Local structure of CO coadsorbed with O on Ni(111): A temperature-dependent study

Using C Is scanned-energy mode photoelectron diffraction (PhD), the local g eometry of CO adsorbed onto a Ni(111)(2x2)-O surface has been investigated as a function of the temperature of dosing or subsequent annealing. Around room temperature the CO adopts local atop sites in agreement with the previ ous interpretation of vibrational spectroscopy, but at low temperature a su bstantial fraction of the CO molecules adopt the hollow sites occupied in t he absence of the preadsorbed O, generally consistent with the results of a n earlier PhD study. Heating such a surface leads to a new state in which o nly atop sites are occupied, but this appears to be a result of desorption of the hollow species rather than any transformation of sites. The results confirm the qualitative site occupations deduced in a recent study of this system which used C Is and O Is photoelectron binding energy shifts to fing erprint the local site changes.