Cesium adsorption on refractory metals

Abstract

The measurement of the effective work function of a cesium-covered emitter as a function of emitter temperature and cesium pressure provides an important insight into the adsorption phenomena. In this paper, experimental data on the work function of tantalum, molybdenum, rhenium, iridium and other refractory metals, over a range of emitter temperature (1500–2000°K) and cesium pressures (0·01–3 mmm Hg), is presented. A model of the adsorption process, in good agreement with the trends shown by the data, is also given. A useful manner of presenting the experimental data is provided by the fact that curves of constant work function are straight lines when plotted on a plane of logarithm of the pressure vs. reciprocal temperature. The data shows that over a wide range of temperature and pressure the effective work functions are in the order φ(Ir) < φ(Re) < φ(Mo) < φ(Ta) This is just the opposite of the ordering of the vacuum work function. The data can be explained on the basis of a model which assumes the adsorbed cesium to be an ion attracted to the surface by its mirror image charge. The difference of potential which gives the energy for ion evaporation has three contributions, namely, the mirror image potential, the potential at the surface due to the neighboring adsorbed ions, and the potential at a great distance from the surface due to the neighbors. The energy for atom evaporation can be determined from the energy for ion evaporation, and it is concluded that the energy of atom evaporation, V α ( θ), is given by V α ( θ) = V m + φ m − V 1 − V s ( θ) where V m is the mirror-image potential, φ m is the vacuum work function of the surface, V i is the ionization potential of cesium, and V s ( θ), the potential of an ion at the surface due to the neighboring adsorbed ions, is an increasing function of the fractional coverage, θ.