Infrared and vapor flux studies of vapor-deposited amorphous and crystalline water ice films between 90 and 145 K

Three methods were simultaneously employed to investigate vapor-deposited water ice films (90-145 K) in a high-vacuum chamber: grazing-angle Fourier transform infrared (FTIR)-reflection absorption spectroscopy to characterize ice phase; optical interference methods to monitor ice density, growth rate, and film thickness; and gas phase mass spectrometry to monitor sublimation flux during annealing. The results of this work were used to further describe the physical properties of amorphous and crystalline ice, with the goal of achieving greater consensus regarding the structure, density, and vapor flux of amorphous ice. Amorphous and crystalline ice were clearly distinguishable using grazing-angle FTIR spectroscopy. Vapor-deposited films (deposited between 0.1 and 10 nm s^-1) appeared amorphous at temperatures below 105 K and crystalline above 120 K. Between 110 and 115 K, amorphous ice, crystalline ice, or a mixture of both was observed, inconsistent with the prediction that ice will be crystalline in this regime [Kouchi et al., 1994]. Upon annealing (1-2 K min^-1), amorphous ice underwent a phase change to crystalline ice between 147 and 154 K. The density of ice at 90 K was slightly greater (0.94 ± 0.01 g cm^-3) than the density of ice above 110 K (0.93 ± 0.01 g cm^-3), consistent with literature values for bulk amorphous (excluding micropores) and crystalline ice. Sublimation flux was examined in light of the proposed existence of two metastable phases of amorphous ice: low-density amorphous and restrained amorphous [Jenniskens et al., 1998]. No supporting evidence was provided; however, our techniques cannot rule out their proposed existence.