The release of trapped gases from amorphous solid water films. II. "Bottom-up" induced desorption pathways

In this (Paper II) and the preceding companion paper (Paper I; R. May, R. Smith, and B. Kay, J. Chem. Phys. 138, 104501 (2013)), we investigate the mechanisms for the release of trapped gases from underneath amorphous solid water (ASW) films. In Paper I, we focused on the low coverage regime where the release mechanism is controlled by crystallization-induced cracks formed in the ASW overlayer. In that regime, the results were largely independent of the particular gas underlayer. Here in Paper II, we focus on the high coverage regime where new desorption pathways become accessible prior to ASW crystallization. In contrast to the results for the low coverage regime (Paper I), the release mechanism is a function of the multilayer thickness and composition, displaying dramatically different behavior between Ar, Kr, Xe, CH4, N2, O2, and CO. Two primary desorption pathways are observed. The first occurs between 100 and 150 K and manifests itself as sharp, extremely narrow desorption peaks. Temperature programmed desorption is utilized to show that these abrupt desorption bursts are due to pressure induced structural failure of the ASW overlayer. The second pathway occurs at low temperature (typically