Conformational changes in chemisorbed CO on Ni(110) due to molecular interactions: An ESDIAD study

Abstract

The surface bonding geometry of CO chemisorbed at 83 K on the Ni(110) surface was studied as a function of CO coverage using the ESDIAD technique. A study of the O + angular distribution from CO indicated that at CO coverages below about 0.75 CO/Ni, a normal orientation of the CO bond exists. Above this coverage, CO species begin to be tilted approximately 19° away from the normal in a plane perpendicular to the Ni atom rows. The O + ion angular distribution evolves from a single normal beam to a split beam during this transformation. The yield of positive ions from chemisorbed CO was studied as a function of electron energy. Since a sharp threshold for O + emission occurs at the O(1s) core level energy, measurements above this threshold are particularly effective in observing the conformational transformation. In addition, smaller yields of O 2+ and CO + have been characterized as functions of the electron energy. The conformational behavior of CO has been correlated with thermal desorption spectra and LEED behavior for the overlayer. The work was carried out in a new ESDIAD analyzer which acquires digital ion angular distribution data. The new apparatus is able to eliminate a serious soft X-ray background present in all photographic ESDIAD measurements. Coherent accumulation of ESDIAD data and subsequent background subtraction yields excellent signal-to-noise ratios for ion angular distributions, and facilitates quantitative analysis of ion beam orientations.