Vertical diffusion of water molecules near the surface of ice.

Abstract

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.