PHOTOELECTRON DIFFRACTION INVESTIGATION OF THE ADSORPTION SITE AND LOCAL-STRUCTURE FOR POTASSIUM ON NI(111)

A recent LEED study of the Ni(111)(2 X 2)-K structure indicates that this is one of a small number of examples of alkali metal adsorption on close-packed metal surfaces now believed to involve atop adsorption. In order to provide an independent test of this surprising result we have conducted scanned energy mode photoelectron diffraction experiments on the K 2s and 2p emission at a range of polar emission angles. A simple Fourier transform mapping of the nearest neighbour contribution to these spectra provides direct evidence for the atop adsorption site, and subsequent theory-experiment comparisons using multiple scattering calculations confirm this site and give a nearest neighbour bondlength of 2.86 +/- 0.03 angstrom, essentially identical to the LEED result (2.82 +/- 0.04 angstrom). The new results also indicate a significant contraction (0.17 +/- 0.06 angstrom) of the outermost Ni atom layer, but find essentially none of the ''rumpling'' obtained in the LEED study.