Surface science lettersRapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm-2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.