STRUCTURE DETERMINATION OF A CO O COADSORPTION PHASE ON NI(111)/

The structure of the Ni(111)(2 x 2)-O-CO adsorption phase, formed by C O adsorption onto a Ni(111)(2 x 2)-O phase, has been determined by sca nned-energy mode photoelectron diffraction of both the C 1s and O 1s s ignals. Contrary to earlier interpretation of vibrational spectroscopi c data, the CO molecules occupy the same threefold coordinated hollow sites which they adopt on the clean Ni(111) surface, with a Ni-C bondl ength of 1.95+/-0.04 Angstrom and a C-O bondlength of 1.17+/-0.03 Angs trom. Changes in both Ni-C bondlength and the vibrational amplitude of the CO molecule against the surface are related to the weaker CO-meta l bonding in the presence of the atomic oxygen. The chemisorbed oxygen atoms occupy the same ''fcc'' hollow sites, directly above third laye r Ni atoms, both before and after CO adsorption.