Proton-Induced Crystallization of Amorphous Solid Water

Abstract

Amorphous solid water (ASW) is a metastable solid form of water. It has attracted the ice researchers’ attention because of the astrochemical significance and its similarities to supercooled liquid water. Metastable ASW changes its phase into crystalline ice (CI) at enough high temperature. In this poster, we report that excess protonic impurities can promote the crystallization of ASW, unlike other impurities that disturb the crystallization. ASW films were doped with excess protons from dissociated HCl and its crystallization fractions were determined from OH stretch vibration changes using a reflectionabsorption infrared spectroscopy (RAIRS) instrument. By comparison of HCl-doped and NaCl-doped ASW samples, facilitated crystallization by excess protons was confirmed. Kinetic analysis showed the lowering of the apparent activation energy from 63.4 kJ·mol1 without HCl to 48.5 kJ·mol1 with 0.1 ML HCl. Such accelerated crystallization effect was observed when HCl was adsorbed not only on the ASW films but also in the films, and the crystallization initiated where HCl was located. This study shows one of the uncommon properties of protons in water and ice and will contribute to understanding ices at very low temperatures.