HBr Uptake on Ice:  Uptake Coefficient, H₂O/HBr Hydrate Formation, and H₂O Desorption Kinetics

HBr uptake on thin ice films was examined using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy techniques. LITD was used to determine the uptake coefficient in situ during low HBr exposures as a function of HBr pressure and ice film temperature. The HBr uptake coefficient on ice was γ = 0.61 ± 0.06 at 140 K and γ = 0.24 ± 0.05 at 100 K at low HBr coverages for HBr pressures ranging from 3 × 10 -8 to 1.4 × 10 -7 Torr. TPD and FTIR were used to examine HBr uptake on ice at higher HBr exposures. TPD studies observed that thin ice films exposed to HBr at 140 K saturated with HBr after large exposures and formed an HBr hydrate with an H 2 O:HBr ratio of ∼3.6:1. FTIR measurements monitored the development of the H 3 O + bending vibration versus HBr exposure on ice. Saturation of the integrated absorbance for this H 3 O + bending vibration was observed after large HBr exposures. The uptake of HBr on ice also had a profound effect on H 2 O desorption from ice. H 2 O desorbed at higher temperatures in the presence of HBr. The zero-order kinetics for H 2 O desorption from pure ice were E d = 13.4 kcal mol -1 and ν 0 = 1.9 × 10 32 cm -2 s -1 . The zero-order kinetics for H 2 O desorption from the HBr hydrate formed after saturation HBr exposures were E d = 10.6 kcal mol -1 and ν 0 = 6.3 × 10 27 cm -2 s -1 .