and small clusters (SO 2) x (H 2O) y. States of mixed H 2O/SO 2 films with excess of water are dependent upon deposition conditions. With He as carrier gas, a metastable (H 2O) m (SO 2) n phase is observed. Deposition without carrier gas leads to a solid solution of dioxide sulfur in water ice. Above 120 K a large part of sulfur dioxide diluted in water ice escapes, and one observes changes in the frequency and the profile of sulfur dioxide absorptions which probe the gradual transformation of water ice. Sulfur dioxide which remains in the lattice escapes at 170 K when water ice sublimates.