DIFFUSION OF HYDROGEN AND DEUTERIUM ON STEPPED TUNGSTEN SURFACES .1. W(123)

The diffusion of H-1 and H-2 on a (123) surface of a tungsten field emitter was studied by the fluctuation method. The surface consists of (110) terraces, separated by (110)-oriented steps; step edges thus correspond to close-packed rows of W atoms running in the (111) direction. Activation energies are similar to what is seen on W(110), suggesting that steps offer very little impedance to diffusion. At certain coverages dips in D are seen in the activated regimes and interpreted as corresponding to phase transitions. The coverages at which these are observed differ for H-1 and H-2. There is also evidence from the mean square fluctuation versus temperature and coverage results that the thermodynamics of H-1 and H-2 are different. There is very little isotope effect in tunneling and this is rationalized in terms of the mass dependence of lattice distortions accompanying tunneling, which counteracts the mass dependence of pure hydrogen tunneling.