Interaction of chlorodifluoromethane with ultrathin solid water films

The kinetics and dynamics of chlorodifluoromethane (CHF2Cl) adsorption on a nd desorption from 30-bilayer-thick amorphous solid water (ASW) and crystal line ice (CI) films were studied utilizing molecular beam techniques and th ermal desorption spectroscopy. The adsorbed state and desorption kinetics o f CHF2Cl are sensitive to the surface structure and, hence, the degree of c rystallinity of ice. Chlorodifluoromethane binds more strongly to Cl than A SW and desorbs at a higher temperature during thermal desorption spectrosco py (TDS), altering the spectrum line shape and allowing differentiation bet ween the two phases. Adsorbed states and thus the TDS line shape also respo nd to structural changes on the ASW surface prior to the onset of crystalli zation. The value of the condensation coefficient (a), a measure of CHF2Cl adsorption kinetics and dynamics, is high on both surfaces (similar to 1). However, a small but reproducible variation of a with the phase of solid wa ter can be used to distinguish ASW from Cl. The sensitivity of CHF2Cl adsor ption and desorption to ice surface structure can contribute information ab out the phase and morphology of bulk ice, provided that the structure of th e surface reflects that of the bulk. (C) 2001 American Vacuum Society.