Surface and Bulk Diffusion of H218O on Single-Crystal H216O Ice Multilayers

Abstract

The surface and bulk diffusion of H218O on single-crystal H216O ice multilayers grown epitaxially on Ru(001) were measured using laser-induced thermal desorption (LITD) techniques. Low-energy electron diffraction studies confirmed that the ice multilayers on Ru(001) were crystalline to thicknesses of at least 105 bilayers (i.e., ∼385 A). Measurements of H218O surface diffusion on the crystalline ice multilayers were performed using a scanning LITD method with mass spectrometric detection. Using a mask, one bilayer of isotopically-labeled H218O was deposited on half of a crystalline H216O ice multilayer. LITD measurements were then used to scan the coverage profile as a function of diffusion time at 140 K. No H218O LITD signals were observed on the masked half of the crystal for diffusion times as long as 60 min. These measurements were consistent with an upper limit for H218O surface diffusion of DS ≤ 5 × 10-9 cm2/s at 140 K. The lack of measurable surface diffusion may indicate that H218O is diffusing in...