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[PCG20-P03] Mechanisms of structural changes of amorphous ice
In interstellar molecular clouds, water molecules exist on mineral particles as amorphous ice [1]. Various elements deposit on the surface of amorphous ice, and form various molecules such as ammonia, formaldehyde, and ethanol through surface and photochemical reactions [2]. It is essential to understand the structure of amorphous ice, because amorphous ice acts as a reaction field of molecular evolution in space [1]. To investigate the effects of formation condition and thermal history after the formation on structures of amorphous ice, we measured infrared (IR) spectra and reflection high energy electron diffraction (RHEED) of amorphous ice. To obtain a detailed structure from the diffraction pattern, we supplement the measured data with calculations using molecular dynamics (MD) method. Amorphous ice was prepared with a vapor deposition of distilled and degassed water on a substrate of oxygen-free copper at a temperature range of 5–175 K. The total pressure in the vacuum chamber was kept at about 2.0×10–5 Pa during the deposition. After the deposition of amorphous ice, the substrate was cooled to 5 K. Then, the sample was heated to 165 K. The IR spectra and electron diffraction patterns of amorphous ice were measured during deposition, cooling, and heating. The MD calculations were performed using an atom-atom potential model with MXDORTO program [3]. The fundamental cell consisting of 360 water molecules with three-dimensional periodic boundary conditions was used. The amorphous ice with a density of 1.08 g cm−3 was prepared by quenching the liquid water phase (1.12 g cm−3 ) from 380 to 10 K at a cooling rate of 2.5 K fs−1 . The MD code was run with NTP ensemble in the temperature range of 10−170 K at 0.1 MPa and the radial distribution function (RDF) and structure were analyzed. From the correlation between the calculated RDF and the measured diffraction patterns, the detailed structural of amorphous ice were analyzed. The IR spectral futures of the O–H stretching modes of the deposited amorphous ice change in heating process due to a structural change. We decomposed the vibrational band into three modes [4], and attempted to classify the changing processes from the variation processes of the wave number, half-width, and intensity ratio of the decomposed three peaks. The results show that the transition process with heating is classified into the following four processes (i) transition from high density amorphous (HDA) ice to an intermediate structure at around 60 K, (ii) transition from the intermediate structure to low density amorphous (LDA) ice at around 100–120 K via the glass transition, (iii) crystallization to cubic ice (Ic) at around 145 K, and (iv) phase transition from Ic to hexagonal ice (Ih) at around 175 K. We discuss the detailed processes using the diffraction patterns and MD calculations. [1] J. Klinger, D. Benest, A. Dolfus, and R. Smoluchowski : Ices in the Solar System, (Reidel, 1985). [2] E. L. Gibb, D. C. B. Whittet, A. C. A. Boogert, and A. G. G. M. Tielens : Interstellar ice : the infrared space observatory legacy , Astrophys. J., Suppl. Ser., 151 (2004) 35. [3] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange, #029 (1990). [4] E. Whalley, D. Klug, and Y. P. Handa : Entropy of amorphous ice , Nature 342 (1989) 782–783