Chemisorption of carbon monoxide on tungsten. Part 1.—Desorption spectra and electron stimulated desorption

Abstract

Desorption kinetics and the surface stoichiometry for carbon monoxide on tungsten are reported, with careful attention to the minimising of experimental errors. Desorption spectra at high coverages show an α and two β peaks, the α state desorbing with apparent first order kinetics (activation energy for desorption ∼100 kJ mol^–1); the β₁ state also with first order kinetics (∼220 kJ mol^–1); and the β₂ state with higher order kinetics (∼420 kJ mol^–1). The isosteric adsorption energy for the β₂ state is independent of coverage in this state, within experimental error. It is shown that pumping at room temperature reduces the magnitude of the α state, but not of the β states (contrary to previous results), and it is concluded that α to β conversion during flash desorption is not appreciable. CO₂ desorption spectra were recorded from CO on W, but the total amount was ≲ 10⁴ times the amount of CO desorbed. The total β coverage at saturation is 5.2(±0.8)× 10¹⁸ molecule m^–2. In conjunction with LEED results in the literature, this coverage is shown to be consistent with dissociative adsorption in the β state. This postulate is also consistent with the desorption spectra, as shown in Part 2. Electron stimulated desorption shows that the α state is composed of two sub-states: α₁, yielding low energy CO⁺ ions on bombardment with 100 eV electrons; and α₂, yielding high energy O⁺ ions.