Surface analysis of thermionic dispenser cathodes

Surface analysis of thermionic Ba dispenser cathodes with W, Re, Ir and Os/Ru substrates 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 substrates: 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. It was found that the work functions of the cathodes are directly determined by the Ba–O coverage of the surface, with a larger coverage resulting in a lower work function of 2.02, 1.93, 1.89 and 1.85 eV for W, Re, Ir and Os/Ru substrates, respectively. The relationship between Ba–O coverage and work function can be described by a simple relation using the density of Ba–O dipoles and a single value for the dipole moment for all cathodes. For all substrates, the density of Ba–O dipoles is below the value that would result in the lowest work function. The work function of the cathode is thus limited by the density of the Ba–O dipoles, which 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.