DIFFUSION OF OXYGEN ON THE MO(110) PLANE

The diffusion and mean square fluctuations [(deltaN)2]/[N] of O on Mo(110) have been investigated as functions of coverage and temperature over the range 0.05 less-than-or-equal-to theta less-than-or-equal-to 0.9. The diffusion coefficient D, multiplied by [(deltaN)2]/[N] yields the jump diffusion coefficient D(j), which is similar to the single particle or tracer coefficient. The forms D = D0 exp(-betaE) and D(j) = D(j0) exp(-betaE(j)) with beta = 1/k(B)T are used to analyze the results. Except at theta = 0.9 [(deltaN)2]/[N] is temperature independent, so that E = E(k). E increases from 17 kcal at theta = 0.07 to 26 kcal at theta = 0.5, then drops to 18 kcal at theta = 0.7 and finally rises to approximately 20 kcal at theta = 0.9. At this coverage E(j) congruent-to 28 kcal. D0 varies from 1 cm2 s-1 at low coverage to 10(-2)-10(-1) cm2 s-1 at theta = 0.5, then drops to 10(-5) cm2 s-1 at theta = 0.9. D(j0) values range from 10(-2) cm2 s-1 at theta = 0.05, drop to 2 x 10(-4) cm2 s-1 at theta = 0.15, then rise to 10(-2) cm2 s-1 at theta = 0.5 and finally decrease to 10(-5) cm2 s-1 at higher coverages. There are strong compensation effects between E and D(j0). The variation in E with coverage can be explained in terms of repulsive nearest and attractive farther-than-nearest-neighbor O-O interactions, which are suggested by the low theta phase diagram. The coverage dependence of [(deltaN]2/[N] is complicated, with a minimum at theta congruent-to 0.25 and a broad maximum near theta = 0.5.