Field ion microscopic observations showed that the rate of surface self-diffusion of individual atoms on the (100) and (311) planes was appreciably increased by exposing the surface to H. In the case of (100), H partial pressures of the order of 10-9Torr caused the onset temperature for the migration of a Rh adatom to decrease from 290 to 240K. When the adatom was mobile, its mean-square displacement was constant as a function of time. A similar H exposure lowered the onset temperature, for self-diffusion on (311), from 180 to 120K. Here, the mean-square displacement increased monotonically as a function of time following H exposure. This continuous increase indicated that the H-enhanced diffusion rate was coverage-dependent. The difference between the results for (100) and (311) was attributed to the higher temperatures which were required for self-diffusion on (100). At higher temperatures, a significant amount of H was thermally desorbed during diffusion, and the coverage remained relatively constant as a function of time. The observed coverage dependence indicated that the H-induced enhancement involved more than a simple lowering of the activation barrier to surface diffusion by, for instance, the attachment of a H atom to a Rh atom.

G.L.Kellogg: Physical Review B, 1997, 55[11], 7206-12