ADSORPTION SITES IN COADSORPTION SYSTEMS DETERMINED BY PHOTOEMISSION SPECTROSCOPY - K AND CO COADSORBED ON RH(111)

The adsorption sites of coadsorbed K and CO on the Rh(lll) surface hav e been determined using high-resolution core-level spectroscopy, low-e nergy electron diffraction and site-resolved photoelectron diffraction . For both a (2x2)-2CO-1K and a (2 root 3 x 2 root 3-6CO-1K structure, we find that the CO molecules occupy threefold hollow sites and the K atoms on-top sites, contrary to the adsorption sites of K (threefold hollow site) and CO ton-top site below 0.5 monolayers) if adsorbed alo ne on Rh(lll). Deposition of K onto a CO precovered surface is found t o induce large shifts towards lower binding energy of the C and O Is c ore levels (similar to 0.7 eV for C Is and similar to 1.5 eV for O Is) . The major part of these shifts is shown to arise from the K-induced site change of the CO molecules. This finding may be of importance in the interpretation of XPS data of related co-adsorption systems. Final ly, it is suggested that the C and O Is binding energies provide usefu l fingerprints of the CO adsorption site also for co-adsorption system s. (C) 1998 Elsevier Science B.V. All rights reserved.