Mobility of oxygen on the (110) plane of tungsten

Abstract

The correlation function of field emission current fluctuations from the (110) plane of tungsten field emitters covered with oxygen has been measured in the range 0.15 ⩽ O/W ⩽ 0.56 for 300 ⩽ T ⩽ 800 K. At intermediate temperatures the experimental correlation functions agree with a theoretical model based on single particle diffusion, and yield diffusion coefficients and activation energies of diffusion of O atoms on this plane. E dif = 14 kcal for O/W ⩽ 0.20 and then increases to 22 kcal for O/W = 0.56. The preexponential D 0 = 10 −7 cm 2 sec −1 at low coverage and increases to 10 −4 cm 2 sec −1 at high coverage. At high temperatures (600–700 K, depending on θ) the decay of the correlation function is slower than predicted by theory. It is suggested that this results from dynamical correlations, or multi-particle density fluctuations, which can occur when several adsorbate particles are simultaneously in motion. At low temperatures (300–500 K, depending on θ) exponential decay of the correlation function is observed, and explained as a prediffusive flip-flop of O atoms between binding locations. The temperature dependence of the mean square fluctuations in the diffusive regime indicates mean repulsive interactions of 5–8 kcal between O atoms.