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-temperature adsorption behaviour and the structure of K and Cs on Cu(OO1) at submonolayer coverages. Adsorption of K takes place in fourfold hollow sites up to coverages of about 0.25 monolayers (ML), where 1 ML corresponds to 1.53 X lOI atoms/cm2. At higher coverages the formation of a quasi-hexagonal incommensurate adlayer is observed. In contrast, for Cs adsorption we observe from the very beginning the formation of the quasi-hexagonal structure up to the completion of the adlaytr at about 0.30 ML. For K adsorption in the hollow sites we determine an adsorption height, d, = 2.25(H) A, corresponding to an effective K radius of reff = 1.6(l) w close to the ionic radius of 1.33 A. We do not observe a change in the effective radiu! as a function of coverage. For the quasi-hexagonal Cs strucJure we find an (average) adsorption height d I = 2.94 A corresponding to an effective radius of reff = 2.18 and 1.93 A, for the limiting cases of hollowand bridge-site adsorption, respectively. The analysis of the superlattice reflections corresponding to the quasi-hexagonal incommensurate structures indicated that the K adlayer is strongly modulated. The first Fourier component of the substrate-induced modulation was determined to uol = 1.29(3) A. In contrast, for Cs/Cu(OOl) static modulation is much less important (uol I 0.2 A>. Variation of the Cs adlayer density by changing the substrate temperature allows continuous expansion and contraction of the adsorbate unit cell. No commensurate-incommensurate transition has been observed.