Covalent HCl at the Surface of Crystalline Ice at 125 K: The Stable Phase at Submonolayer Levels

FT-IR spectroscopy of ice nanocrystals, within micrometer thick assemblies, allows observation of the complete stretch-mode vibrational spectrum of the ice surface bilayer with a high level of signal-to-noise. For these high surface area samples, the influence of submonolayer quantities of DCl on the D 2 O ice surface is clearly observable. The effects for DCl are analogous to those for strongly H-bonding covalent adsorbates in the following respects: (a) the extent of shifting of the ice surface-mode vibrational frequencies, (b) the induction of order at the ice surface followed by relaxation of the subsurface to oxygen-ordered interior ice, (c) the partial reversibility of the DCl adsorption upon exposure to a second strong adsorbate, and (d) the observation of the internal vibrational mode of the adsorbed DCl. These results identify covalently adsorbed DCl as the thermodynamically stable form for submonolayer exposures at 125 K.