[M-IS13] 結晶成⻑、溶解における界⾯・ナノ現象

[MIS13-07] Size-dependent structure and Ostwald’s rule of stages in vapor-deposited ice at 120 K

*羽馬 哲也¹、佐藤 玲央¹、野口 賢太郎¹、石橋 篤季¹ (1.東京大学先進科学研究機構)

キーワード：極域中間圏雲、アモルファス氷、立方晶氷、六方晶氷

Polar mesospheric clouds, also known as noctilucent clouds, occur within the summer mesopause region at mid and high latitudes, at altitudes about 83 km. Satellite observations show that polar mesospheric clouds are mainly composed of water ice particles with small amounts of meteoric smoke (0.01–3% by volume). The formation mechanism of polar mesospheric clouds is a topic of intense research, because they can be used to study physicochemical processes occurring in the mesosphere/lower thermosphere region and also regarded as potential indicators of global climate change. A recent theoretical study showed heterogeneous nucleation on nanometer-sized meteoric smoke particles can be a formation route of water ice particles in polar mesospheric clouds [1]. At low temperatures and pressures relevant to the mesosphere, metastable cubic ice I (ice I_c) and amorphous water can be formed in addition to stable hexagonal ice I (ice I_h) by vapor deposition on a substrate, i.e., heterogeneous nucleation [2]. However, the formation conditions of ices I_h and I_c and amorphous water by vapor deposition is still poorly understood. In this study, we developed a new apparatus for in situ structural analysis of vapor-deposited water ice using a combination of reflection high-energy electron diffraction (RHEED) and infrared reflection-absorption spectroscopy (IRRAS). In situ RHEED measurements revealed that the structure of vapor-deposited ice on an aluminaum substrate sensitively depended on the size (thickness) under a typical mesospheric temperature (120 K) and water vapor pressure condition (4.0 × 10^-7 Pa), that is, amorphous water first appeared and ice I_c and ice I_h were subsequently formed as the ice thickness increased, providing Ostwald’s rule of stages [3]. Using IRRAS, the threshold thicknesses for the formation of ice I_c and ice I_h are determined as 17 ± 2 nm and 62 ± 2 nm, respectively. At a high water vapor pressure condition (4.0 × 10^-6 Pa), only amorphous water was formed at 120 K. The present study suggests that amorphous water, ice I_c, and ice I_h can coexist in polar mesospheric clouds, depending not only on the temperature but also on the water vapor pressure and ice sizes.

[1] K.K. Tanaka, I. Mann, and Y. Kimura, Atmos. Chem. Phys. 22, 5639 (2022).
[2] T. Yamazaki, A. Kouchi, K.I. Murata, H. Katsuno, H. Nada, T. Hama, and Y. Kimura, Mon. Not. R. Astron. Soc. 527, 2858 (2024).
[3] P. T. Cardew, Cryst. Growth Des. 23, 3958 (2023).