Browsing by Author "King, David A."

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Chemisorption of Carbon Monoxide on Tungsten: Part 1.—Desorption Spectra and Electron Stimulated Desorption
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (4), 1973) Goymour, Clarence G.; King, David A.
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.
Chemisorption of Carbon Monoxide on Tungsten: Part 2.—Lateral Interaction Model for Desorption Kinetics
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (4), 1973) Goymour, Clarence G.; King, David A.
Ordered structures formed by the β states of carbon monoxide on single crystal tungsten surfaces imply strong lateral interactions between adatoms. A statistical model, based on the quasi-chemical approach of Fowler and Guggenheim, is used to derive a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account the existence of lateral interactions between nearest-neighbour adatoms in the overlayer. The model thus provides a link between structural and kinetic studies of chemisorption. It is successfully applied to the β desorption spectra for CO on W reported in Part 1, and a pairwise lateral repulsive interaction energy of 20 kJ mol–1 between C and O adatoms is derived. The kinetic equation is extended to allow for a variation in the C/O adatom ratio, and again the predictions of the model are in close agreement with the experimental CO desorption spectra of Goymour and King from mixed O2+ CO adlayers on W.
Field Emission Study of the Formation and Desorption of Oxide Layers on Tungsten Surfaces
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (2), 1972) Goymour, Clarence G.; King, David A.
The interaction of oxygen with tungsten has been re-examined in the field emission microscope (FEM) as a sequel to a detailed temperature-programmed flash-desorption study of the kinetics of adsorption and desorption in the interaction of oxygen with polycrystalline tungsten. FEM patterns and Fowler-Nordheim average work functions which accompany the four identifiable stages in the formation and high-temperature evaporation of oxide layers on tungsten have been observed and distinguished from changes due to redistribution or rearrangement processes in the adlayer. Using distinctive FEM patterns as end-points for each of the stages of desorption of the adsorbate, activation energies Ed, and first-order rate-constant pre-exponentials ν1 were determined for the loss of each layer; the overall work function change accompanying the loss of each layer was also characterized: layer I (desorption product: O atoms, 1900–2500 K), Ed= 543 kJ/mol; log ν1= 13.5; Δϕ∼–0.2 eV. layer II (desorption product: W oxides, 1600–1800 K), Ed= 506 kJ/mol; log ν1= 16.5; Δϕ=–1.0 eV. layer III (desorption product: W oxides, 1300–1500 K), Ed= 458 kJ/mol; log ν1= 16.0; Δϕ=–0.9 eV.
Interaction of Oxygen with Poly crystalline Tungsten: Part 3.—Electron Stimulated Desorption
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (7), 1972) King, David A.; Madey, Theodore E.; Yates, John T.
This paper is a sequel to a detailed study of the kinetics of adsorption and desorption in the interaction of O2 with polycrystalline tungsten, in which a number of oxide and O atom states were identified by thermal desorption. In electron stimulated desorption (ESD) studies two states of adsorbed oxygen were distinguished, β1 and β2; the former has a high ESD cross section and the latter a low cross section. In this paper an attempt is made to correlate the states observed in the thermal desorption study with these β1 and β2 states. The ESD properties of the β1 state are determined, and the sticking probability for readsorption onto an oxygen-saturated surface depleted of β1 by ESD is determined as a function of the incremental oxygen coverage. The steady state between electronic desorption and readsorption from the gas phase is examined, and isotopic exchange between 16O and 18O in the β2 and β1 states is observed and characterized. The appearance of the β1 state is related in a complex way to the formation of tungsten oxides which can be thermally desorbed in the temperature range 1200–1600 K.