Dynamics and structure of chemisorbed CO on Cu(110): An electron-stimulated desorption ion angular distribution study

Abstract

The adsorption of CO on a Cu(110) surface has been studied in the temperature range between 32 K and 200 K by the ESDIAD (electron-stimulated desorption ion angular distribution) technique. Here a desorbing neutral excited CO* species is spatially imaged. At coverages up to θ=0.2, a pronounced anisotropy in the librational motion of CO adsorbed in the atop configuration has been observed. The higher amplitude of libration is along the copper atom rows ([11̄0]-direction). Large changes in the observed CO* pattern in the low temperature range between 32 K and 70 K give an indication of a shallow potential well and the thermal population of the higher librational levels. With increasing CO coverage three distinct ESDIAD patterns are observed. Starting from a single normal beam with an elliptical cross-section having the major axis in the [11̄0]-direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the [11̄0]-directions. With further CO coverage increase, an additional central beam grows with a slight elliptical broadening in the [001]-direction. Based on ESDIAD and digital low-energy electron diffraction measurements, a linear-chain model for CO adsorption on atop sites of Cu(110) is proposed.