Quantitative structural study of the coadsorption of CO and K on Ni(111) using photoelectron diffraction

Scanned energy mode photoelectron diffraction measurements from the K 2p, O 1s and C 1s states of a Ni(111)(2 x 2)-K/nCO (n approximate to 2) coadsorption structure have been coupled with multiple scattering simulations to determine the local adsorption structure. This structure is compared with the structure of the Ni(111)c(4 x 2)-CO phase obtained from a similar analysis of C 1s and O 1s photoelectron diffraction data (which extends earlier work based on the C 1s signal alone), and of the structure of the Ni(111)(2x2)-K phase based on a previously-published analysis of K 2p and K 2s photoelectron diffraction. In the CO coadsorption phase the K atoms retain the atop site of the (2 x 2)-K phase but with a K-Ni bondlength increased by 0.15+/-0.05 Angstrom, and with the outermost Ni layer contraction of the (2 x 2)-K phase replaced by an outer layer expansion. The CO molecules occupy both types of hollow site on the surface with a C-Ni outermost layer spacing of approximately 1.30 Angstrom, but the inferred C-O bondlength shows no statistically significant change from its value in the absence of coadsorbed K. (C) 1997 Elsevier Science B.V.