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Abstract

The mean residence time τi of alkali ions on both atomically clean and strongly gas-covered surfaces of polycrystalline wolfram has been studied with a recently developed technique of combining modulated atomic and/or molecular beams from vapors of alkali metals and alkali halides with phase-sensitive mass-spectrometic detection. The experimental arrangement allows the direct determination of the mean residence time τi of an ion on a metal surface under extremely clean surface conditions and at very small incident beam intensities (109− 1012 particles cm.−2 sec.−1) to avoid an influence of surface coverage by the beam material itself. In addition, it allows mass-spectrometric determination of the effect of the composition of the incident beam on τi. The dependence of τi on the surface temperature T is given by Frenkel's equation τi = τmath image exp (Ei/kT), where Ei is the ion desorption energy. With alkali chloride sources, the values tabulated below were determined over ranges within the temperature region 1100 °K < T < 1700 °K. The atomic and molecular composition of the beam significantly affected τi, e.g., for an incident beam of neutral sodium atoms, τmath image = (0.85 ± 0.05) × 10−13 sec. Ei = 2.69 ± 0.03 eV for Na+ on clean W.

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The present experiments are being extended to determine the mean residence time τα of neutral alkali atoms on wolfram surfaces. Then with the aid of the more precisely determined values of Ei and Eα and with additional knowledge of the effective work function of the surface and of the ionization energy of the incident particle, it will be possible to decide such an important question as whether or not the surface states of adsorbed atoms differ from those for ions.