Untersuchungen der Oxidbildung und Adsorption von Erdalkalien an einer sauerstoffvorbelegten Wolframoberfläche mit Hilfe der Oberflächenionisation

The surface ionization of alkaline-earth elements on tungsten has been studied in dependence on the temperature T and the surrounding oxygen partial pressure po₂; the values of the ionization efficiency β together with those of the change of the work function ΔΦ of the surface have been applied to get information about chemical reactions of the incident alkaline-earth atoms with adsorbed oxygen and about the adsorption of alkaline-earth elements on tungsten.Whereas in the high temperature range the tungsten surface is clean, towards lower temperatures (i.e. below ≈ 2500 K at po₂ = 1 × 10^?6 Torr or below ≈ 2000 K at po₂ = = 1 × 10^?9 Torr), an adsorption of oxygen increases the work function Φ and, consequently, the ionization efficiency β of incident metal atoms. A characteristic feature of the surface ionization of the alkaline-earth elements, however, is a rapid re-decrease of β with further decreasing temperature, which occurs at T ≈ 1400 K for Mg/W, T ≈ 1600 K for Ca/W, T ≈ 1800 K for Sr/W, and at T ≈ 2000 K for Ba/W. It is shown that this behaviour of β is caused by two different reasons: Whereas in the case of Mg/W a substantial Mg adsorption leading to a reduction of the work function is responsible for the decrease of β solely, the β values of Ca and Sr are additionally influenced by chemical reactions of the incident metal atoms with adsorbed oxygen resulting in an alkaline-earth oxide desorption. In the system $\text{Ba}\text{W}$ the decrease of the ionization efficiency β can be referred to BaO formation exclusively.Assuming a thermodynamic equilibrium between the different adparticles and using experimental values of the dissociation energy of the alkaline-earth oxides (in the gas phase), the results are in good agreement with theoretical calculations.