Adsorption isotherms of oxygen on tungsten (100), (110) and (111) surfaces in the pressure range 10 −9 to 10 −4 torr and temperature range 1500 to 2600 k, as measured by AES

Abstract

This paper is a continuation of a previous investigation of oxygen adsorption on tungsten at high temperature using Auger electron spectroscopy. In this paper the adsorption isotherms of oxygen on (100), (110) and (111) faces of tungsten are reported. It is shown that these isotherms can be described by an equation of the form pO 2 = AF( θ) exp [− q( θ)/ kT]. The coverage depended functions F( θ) and q( θ) evaluated from the isotherms are different for all three investigated faces. The isosteric adsorption energy q has following initial values at very low oxygen coverage: q 100 = 6.1 eV, q 110 = 6.8 eV and q 111 = 6.5 eV. Increasing the oxygen coverage has only small influence on q 111; it changes from the initial value to q 111 ≈ 6.0 eV at θ ≈ 0.3 and remains constant at this value up to θ ≈ 1. q 110 shows the strongest dependence on oxygen coverage. It decreases rapidly at low coverages, slowly at moderate coverages and reaches the value q 110 = 5.0 at θ ≈ 1. The variation of q 110 with increasing oxygen coverage is monotonie from the initial value to q 111 = 4.9 eV at θ ≈ 1. Assuming that the atomic oxygen is the dominant species leaving the tungsten surface at high temperatures the functions F( θ) are used to calculate the oxygen equilibration probability ζO 2 (high temperature sticking probability) as a function of oxygen coverage θ. The main characteristic of ζO 2( θ) for all three faces is that it shows a maximum for (100) and (111) faces at θ = 0.3 and for (110) face at θ = 0.55.