COVERAGE-DEPENDENT ADSORPTION SITES FOR K CU(001) AND CS/CU(001) DETERMINED BY SURFACE X-RAY-DIFFRACTION/

Surface X-ray diffraction has been used to analyze in situ the room-te mperature adsorption behaviour and the structure of K and Cs on Cu(001 ) at submonolayer coverages. Adsorption of K takes place in fourfold h ollow sites up to coverages of about 0.25 monolayers (ML), where 1 ML corresponds to 1.53 x 10(15) atoms/cm2. At higher coverages the format ion of a quasi-hexagonal incommensurate adlayer is observed. In contra st, for Cs adsorption we observe from the very beginning the formation of the quasi-hexagonal structure up to the completion of the adlayer at about 0.30 ML. For K adsorption in the hollow sites we determine an adsorption height, d(perpendicular-to) 2.25(15) angstrom, correspondi ng to an effective K radius of r(eff) = 1.6(1) angstrom close to the i onic radius of 1.33 angstrom. We do not observe a change in the effect ive radius as a function of coverage. For the quasi-hexagonal Cs struc ture we find an (average) adsorption height d(perpendicular-to) = 2.94 angstrom corresponding to an effective radius of r(eff) = 2.18 and 1. 93 angstrom, for the limiting cases of hollow- and bridge-site adsorpt ion, respectively. The analysis of the superlattice reflections corres ponding to the quasi-hexagonal incommensurate structures indicated tha t the K adlayer is strongly modulated. The first Fourier component of the substrate-induced modulation was determined to u01 = 1.29(3) angst rom. In contrast, for Cs/Cu(001) static modulation is much less import ant (u01 less-than-or-equal-to 0.2 angstrom). Variation of the Cs adla yer density by changing the substrate temperature allows continuous ex pansion and contraction of the adsorbate unit cell. No commensurate-in commensurate transition has been observed.