Diffusion of HDO into Single-Crystal H216O Ice Multilayers: Comparison with H218O

The diffusion of HDO into ultrathin single-crystal H 2 16 O ice multilayers was investigated using a novel combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a Ru(001) metal substrate, and the diffusion coefficients were measured perpendicular to the basal (0001) facet. The measured HDO diffusion coefficients ranged from D = (2.2 ± 0.3) × 10 -16 to D = (3.9 ± 0.4) × 10 -14 cm 2 /s over the temperature range 153−170 K. Arrhenius analysis of the temperature-dependent diffusion coefficients yielded a diffusion activation energy of E A = 17.0 ± 1.0 kcal/mol and a preexponential factor of D 0 = (4.2 ± 0.8) × 10 8 cm 2 /s. The similarity of the diffusion coefficients for HDO and H 2 18 O indicates that H/D exchange does not contribute significantly to HDO diffusion in ice. The agreement between the diffusion kinetics for HDO and H 2 18 O argues that the HDO diffusion occurs via a molecular transport mechanism. The large diffusion preexponentials for both HDO and H 2 18 O diffusion into the ultrathin ice multilayers also suggest that bulk transport properties in ice may be perturbed by close proximity to the ice surface.