Interaction of Oxygen with Polycrystalline Tungsten. I. Sticking Probabilities and Desorption Spectra

Abstract

The interaction of oxygen with a polycrystalline W filament has been studied in the temperature range 300–1200°K and for exposures between 10⁻⁷ and $\text{1\hspace{0.17em}}\mathrm{torr}·\sec$ . Both sticking probabilities and desorption spectra have been measured. After low exposures ${\text{(〈2×10}}^{\text{−6}}\hspace{0.17em}\mathrm{torr}·\sec )$ the adsorbate is all desorbed as O atoms with first order kinetics. With higher exposures, oxygen is removed as oxides: WO, WO₂, WO₃, W₂O₆. The desorption spectra for each of these species are complex, and nine distinct and reproducible oxide states are identified. Saturation coverages in each of these states are independent of adsorption temperature in the range 300–1200°K. At a total coverage of ${\text{8.5×10}}^{14}\hspace{0.17em}{\mathrm{O}}_2$ molecules cm⁻², ${\text{2.2×10}}^{14}\hspace{0.17em}{\mathrm{O}}_2$ molecules cm⁻² are desorbed as oxides; after exposures ${\text{〉 10}}^{\text{−2}}\hspace{0.17em}\mathrm{torr}·\sec$ at 300°K, the total uptake rises to ${\text{15×10}}^{14}\hspace{0.17em}{\mathrm{O}}_2$ molecules cm⁻². At temperatures between 500 and 1000°K and exposure up to $\text{1\hspace{0.17em}}\mathrm{torr}·\sec$ , a multilayer oxide film is formed of unlimited thickness. The desorption product from this oxide film is predominantly WO₂, which desorbs in the region of 1200°K. The kinetics of adsorption and desorption are evaluated and discussed.