Local Structure of CO Coadsorbed with O on Ni(111):  A Temperature-Dependent Study

Using C 1s scanned-energy mode photoelectron diffraction (PhD), the local geometry of CO adsorbed onto a Ni(111)(2×2)-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 previous interpretation of vibrational spectroscopy, but at low temperature a substantial fraction of the CO molecules adopt the hollow sites occupied in the absence of the preadsorbed O, generally consistent with the results of an earlier PhD study. Heating such a surface leads to a new state in which only 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 1s and O 1s photoelectron binding energy shifts to fingerprint the local site changes.