Anisotropy in surface ace diffusion: Oxygen, hydrogen, and deuterium on the (110) plane of tungsten

Abstract

Abstract The field emission fluctuation method of measuring surface diffusion coefficients has been extended to a determination of anisotropy by using as probehole a long, thin, rectangular slit, which can be rotated in situ. For oxygen on W(110), D (110) D (100) = 2 (where the subscripts indicate directions in the plane) under almost all conditions of coverage and temperature. This is the expected result if actual jumps occur along (111) directions. For 1H and 2H very little anisotropy was found for θ > 0.25, and D (110) D (100) ⩽1.3 at θ ⩽ 0.25. This result is tentatively explained by symmetry breaking: It has recently been found by Estrup that H adsorption causes surface reconstruction consisting of a shifting of the top layer along the (110) direction. This Jahn-Teller effect leads to inequivalence of the two asymmetric bridge sites at the ends of the “hourglass” wells on (110) and this can be shown to favor diffusion along (100) over that along (110), relative to the unreconstructed plane, thus decreasing D (110) D (100) . Anisotropies in the mean square fluctuations were also seen in all cases. These can be explained by assuming that the correlation lengths of the fluctuations differ along the (110) and (100) directions and that the correlation length along the former is at least 40 A. A mean field theory calculation in terms of nearest, next nearest and next-next nearest neighbor interaction energies is carried out, and it is shown that values of these parameters in good agreement with those invoked to explain the O/W(110) phase diagram and the increase in activation energies of diffusion for that system give the required correlation lengths and anisotropies. In particular it is shown that the anisotropies in interactions along the (110) and (100) directions are largely responsible for the mean square fluctuation anisotropy for oxygen. For 1H and 2H similar but even larger mean square fluctuation anisotropies are found, and vary more strongly with coverage and temperature than for oxygen, suggesting even more complex interactions.