DIFFUSION OF HYDROGEN AND DEUTERIUM ON STEPPED TUNGSTEN SURFACES .2. W(023)

The diffusion of H-1 and H-2 on a (023) surface of a tungsten field emitter has been studied by the fluctuation method. This surface consists of (110) oriented terraces separated by (100)-oriented steps. Activated and tunneling diffusion are observed both parallel and perpendicular to steps with very little anisotropy. The activation energies and prefactors for diffusion differ considerably from those found on W(110) and it is hypothesized that the observed values are dominated by diffusion across steps. The activation energies for H-1 are higher than those for H-2 by approximately 2 kcal and prefactors for H-1 are also much higher than for H-2. This is interpreted as activated polaronic diffusion largely controlled by substrate deformation, with the energy differences arising from zero point energy differences for the two isotopes during lattice distortion. At half coverage a switch from an initial activated to a slower one is seen at 151 less-than-or-equal-to T less-than-or-equal-to 166 K and interpreted as a hydrogen-assisted reconstructive phase transition involving the non-close-packed (100) oriented steps. Tunneling diffusion shows only very small isotope effects probably because of polaronic contributions which counteract the isotope effect for pure hydrogen tunneling.