5:15 PM - 6:30 PM
[PCG19-P05] Effects of deposition temperature on structural changes of amorphous ice in heating process
Keywords:Amorphous ice, Infrared spectroscopy
In interstellar molecular clouds, various molecules such as H2O, CO2, NH3, CH4, H2CO, and CH3OH are formed from deposited elements such as hydrogen, oxygen, carbon, and nitrogen. The deposited H2O exists as amorphous ice. The deposited molecules undergo chemical evolutions through various processes on amorphous ice. Amorphous ice formed from a vapor deposition is principally classified into two types: low-density amorphous (LDA) and high-density amorphous (HDA) ices. HDA ice is transformed into LDA ice with heating [1]. However, the mechanism of the phase transition is less conclusive, because the transition temperature depends on experiments [2]. To investigate the effects of deposition temperature on structural change in heating process, infrared spectra and temperature programmed desorption (TPD) curves of amorphous ice with heating were analyzed.
Amorphous ice was prepared with vapor deposition of distilled and degassed water on a substrate of oxygen-free copper at a temperature of 43.5–120.5 K. The total pressure in the vacuum chamber was kept at about 5.0×10–5 Pa during the deposition. After the deposition of amorphous ice, the substrate was cooled to 43.5 K at a rate of 2 K/min. Then, the sample was heated to 176 K at a rate of 1–4 K/min. The IR spectra were measured using Shimadzu IRPrestage-21 at every 15 seconds during deposition, and measured at 2 K intervals during cooling and heating. The TPD curves were measured using a quadrupole mass spectrometer. Q-mass (Pfeiffer QME220) during heating.
The wave number of the O–H stretching mode of the deposited amorphous ice changes in heating process due to a structural change. The result shows that the variation process of the wave number in heating process depends on the deposition temperature. The transition points can be analyzed from the variation in a changing rate of wave number with heating. For the ice deposited at temperatures below 82 K, three transition points exist in the temperature range of 43.5–160 K. The transitions observed at ~60 and ~100 K are attributed to the transition from HDA ice into LDA ice via an intermediate structure. The transition observed at around 150 K is crystallization to the cubic ice Ic from the LDA ice. For the ice deposited at temperatures above 85 K, only two transitions occur from the intermediate structure into LDA ice at ~100 K and the crystallization to Ic at ~150 K. The transition points depend on the deposition temperature before the heating. On the other hand, the beginning of the desorption of H2O analyzed from TPD curves are ~50, ~90 and ~140 K. These values are approximately independent of the deposition temperature. This suggests that the transition of amorphous ice is induced by the desorption of H2O due to a rearrangement of H2O.
Amorphous ice was prepared with vapor deposition of distilled and degassed water on a substrate of oxygen-free copper at a temperature of 43.5–120.5 K. The total pressure in the vacuum chamber was kept at about 5.0×10–5 Pa during the deposition. After the deposition of amorphous ice, the substrate was cooled to 43.5 K at a rate of 2 K/min. Then, the sample was heated to 176 K at a rate of 1–4 K/min. The IR spectra were measured using Shimadzu IRPrestage-21 at every 15 seconds during deposition, and measured at 2 K intervals during cooling and heating. The TPD curves were measured using a quadrupole mass spectrometer. Q-mass (Pfeiffer QME220) during heating.
The wave number of the O–H stretching mode of the deposited amorphous ice changes in heating process due to a structural change. The result shows that the variation process of the wave number in heating process depends on the deposition temperature. The transition points can be analyzed from the variation in a changing rate of wave number with heating. For the ice deposited at temperatures below 82 K, three transition points exist in the temperature range of 43.5–160 K. The transitions observed at ~60 and ~100 K are attributed to the transition from HDA ice into LDA ice via an intermediate structure. The transition observed at around 150 K is crystallization to the cubic ice Ic from the LDA ice. For the ice deposited at temperatures above 85 K, only two transitions occur from the intermediate structure into LDA ice at ~100 K and the crystallization to Ic at ~150 K. The transition points depend on the deposition temperature before the heating. On the other hand, the beginning of the desorption of H2O analyzed from TPD curves are ~50, ~90 and ~140 K. These values are approximately independent of the deposition temperature. This suggests that the transition of amorphous ice is induced by the desorption of H2O due to a rearrangement of H2O.