LARGE FLUXES OF HIGHLY EXCITED CESIUM IONS FROM A DIFFUSION SOURCE

The flux of caesium ions desorbing from a hot Ir surface has been studied by electrostatic means at energies below 400 eV, and with detection in a well shielded Faraday collector. The caesium diffuses through the iridium metal foil at temperatures of 1300-1400 K. Two grids at variable voltages are used to analyse the flux. Signals up to 10-4 A cm-2 are measured under varying conditions. In the flux, positive ions are observed with positive emitter voltage, while negative ions are found with negative emitter voltage. The positive ions may change sign to negative ions in contact with surfaces with a large probability, and their transport is strongly influenced by the electric fields. The authors conclude that the positive ions are in excited states. The negative ions may eject electrons in contact with the collector surface and appear as positive ions, which shows that they are excited.