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, It 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. (C) 2002 Elsevier Science B.V. All rights reserved.