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 photoelectron diffraction from the C ls state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing 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 indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.