ESCA study of carbon monoxide and oxygen adsorption on tungsten

Abstract

The chemisorption of both CO and O 2 on a clean tungsten ribbon has been studied using an ultrahigh vacuum X-ray photoelectron spectrometer. For CO, the energy and intensity of photoemission from O(1s) and C(1s) core levels have been studied for various adsorption temperatures. At adsorption temperatures of ∼100 K., the “virgin”-CO state was the dominant adsorbed species. Conversion of this state to more strongly-bound β-CO is observed upon heating the adsorbed layer to ∼320K. Thermal desorption of CO at 300⩽ T⩽640 K causes sequential loss of α 1-CO and α 2-CO as judged by the disappearance of O(1s) and C(1s) photoelectron peaks characteristic of these states. Oxygen adsorption at 300K gives a single main O(ls) peak at all coverages, although at high oxygen coverages there exist small auxiliary peaks at ∼2eV lower kinetic energy. The photoelectron C(1s) and O(1s) binding energies observed for these adsorbed species are all lower than for gaseous molecules containing C and O atoms. For CO adsorption states there is a systematic decrease in photoelectron binding energy as the strength of adsorption increases. These observations are in general accord with expectations based on electronic relaxation effects in condensed materials.