Effects of Microstructure on the Electrical Properties of Amorphous Solid Water

Abstract
This dissertation describes experimental efforts intended to investigate the effects of microstructure on electrical properties of amorphous solid water (ASW) films grown by vapor deposition on cold substrates (≤140 K). We have approached this problem using several experimental techniques, including vacuum microbalance, mass spectrometry, reflectance spectroscopy, optical microscopy, electrostatic charging with low energy ions, and a Kelvin probe.
We grew thin ASW films (~1100 ML) on cold substrates (< 110 K) in an ultrahigh vacuum environment and studied the influence of growth conditions (e.g., growth temperature, incident angle of the vapor flux, and substrate materials) and subsequent heating on the surface potentials of the ice films. Measurements of the surface potentials of the films suggested that the ice films are spontaneously polarized during condensation, and that this spontaneous polarization strongly depends on the microscopic pore structure. We propose a model, connecting the microstructure of the pores with the observed polarization, to explain the obtained results.
We also subjected ASW films to low-energy (500 eV) ions (He+ / Xe+) at low temperatures (≤140 K) to study the electrostatic charging/discharging of the ASW films and their dependences on film microstructure. When an irradiated ASW film was heated, our data suggested that the microstructure of the pores plays a significant role in the motion of ions in the water ice films at low temperatures.
In addition, optical microscopy showed spontaneous cracking during growth of ASW films. Our measurements showed that an ASW film will crack if its thickness exceeds a threshold that depends on the growth temperature and incident angle of the vapor flux. At about 44 K, cracking also occurred during the heating (10 K – 200 K) of a transparent ASW film (~ 1100 ML) deposited atop a Xe film. Surface potentials of the cracked films at the growth temperature and subsequent heating suggested that the cracking-induced defects affect the electrical properties of the ASW films at temperatures below ~100 K, by retaining the polarization or hindering the motion of ions at elevated temperatures.
The polarization and motion of ions in water ice have been studied for decades and are still controversial. Most of the earlier studies were done with the crystalline ice and the proposed models were relative to the proton tunneling across the hydrogen bond network restricted by the water rules. In the scattered reports on ASW, the microstructure of the pores was not explicitly taken into account. The results in this dissertation show that the electrical properties of ASW are determined by the microstructure (pores, cracks), rather than by the intrinsic dielectric behavior of the ice itself. ASW films serve as laboratory analogues of icy surfaces in space environments. However, the microstructure of the ASW strongly depends on the growth conditions, and the findings in this work suggest that care should be used in applying laboratory data to astrophysical problems for which growth conditions are not well known.