Correlation of electron emission with changes in the surface concentration of barium and oxygen on a tungsten surface

Abstract

Techniques, developed to determine the surface concentration of monolayer adsorbates on metal substrates by means of chemical mapping Auger spectrometry, have been used to measure desorption of barium from Ba-W, Ba-C-W, and Ba-O-W surfaces. Desorption data were obtained by measuring the change in surface concentration with time at a constant temperature. By making these measurements over the temperature range 900 to 1400 K, activation energies for desorption were acquired. Higher barium desorption rates were obtained from Ba-W and Ba-C-W surfaces than from Ba-O-W surfaces. The activation energy for barium desorption from a Ba-O-W surface with about a monolayer of oxygen on the tungsten substrate was 4.8 eV and was about 1 eV lower for the other two types studied (Ba-W, Ba-C-W). To correlate electron emission with surface chemistry, time sequential measurements of both electron emission and barium (and oxygen) desorption were made on both barium-covered oxygenated and clean tungsten surfaces. The most stable thermionic emission and minimum desorption rate were obtained from Ba-O-W surfaces with a chemisorbed monolayer of oxygen. Exposing the tungsten surface to higher oxygen concentrations before monolayer barium deposition created an apparent subsurface oxygen concentration which led to poor electron emission characteristics. This effect increased with oxygen content on the Ba-O-W surfaces. Similar measurements of desorption and electron emission were made on an unimpregnated tungsten cathode, covered by a barium monolayer, and these results were collated with those obtained from Ba-O-W surfaces.