COVERAGE-DEPENDENT ADSORPTION SITES IN THE K CU(100) SYSTEM - A CRYSTAL TRUNCATION ROD ANALYSIS/

The analysis of the intensity along the diffraction rods normal to the crystal surface (crystal truncation rods) has been used to analyse th e adsorption sites of potassium atoms adsorbed at T = 330 K on atomica lly clean Cu(100) under ultra high vacuum conditions. At potassium cov erages below about THETA = 0.18 ML (1 ML = 1.53 x 10(15) atoms cm-2), where no ordered adsorbate superstructure forms, the potassium atoms a re adsorbed in the fourfold hollow sites of the Cu(100) substrate at d (perpendicular-to) = 2.1(4) angstrom above the first Cu layer correspo nding to an effective potassium radius of r(eff) = 1.5(3) angstrom whi ch is close to the ionic radius of 1.33 angstrom. Within about 1-2% th e substrate interlayer spacings, d12 and d23 are not affected. At high coverage (THETA almost-equal-to 0.30 ML) where an ordered incommensur ate overlayer forms, the adsorption site is partially on-top. These re sults are in correspondence with the model of alkalimetal adsorption o n metals supposing extensive charge transfer from potassium to the sub strate at low coverage.