Interaction of oxygen with polycrystalline tungsten. Part 3.—Electron stimulated desorption

Abstract

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.