Variation du travail de sortie du W (100) par adsorption d'oxyge`ne,de ce´sium et co-adsorption d'oxyge`ne et de ce´sium

The work-function determination by the Kelvin method is used to study the adsorption of oxygen, cesium and co-adsorption of oxygen and cesium on a (100) tungsten surface, at room temperature. The work-function change of the clean surface with the oxygen exposure is used to estimate the sticking coefficient and the dipole moment of adsorbed oxygen. During cesium deposition on the clean surface, a minimum at 1.58 eV and a plateau at 1.80 eV are obtained. Starting from the minimum obtained with cesium, oxygen adsorption leads to a decrease of the work function down to 1.17 eV, when cesium adsorption on a previously oxygenated surface gives a 1.12 eV minimum and an increase of the plateau up to 2.20 eV. This last variation is shown to be consistent with the observed increase of the dipole moment of cesium adsorbed on a partially oxygenated surface, which accounts also for the lowering of the work function minimum.