EXAFS and Near Edge Structure IV

J. Phys. Colloques 47 (1986) C8-529-C8-532
DOI: 10.1051/jphyscol:1986899

SURFACE EXTENDED X-RAY ABSORPTION FINE STRUCTURE APPLIED TO THE POLYCRYSTALLINE SURFACES OF REAL THERMIONIC CATHODES

D. NORMAN¹, R.A. TUCK², H.B. SKINNER², P.J. WADSWORTH², T.M. GARDINER², I.W. OWEN^{3, 4}, C.H. RICHARDSON^{3, 5} et G. THORNTON<sup>3, 6

1</sup>  SERC Daresbury Laboratory, GB-Warrington WA4 4AD, Great-Britain
²  Thorn-EMI-Varian Ltd, 248 Blyth Road, GB-Hayes UB3 1HR, Middlessex, Great-Britain
³  Department of Chemistry, University of Manchester, GB-Manchester M13 9PL, Great-Britain
⁴  Vacuum Generators Ltd, Menzies Road, GB-Hastings, TN34 1QY East Sussex Great-Britain
⁵  V.G. Scientific Ltd., The Birches Industrial Estate, Imberhorne Lane, GB-East Grinstead, RH19 1UB West Sussex, Great-Britain
⁶  Also at SERC Daresbury Laboratory, GB-Warrington WA4 4AD, Great-Britain

Abstract
We have used Surface Extended X-ray Absorption Fine Structure (SEXAFS) to study the local geometry around barium atoms on thermionic emitting cathodes at their normal operating temperatures. 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 almost the same for both cathodes, but the alloy cathode has Ba bonded to two oxygen near neighbours (compared to one for the tungsten cathode), which lie in hollow sites of the substrate. This provides an explanation of the lower work function observed for the alloy cathode.