Kinetic Ejection of Electrons from Tungsten by Cesium and Lithium Ions

The total yield and the kinetic-energy distribution have been measured for electrons ejected from both atomically clean and gas-covered tungsten surfaces by impact of Li⁺ and Cs⁺ in the kinetic-energy range 150 to 1500 ev. The results indicate that the probability of kinetic emission becomes vanishingly small, {~}10^-6, in the vicinity of 150-ev ion kinetic energy and increases to 6.2×{}10^-2 and 1.2×{}10^-2 at 1500 ev for Li⁺ and Cs⁺, respectively, when the tungsten surface is atomically clean. The adsorption of a monolayer of either O₂ or N₂ increases the efficiency of this process by at least an order of magnitude below 350-ev ion energy, and by a factor of 2 or 3 at energies above some 500 ev. In addition, it was observed that the energy distribution of the electrons ejected does not change significantly as the result of such adsorption. An attempt to measure the probability of formation of Li^- from Li⁺ striking an atomically clean tungsten surface showed that such a process, if it occurs at all, occurs with a probability less than about 2×{}10^-8 for Li⁺ incident with 150- to 1500-ev kinetic energy, while negative ions of undetermined constituency were observed to be formed with a probability of about 2×{}10^-3 under the same conditions when the tungsten surface was thoroughly contaminated.