Using neutrons to ascertain the impact of deposition temperature on amorphous solid water

Abstract

Amorphous solid water (ASW), formed via vapour deposition under low temperature and pressure conditions, has been the focus of physical- and astro-chemists for some time as it represents the most likely formation process for interstellar ices. The porous structure of ASW has been found to be significantly impacted by deposition conditions, with little literature on the specific impacts of deposition temperature. This work utilises total neutron scattering (TNS) and small angle neutron scattering (SANS) to provide direct experimental evidence that deposition temperature does indeed have a significant impact on the structure of grown ASW. At low deposition temperatures, the ASW structure is highly porous and seemingly in the form of nanoporous islands/grains with voids between them – with these two populations of pores making up the total porosity. With increasing deposition temperatures, the nanopores in the islands become smaller until they are no longer present at temperatures above 80 K, whereby the voids start to dominate. Therefore, even at higher deposition temperatures, there is still porosity present from void volume, rather than being a fully compact ice.