Crystallization Mechanisms of Porous and Compact Amorphous Solid Water Films

Abstract

Amorphous solid water (ASW), formed by vapor deposition on cryogenic substrates (<130 K), transforms into crystalline ice at high temperatures. However, some aspects of the crystallization mechanism, such as the nucleation process, remain unclear. In this study, the crystallization kinetics of porous ASW (pASW) and compact ASW (cASW) were evaluated to identify their nucleation sites. ASW films were grown under ultrahigh vacuum conditions by background and tube-doser deposition, and their porosities were controlled by adjusting the deposition temperature and angle. In addition, the crystallized fractions were measured using desorption spectrometry. The results reveal that the pASW crystallization is triggered within the film interior, whereas the nucleation of cASW films occurs on their surfaces. Owing to the presence of pores, the pASW films crystallize faster than the cASW ones, and the crystallization activation energy of the former (68.9 kJ mol^–1) is lower than that of the latter (79.8 kJ mol^–1). These findings highlight the role of porosity in the crystallization of ASW, providing insights into the behavior of water molecules in cryogenic environments including polar atmospheres and the surfaces of planets and satellites.