Gaseous "nanoprobes" for detecting gas-trapping environments in macroscopic films of vapor-deposited amorphous ice.

Abstract

Vapor-deposited amorphous ice, traditionally called amorphous solid water (ASW), is one of the most abundant materials in the universe and a prototypical material for studying physical vapor-deposition processes. Its complex nature arises from a strong tendency to form porous structures combined with complicated glass transition, relaxation, and desorption behavior. To gain further insights into the various gas-trapping environments that exist in ASW and hence its morphology, films in the 25-100 μm thickness range were codeposited with small amounts of gaseous "nanoprobes" including argon, methane, helium, and carbon dioxide. Upon heating in the 95-185 K temperature range, three distinct desorption processes are observed which we attribute to the gas desorption out of open cracks above 100 K, from internal voids that collapse due to the glass transition at ∼125 K and finally from fully matrix-isolated gas induced by the irreversible crystallization to stacking disordered ice (ice Isd) at ∼155 K. Nanoscale films of ASW have only displayed the latter desorption process which means that the first two desorption processes arise from the macroscopic dimensions of our ASW films. Baffling the flow of water vapor toward the deposition plate greatly reduces the first desorption feature, and hence the formation of cracks, but it significantly increases the amount of matrix-isolated gas. The complex nature in which ASW can trap gaseous species is thought to be relevant for a range of cosmological processes.