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.
