Site exchange in thermally disordered adsorbate layers

The probability for thermally activated occupation of sites, which are not occupied at temperatures far below the order-disorder phase transitions, was determined for the adsorbate systems S Ru(0001) and O Ni(111) at a coverage of 0.25 usin analysis. We concentrated on the two threefold coordinated sites, which are energetically the most favourable on these surfaces at low coverage, and determined their occupation probabilities as a function of temperature at constant coverage. We found significant occupation of the less favoured hcp site already below the order-disorder phase transition for the system O Ni(111) - p(2×2) , w for S Ru(0001) - p(2×2) only hcp sites are occupied even at temperatures far above the order-disorder phase transition temperat In addition to the increase of diffuse background intensity caused by the Debye-Waller factor, an S-shaped contribution close to the order-disorder phase transitions was found far away from the Bragg peaks, if a temperature dependent change of site occurs. Data for the (√3×√3)R30° structure of S Ru(0001) , where a thermally activated change of sites has been found recently [8 interpretation of enhanced background intensities.