TOP-SITE ADSORPTION FOR K ON CU(111) AND NI(111) SURFACES

Surface-extended x-ray-absorption fine-structure (SEXAFS) measurements have been obtained from p (2 X 2) overlayers of K on Ni(111) and Cu(111) surfaces. The data show that at 65-70 K, potassium occupies the atop site on both substrates, with chemisorption bond lengths of 2.92+/-0.02 angstrom on Ni and 3.05+/-0.02 angstrom on Cu. The identical adsorption site and small change in bond length, only slightly larger than that predicted from the difference in Cu and Ni lattice constants, are consistent with the expected dominance of adatom-adatom interactions in these near-saturation metallic adlayers. The inability to obtain SEXAFS data from K at lower coverages on these surfaces and at these temperatures, indicative of disorder/multisite occupation, is further evidence of a relatively weak alkali-metal substrate interaction even in the dilute-adatom, nonmetallic-overlayer regime.