Temperature dependence of electric-field gradients at isolated Se adatoms on (001) and (111) surfaces and derived adatom distances

Increasing electric-field gradients (EFG) with increasing temperature were observed at isolated Se adatoms on Ni(111), Pd(111), and Co(0001) surfaces applying perturbed angular correlation spectroscopy. This is in contrast to EFG's at probes on surfaces with (001) orientation and in metallic bulk systems. We correlate the results with the EFG dependence on the adatom distance from the surface, as calculated by the discrete variational method molecular/cluster method. The sign changes from negative EFG's on (001), to positive EFG's on (111) surfaces. The distance of isolated Se atoms will be compared with measurements of concentrated Se coverage.