QUASI-LIQUID MOLECULAR LAYER AT SOLID HYDROGEN SURFACES

Quasi-elastic neutron scattering has been applied to characterize the surface-mobile layer of films of HD (an isotope of molecular hydrogen) condensed on MgO powder in the coverage range 1-5 layers and the temperature range 7-15 K (T(m) = 16.6 K). The close-packed surface has a diffusivity from about 8 K, with 0.3 mobile layers and a diffusion coefficient of 1/3 that of bulk liquid HD at T(m). The mobile quantity increases with temperature. The diffusion coefficients are in the range (0.8-2.0) x 10(-5) cm2/s, and follow an Arrhenius-type temperature dependence with an activation energy of 16.1 K. The partial order remains in the mobile layer where HD molecules jump from site to site on a hexagonal lattice.