XPS/ISS investigation of the activation of M-type impregnated dispenser cathodes

Abstract

The work function of impregnated tungsten dispenser cathodes can be lowered by several tenths of an electron volt by sputter coating the emitting surface with an 80% Os, 20% Ru coating approximately 0.5 μ thick. The lower work function would be expected if the thin film model of dispenser cathode activation is correct. The Os/Ru coating would have a higher work function than bare W, giving rise to a stronger dipole moment when the surface is activated by barium adsorption. This stronger dipole moment results in a lower net work function of the cathode. In this work we report results of an XPS/ISS investigation of the activation of two M type cathodes: a 4:1:1 and a 5:3:2 BaO:CaO:Al 2O 3 mole impregnant ratio. The Os and Ru XPS peaks show relatively little change in energy during activation, indicating small or no chemical state changes. The XPS Ba peaks, on the other hand, did show changes indicating the adsorption of metallic Ba on the surface. These changes were most pronounced on the X-ray excited Ba MNN Auger peaks and indicated a more metallic Ba layer than was observed on the uncoated impregnated dispenser cathodes. Similar changes are also seen in the ISS data. These changes are probably the result of the higher binding energy, hence the lower evaporation rate, of metallic Ba from a higher work function substrate.