Coadsorption of oxygen and carbon monoxide on tungsten: Desorption spectra, electron stimulated desorption and field emission microscopy

• Published in: Surface science 1973-03, Vol.35, p.246-270
• Main Authors: Goymour, Clarence G., King, David A.
• Format: Article
• Language: English
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/0039-6028(73)90217-3

CO/W desorption spectra are characterized by an α state and multiple β states; using electron stimulated desorption (ESD) the α state was shown to comprise two sub-states, α 1 and α 2. In this paper the consecutive interactions of O 2 and CO on W are investigated using ESD, flash desorption and field emission microscopy (FEM). Desorption spectra show that the α-CO state is displaced by O 2, in two stages. The ESD probe provides an identification of the first stage with the removal of the α 1-CO state, and energy analysis of ESD ions reveals a large energy shift (∼ − 1.5 eV) during O 2 coadsorption which can be attributed to an incresae in the α 1-CO WC bond length of ∼ 0.15 Å. During this O 2-induced displacement, the two β peaks converge into a single peak at the β 1 position; this is ascribed to adatom interactions in the mixed O and C adlayer. Isotope exchange experiments with 28CO and 36O 2 reveal (i) no exchange in the α-CO states, and (ii) complete exchange in the β-CO states, which is consistent with dissociative adsorption in the latter. The amount of coadsorbed O 2 is estimated from these results, and from FEM data: a full monolayer of O adatoms can be coadsorbed on CO-saturated W, but CO pre-adsorption inhibits the formation of W oxides. The β 1-O 2 (ESD active) state also forms on the CO-covered surface: this state is identical in population, ESD cross section and ion energy distribution to β 1-O 2 on clean W, and retains its identity in the mixed layer (it does not undergo isotopic exchange). CO 2 desorption spectra from the mixed layer were also characterised, complete isotopic scrambling being observed. Pre-exposure of tungsten to O 2 inhibits CO adsorption: a monolayer of O 2 is sufficient to prevent CO adsorption, and at low O 2 coverages, every O 2 molecule preadsorbed prevents one CO molecule from adsorbing. Isotopic exchange is again complete in the β states, and a lateral interaction model for desorption kinetics, based on dissociative adsorption in the β-CO state, quantitatively describes the CO desorption spectra.