Surface structure of thermionic-emission cathodes.
We have used surface extended x-ray absorption fine structure (SEXAFS) to study the local geometry around barium atoms on thermionic cathodes. On the surfaces of tungsten and tungsten-osmium alloy dispenser cathodes Ba is bonded to oxygen with a well-defined short-range order. The Ba-O distance is similar (2.62\ifmmode\pm\else\textpm\fi{}0.04 A\r{}) for the two cathodes, with oxygen atoms occupying hollow sites of the substrate. However, the alloy cathode has Ba bonded to two oxygen near neighbors (compared to one for the tungsten cathode), which will enhance the surface dipole, thus explaining the observed lower work function.