HBr uptake on ice: Uptake coefficient, H2O/HBr hydrate formation, and H2O desorption kinetics

HBr uptake an thin ice films was examined using laser-induced thermal desor ption (LITD), temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy techniques. LITD was used to determine the up take coefficient in situ during low HBr exposures as a function of HBr pres sure and ice film temperature. The HBr uptake coefficient on ice was gamma = 0.61 +/- 0.06 at 140 K and gamma = 0.24 +/- 0.05 at 100 K at low HBr cove rages for HBr pressures ranging from 3 x 10(-8) to 1.4 x 10(-7) Torr. TPD a nd 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 wit h HBr after large exposures and formed an HBr hydrate with an H2O:HBr ratio of similar to3.6.1. FTIR measurements monitored the development of the H3O + bending vibration Versus HBr exposure on ice. Saturation of the integrate d absorbance for this H3O+ bending vibration was observed after large HBr e xposures. The uptake of HBr on ice also had a profound effect on H2O desorp tion from ice. H2O desorbed at higher temperatures in the presence of HBr. The zero-order kinetics for H2O desorption from pure ice were E-d = 13.4 kc al mol(-1) and nu (0) = 1.9 x 10(32) cm(-2) s(-1). The zero-order kinetics for H2O desorption from the HBr hydrate fanned after saturation HBr exposur es were E-d = 10.6 kcal mol(-1) and nu (0) = 6.3 x 10(27) cm(-2) s(-1).