10:45 AM - 12:15 PM
[PCG20-P07] Effects of deposition temperature on mechanisms of phase transition of amorphous ice
Keywords:Amorphous ice, Dependence of deposition temperature, Water
In interstellar molecular clouds, dust grains from a silicate mineral surrounded by amorphous ice exist [1]. The elements such as H, O, C, and N are condensed on the dust grains and form various molecules such as H2O, CO2, NH3, CH4, H2CO, and CH3OH. The molecules undergo chemical evolutions to complex organic molecules through various reaction processes. Since amorphous ice is a reaction field in interstellar molecular clouds, it is important to understand the mechanisms of structural change of amorphous ice.
Amorphous ice is mainly classified into two types: low density amorphous (LDA) ice and high density amorphous (HDA) ice [2]. HDA ice deposited at temperatures below ~ 40 K undergoes phase transitions to crystalline ice through LDA ice with heating [3]. However, the phase transition temperature is less conclusive, because the transition process depends on the conditions and thermal histories during and after the deposition. To investigate the effect of deposition conditions on the phase transition mechanisms, we analyzed the structural change of ice with heating using infrared (IR) spectroscopy.
Amorphous ice was prepared with vapor deposition of distilled and degassed water on a substrate of oxygen-free copper at a temperature of 5.7–160 K. After the deposition of ice, the substrate was cooled down to 5.7 K and continuously heated to 176 K at a rate of 5 K/min. The IR spectra were measured using Shimadzu IRPrestage-21 at every 3.2 seconds during the deposition, cooling, and heating.
The transition process of HDA ice, which is deposited at 5.7 K, with heating from 5.7 K to 176 K is classified into the following four steps; (i) beginning of the transition from HDA to LDA ice at around 30 K, (ii) the end of the HDA–LDA transition at around 90 K, (iii) crystallization from LDA ice to cubic ice (ice Ic) at around 150 K, and (iv) phase transition from ice Ic to hexagonal ice (ice Ih) at around 165 K. We attempted to analyze these transition temperatures from the change in wave number of the O–H stretching mode of IR spectrum with heating. The result shows that the transition temperatures of the transitions (i) and (ii) depend on the deposition temperature, whereas those of the transitions (iii) and (iv) are almost constant. This indicates that the composition of HDA and LDA ice in the deposited ice depends on the deposition temperature and the composition is kept during cooling. Furthermore, it was found that the transition temperature of the transition (i) depends on surface structure of HDA ice at temperatures below 15 K. The strong peaks of the stretching modes of the O–H dangling bonds are observed for samples with higher transition temperatures of the transition (i). This suggests that the specific surface area is a dominant factor for the temperature of the transition (i). The HDA ice becomes stable because the distorted structure of HDA ice is relaxed with increase in the specific surface area.
[1] S. C. Creighan, J. S. A. Perry, and S. D. Price, J. Chem. Phys. 124 (2006) 114701.
[2] J. Kissel and F. R. Krueger, Nature 326 (1987) 755-760.
[3] P. Jenniskens, D. F. Blake, M. A. Wilson, and A. Pohorille, Astrophys., 455 (1995) 394
Amorphous ice is mainly classified into two types: low density amorphous (LDA) ice and high density amorphous (HDA) ice [2]. HDA ice deposited at temperatures below ~ 40 K undergoes phase transitions to crystalline ice through LDA ice with heating [3]. However, the phase transition temperature is less conclusive, because the transition process depends on the conditions and thermal histories during and after the deposition. To investigate the effect of deposition conditions on the phase transition mechanisms, we analyzed the structural change of ice with heating using infrared (IR) spectroscopy.
Amorphous ice was prepared with vapor deposition of distilled and degassed water on a substrate of oxygen-free copper at a temperature of 5.7–160 K. After the deposition of ice, the substrate was cooled down to 5.7 K and continuously heated to 176 K at a rate of 5 K/min. The IR spectra were measured using Shimadzu IRPrestage-21 at every 3.2 seconds during the deposition, cooling, and heating.
The transition process of HDA ice, which is deposited at 5.7 K, with heating from 5.7 K to 176 K is classified into the following four steps; (i) beginning of the transition from HDA to LDA ice at around 30 K, (ii) the end of the HDA–LDA transition at around 90 K, (iii) crystallization from LDA ice to cubic ice (ice Ic) at around 150 K, and (iv) phase transition from ice Ic to hexagonal ice (ice Ih) at around 165 K. We attempted to analyze these transition temperatures from the change in wave number of the O–H stretching mode of IR spectrum with heating. The result shows that the transition temperatures of the transitions (i) and (ii) depend on the deposition temperature, whereas those of the transitions (iii) and (iv) are almost constant. This indicates that the composition of HDA and LDA ice in the deposited ice depends on the deposition temperature and the composition is kept during cooling. Furthermore, it was found that the transition temperature of the transition (i) depends on surface structure of HDA ice at temperatures below 15 K. The strong peaks of the stretching modes of the O–H dangling bonds are observed for samples with higher transition temperatures of the transition (i). This suggests that the specific surface area is a dominant factor for the temperature of the transition (i). The HDA ice becomes stable because the distorted structure of HDA ice is relaxed with increase in the specific surface area.
[1] S. C. Creighan, J. S. A. Perry, and S. D. Price, J. Chem. Phys. 124 (2006) 114701.
[2] J. Kissel and F. R. Krueger, Nature 326 (1987) 755-760.
[3] P. Jenniskens, D. F. Blake, M. A. Wilson, and A. Pohorille, Astrophys., 455 (1995) 394