Electron Ejection from Ta by He+, He++, and He2+

Abstract

Measurements of total yield (${\gamma }_i$) and kinetic energy distribution are reported for electrons ejected from tantalum by the ions ${\mathrm{He}}^{+}$, ${\mathrm{He}}^{++}$, and ${\mathrm{He}}_2^{+}$ in the kinetic energy range 10 to 1000 ev. The evidence presented indicates that the electrons are released by a collision of the second kind of the ion with the metal surface (potential ejection). One internal secondary electron is produced per incident ion. The probability of this electron escaping is reduced by the possibility of internal reflection at the image barrier at the metal surface. ${\gamma }_i$ for the slowest ions is observed to be 0.14, 0.52, and 0.10 for ${\mathrm{He}}^{+}$, ${\mathrm{He}}^{++}$, and ${\mathrm{He}}_2^{+}$, respectively. The data presented must be considered representative of gas-covered tantalum, since no gas was observed to desorb from the target on heating to 1750°K. ${\gamma }_i$ was found not to vary with time after cooling the target indicating rapid re-establishment of the equilibrium gas layer on the surface from within the metal. The work function of the covered Ta surface is found to be $\mathrm{ca}$ 4.9 ev, some 0.8 ev higher than that of atomically clean Ta. Considerations based on a theory which includes variation of energy levels near the metal surface show resonance neutralization of ${\mathrm{He}}^{+}$ at the covered Ta surface not to be possible. Thus only the so-called direct process of potential ejection occurs, with which conclusion the measured energy limits of ejected electrons are in agreement.