The diffusion of H218O on monocrystalline H216O ice multi-layers which had been grown epitaxially onto (001)Ru was studied by using laser-induced thermal desorption techniques. Low-energy electron diffraction data confirmed that the ice layers on (001)Ru were crystalline up to thicknesses of at least 105 bi-layers (38.5nm). One bi-layer of isotopically-labeled H218O was deposited onto half of a crystalline H216O ice multi-layer. Laser-induced thermal desorption methods were then used to scan the coverage profile as a function of diffusion time at 140K. No H218O signals were detected, from the masked half of the crystal, for diffusion times of up to 1h. The results were consistent with an upper limit, for H218O surface diffusion, of less than 5 x 10-9m2/s at 140K. The lack of measurable surface diffusion was suggested to indicate that the H218O was diffusing into the underlying ice multi-layer. Bulk diffusion was therefore investigated by using a novel technique which involved a combination of isothermal desorption, depth profiling and laser-induced thermal desorption. A single bi-layer of H218O was deposited onto a crystalline H216O multi-layer. Isothermal desorption was then used to depth-profile the ice multi-layer. Laser-induced thermal desorption was simultaneously used to monitor the isotopes, which remained in the multi-layer, as a function of time. The results indicated that H218O diffused readily into the underlying ice multi-layer. The bulk diffusivity of H218O into multi-layers of various thicknesses was deduced to be equal to 1.5 x 10-15cm2/s at 160K. Sandwich experiments, involving stacked multi-layers, confirmed that H218O diffused into the bulk without any involvement of a liquid-like layer at the surface. The bulk diffusion data could be described by the expression:

D (cm2/s) = 9.7 x 107exp[-16.7(kcal/mol)/RT]

This activation energy was within the experimental error of some previous measurements that had been made close to the melting point.

D.E.Brown, S.M.George: Journal of Physical Chemistry, 1996, 100[38], 15460-9