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 trans itions, was determined for the adsorbate systems S/Ru(0001) and O/Ni(l ll) at a coverage of 0.25 using LEED IV analysis. We concentrated on t he two threefold coordinated sites, which are energetically the most f avourable on these surfaces at low coverage, and determined their occu pation probabilities as a function of temperature at constant coverage . We found significant occupation of the less favoured hcp site alread y below the order-disorder phase transition for the system O/Ni(111)-p (2 x 2), whereas for S/Ru(0001)-p(2 x 2) only hcp sites are occupied e ven at temperatures far above the order-disorder phase transition temp erature. In addition to the increase of diffuse background intensity c aused by the Debye-Waller factor, an S-shaped contribution close to th e order-disorder phase transitions was found far away from the Bragg p eaks, if a temperature dependent change of sire occurs. Data for the ( root 3 x root 3)R30 degrees structure of S/Ru(0001), where a thermally activated change of sites has been found recently [8], support this i nterpretation of enhanced background intensities.