Surface analysis of thermionic dispenser cathodes

Surface analysis of thermionic Ba dispenser cathodes with a W matrix and Re, Ir, and Os/Ru surface coatings was performed by means of low-energy ion scattering and Auger electron spectroscopy. It was found that the Ba-O complex responsible for the low work functions of the cathodes has similar properties on the different coatings: the atomic Ba/O ratio is close to unity on all cathodes, with the O atoms positioned in a plane below the Ba atoms. The bonding between the Ba and O atoms as observed in the low-energy Auger spectra is also almost identical on all cathodes. However, a significant difference in the absolute Ba-O coverage was observed for the various cathodes. This density is determined by the strength of the bonding of the O atoms in the Ba-O complex with the substrate atoms. The results are discussed with respect to previous experimental and theoretical investigations of cathode surfaces.