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

Abstract

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 10 8 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.

Delivery of Electrons by Proton-Hole Transfer in Ice at 10 K: Role of Surface OH Radicals

“…We actually observed OH production upon electron irradiation on ice by PSD-REMPI, which eventually disappeared in a time scale of several minutes. The similar exponential-like behavior upon electron exposure was previously reported, and attempts were made to interpret the behavior by charge attenuation in an electrical circuit model in ice. , The subsequently observed small currents can be explained by different mechanisms such as leakage of electrons through the surfaces of pores and cracks or grain boundaries or transmission of solvated electrons in amorphous ice (Figure c) …”

“…The negative charge flow via solvated electrons in ice was examined experimentally by electron bombardment of ice. Although a transient negative current was observed, the mechanism of electron delivery was not clear, and the current immediately diminished below 50 K. 5 As a plausible process for the negative current conductivity, protonhole transfer (PHT), which is the relay of hydroxyl anion (OH − ) in ice, has been proposed. 1 However, quantum chemical calculations for liquid water have shown that PHT by OH − cannot be regarded as a mirror image of the Grotthuss mechanism, and is, thus, unlikely to work.…”

Control of amorphous solid water target morphology induced by deposition on a charged surface

“…It was found that experiments conducted at temperatures above 110 K resulted in flat ASW films, similar to the deposition without accumulated charge on the buffer layer (Figure 3(a)). Sagi et al [26] have shown that at temperatures above 110 K the increased thermal energy of ASW layer allows a faster mobility of electrons in ASW buffer layer, which results in a deeper penetration of the electrons. This process leads to solvation of electrons in ASW matrices resulting in a reduced electric field on the ASW surface.…”

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

“…The transient negative current at the initial electron irradiation was previously reported to be a result of solvated electrons in the ice. 86) Further experiments were performed to understand how OH and electrons work for the negative current. The ice was first exposed only to UV photons for a given number of periods and subsequently exposed only to electrons.…”