Quasi-elastic neutron scattering was used to characterize the mobile surface layers of condensed films of HD (an isotope of molecular H), on MgO powder, with coverages which ranged from 1 to 5 layers. The temperatures ranged from 7 to 15K. The close-packed surface exhibited diffusion at temperatures above about 8K; with 0.3 mobile layers, and a diffusion coefficient which was equal to 1/3 of that of bulk liquid HD at the melting point (16.6K). The mobile fraction increased with temperature. The diffusion coefficients ranged from 8 x 10-6 to 2.0 x 10-5cm2/s, and obeyed an Arrhenius-type temperature dependence with an activation energy of 16.1K. Partial order persisted in the mobile layer, and HD molecules jumped from site to site of an hexagonal lattice.

M.Maruyama, M.Bienfait, F.C.Liu, Y.M.Liu, O.E.Vilches, F.Rieutord: Surface Science, 1993, 283, 333-7