Hydrogen promotion of surface self-diffusion on Rh(100) and Rh(311)

Field ion microscope observations show that the rate of surface self-diffusion for individual atoms on the (100) and (311) planes of Rh is significantly increased by exposure of the surface to hydrogen. On Rh(100) admission of hydrogen at partial pressures in the 10(-9)-Torr range causes the onset temperature for migration of a Rh adatom to decrease from 290 to 240 K. Once the adatom is mobile, its mean-square displacement is constant as a function of time. Similar exposures of hydrogen lower the onset temperature for self-diffusion on Rh(311) from 180 to 120 K. Here the mean-square displacement increases monotonically as a function of time after admission of hydrogen. The continuous increase indicates that the enhancement of the diffusion rate due to hydrogen is coverage dependent. The difference between the results for Rh(100) and Rh(311) is attributed to the higher temperatures required for self-diffusion on Rh(100). At the higher temperatures, a significant amount of hydrogen is thermally desorbed during the diffusion intervals and the coverage remains relatively constant as a function of time. The observed coverage dependence indicates that the mechanism of hydrogen promotion involves more than a simple lowering of the activation barrier of surface diffusion by the attachment of a hydrogen atom to a Rh atom.