Temperature Effect on Transport, Charging, and Binding of Low-Energy Electrons Interacting with Amorphous Solid Water Films

The charging of solid molecular films grown on grains is an important phenomenon observed in astrochemical processes that take place in interstellar space and is relevant in high altitude environmental physics and chemistry. In this work, we present the temperature dependence study of both the conductivity and the capacitance of amorphous solid water (ASW) films (hundreds of monolayers thick) deposited on a Ru(0001) substrate. These layers subsequently interact with low-energy electrons (5 eV) in the temperature range of 50-120 K under ultra high vacuum (UHV) conditions. The charging of the ASW films was measured via contact potential difference (CPD) detection utilizing an in situ Kelvin probe and found to be sensitive to the substrate temperature during film growth, to the substrate temperature during electron irradiation, and to the film thickness. Internal electric fields exceeding 10⁸ V/m are developed. It is shown that solid water conducts and stores electrons with "memory" of the film's thermal history. Furthermore, we propose that trapped electrons discharge during substrate annealing in a process that is driven by the formation and propagation of cracks within the molecular layer, similar to the release of gas molecules embedded inside ASW films, at significantly lower temperatures than the onset of crystallization. Thermal binding energies of electrons to the ASW matrix are obtained from the discharge measurements, in the energy range of 0.26 ± 0.08 eV. These values are 1 order of magnitude smaller than those obtained via photoemission studies.