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The kinetics of HCl adsorption and incorporation into crystalline and amorphous ice were studied by using a thermal molecular beam, with FTIR spectroscopy to characterize the products. Thin films of hexagonal ice were grown on Pt(111) to provide a highly ordered surface on which to test models for HCl adsorption. Absolute HCl uptakes, product H 2 O:HCl stoichiometries, and sticking probabilities were measured as a function of HCl exposure and temperature between 85 and 145 K. Adsorption proceeds via a trapping mechanism, with the barrier to HCl adsorption into the final state being 7 kJ mol -1 lower than for desorption at low coverages. Adsorption becomes less favorable with increasing HCl coverage, saturating with one HCl adsorbed for each surface H 2 O molecule, independent of the ice thickness for T e 120 K. Above 125 K, HCl is incorporated into the ice film, absorption showing a complex exposure and flux dependence. HCl absorption disrupts the ice lattice, causing the rate of HCl uptake to increase as adsorption proceeds. At high HCl fluxes, the creation of favorable adsorption sites on the ice surface is limited by the rate of HCl transport into the film and the sticking probability drops. The saturation product for 130 e T e 140 K is the amorphous trihydrate HCl‚(3.1 ( 0.3)H 2 O and RAIR spectra for this, and for the surface-adsorbed monohydrate species, showed no bands due to molecular HCl. Amorphous ice films show a similar behavior, but with a greater density of water in the surface and more facile HCl transport into the film.

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Chlorine nitrate adsorption kinetics and uptake have been measured on ordered ice and HCl trihydrate films at temperatures below 150 K. Reaction was followed using a thermal molecular beam, with mass spectrometric detection of gas-phase products and temperature-programmed desorption (TPD) and IR to identify adsorbed species. The sticking probability (S) on pure water ice is (0.98 ( 0.03) at 85 K and remains near unity for temperatures up to 145 K. Initially S is independent of the ClONO 2 uptake, indicating a trapping mechanism for reaction. A molecular state which desorbs near 120 K during TPD is identified as a precursor state and above this temperature molecular ClONO 2 is not stable on the surface and adsorption forms HOCl and nitric acid hydrate. On a clean ice surface the reaction probability starts to decrease after 0.1 monolayer of ClONO 2 has adsorbed, reaction ceasing (S < 5 × 10 -2 ) at an uptake of (0.25 ( 0.05) monolayer, independent of temperature. Reaction occurs even at low temperature, where the surface is immobile, indicating that ClONO 2 hydration can occur on the ice surface and does not require an extensive hydrate cage. The saturation stoichiometry is consistent with a surface covered with HOCl and an amorphous nitric acid trihydrate film. The initial reaction probability for ClONO 2 uptake onto pure HCl trihydrate films was similar, (0.98 ( 0.05) for temperatures between 85 and 145 K with some chlorine desorbing promptly into the gas phase even at 85 K. The uptake of ClONO 2 increases from 0.5 to ca. 1 monolayer above 125 K, the temperature at which HCl starts to transport into water ice films, reaction extending beyond the top layer of the HCl trihydrate surface.