An Application of SEXAFS to Sub-Monolayer Complexes on Polycrystalline Surfaces

Abstract

Most applications of Surface Extended X-ray Absorption Fine Structure (SEXAFS) have studied model adsorption systems on single crystal surfaces. The local nature of the SEXAFS probe means that it should be equally well suited to surface systems without long-range order, provided that the adsorbate occupies a unique site. We have shown that this expectation is correct in our study of the local geometry around barium atoms on the surfaces of thermionic emitting cathodes. On 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 for both cathodes, with oxygen atoms deduced to occupy hollow sites of the substrate. However, the alloy cathode has Ba bonded to two oxygen near neighbours (compared to one for the tungsten cathode), with a slightly longer Ba-substrate distance: this enhances the surface dipole and thus explains the lower work function.