LOCAL SITE IDENTIFICATION FOR NO ON NI(111) IN VIBRATIONALLY DISTINCT ADSORPTION STATES

Vibrational spectroscopy of NO on Ni (111) shows two distinct N-O stretching bands for adsorption on the clean surface at different coverages which have been attributed to tilted and perpendicular bridging species at low and high coverage, respectively, while an even higher vibrational energy seen when NO is coadsorbed with oxygen has been attributed to atop NO. We have investigated the local adsorption geometry of these species with scanned energy mode photoelectron diffraction from the N and O 1s states, a method previously found to be very sensitive to site changes and capable of quantitative site identification. We find essentially identical photoelectron diffraction spectra from all three species, indicating that there are no site changes to within a few hundredths of an Angstrom unit. Moreover, a quantitative 'analysis using double scattering model calculations indicates that in all cases the adsorption site is actually a threefold coordinated hollow and not bridge or atop.