Structural transitions of amorphous ice during heating
A. Kinjo, R. Netsu, and T. Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, JapanIn molecular clouds, elements such as hydrogen, oxygen, carbon, and nitrogen deposit on dust
grains, and are transformed into amorphous H2O ice and other various molecules. Because themolecules undergo chemical evolutions through various processes on amorphous ice, theamorphous ice has an important role for the chemical evolutions in the universe. Vapor
deposition method is often used for formation of amorphous ice, because deposition process issimilar to the situation of amorphous ice formation in the universe. When water vapor isdeposited at a low temperature, high density amorphous ice (HDA) ice is formed [1]. HDA ice
is transforms into low density amorphous (LDA) ice with warming. However, the phasetransition dynamics is less conclusive because the structure of deposited amorphous ice
depends on various factors; deposited temperature, growth velocity, impurities, thermal history
after deposition, and so on. To investigate the effects of deposition temperature on phasetransition dynamics, we analyzed sublimation and structural changes of water with heatingprocess using mass spectrometry and infra-red (IR) spectroscopy.
Amorphous ice was prepared with vapor deposition of distilled and degassed water on asubstrate of oxygen-free copper at 42–100 K. The total pressure in the vacuum chamber was
about 2.0 × 10–4 Pa during the deposition. After deposition of amorphous ice, the substrate waswarmed to 180 K at a rate of 2 K/min. The IR spectra were measured using Shimadzu
IRPrestage-21 at 2 K intervals during warming. The mass spectra were measured by Pfeiffer
QME220.
From the analyses of mass spectra, the four steps of gas desorption was observed in thetemperature range of 42–140 K. For the sample from deposition at 42 K, for instance, theobserved gas-desorption temperatures are 70, 100, 110 and 126 K. The desorption temperature
decreases as the deposition temperature increases. From the temperature dependence of wave
number of O–H stretching modes in IR spectra, the crystallization point to cubic ice (Ic) andtransition point from Ic to hexagonal ice (Ih) are confirmed to be around 145 and 175 K,respectively. Thus, the present result suggests that the five types of amorphous phases exist in
the temperature range of 42–140 K. The first phase at <~70 K and the second phase at ~70 < T< ~100 K are assigned to HDA and LDA ice, respectively [1]. The fifth phase at > 126 K isprobably attributed to a precursor of ice Ic due to glass transition of water [2]. The precursor
exists because the crystallization to ice Ic is inhibited. For the third and fourth phases observed
at ~100 < T < ~126 K, the assignments are less conclusive. However, the gas desorption
observed in this temperature range may be due to a collapse of pore structures, because wave
number of the O–H stretching mode in IR spectra increases with warming. This indicates thestrength of hydrogen bonds increases as the temperature rise.
References[1][2]

A. H. Narten, C. G. Venkatesh, and S. A. Rice, 1976, J. Chem. Phys. 64, 1106.
P. Jenniskens, and D. F. Blake, 1996, ApJ 473, 1104.