STRUCTURE DETERMINATION OF NI(111)C(4X2)-CO AND ITS IMPLICATIONS FOR THE INTERPRETATION OF VIBRATIONAL SPECTROSCOPIC DATA

A detailed quantitative structure determination of the Ni(111)c(4 X 2) -CO structure has been undertaken using scanned energy mode photoelect ron diffraction from the C ls state over a wide range of emission angl es. Analyses of these data by approximate direct methods, and by two i ndependent multiple scattering trial-and-error fitting optimisations l ead to a consistent structure in which the CO occupies both types of h ollow site on the surface in equal amounts with a C-Ni top layer spaci ng of 1.29 +/- 0.05 angstrom. This structure is therefore essentially the same as that for Ni(111)c(4 x 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indica tive of bridge site adsorption, is not always a reliable indicator of local adsorption site.