Auger Ejection of Electrons from Tungsten by Noble Gas Ions

Abstract

Experimental investigation of electron ejection from atomically clean tungsten by singly and multiply charged ions of the noble gases is reported. Total electron yield, ${\ensuremath{\gamma}}_{i}$, and distribution in kinetic energy of the secondary electrons have been measured. Ion energies range from 10 to 1000 ev for singly charged ions. ${\ensuremath{\gamma}}_{i}$ is found in each case to be roughly constant over this interval although the variations observed are significant and can be accounted for by theory. ${\ensuremath{\gamma}}_{i}$ values of 0.293, 0.213, 0.094, 0.047, and 0.018 were obtained for 10 ev ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ ions, respectively. Comparison with theory makes it quite clear that for 10-ev ions essentially all electrons observed are ejected by a process of Auger neutralization in which the interaction of two conduction electrons causes one electron to neutralize the ion in the ground state and the other to be excited into the continuum above the filled band. The observed ${\ensuremath{\gamma}}_{i}$ is determined by the probability that these excited electrons escape from the metal. In the case of ${\mathrm{Ne}}^{+}$, indications are that as ion energy increases toward 100 ev a two-stage electronic transition process occurs in a small fraction of the encounters. In this process the ion is first resonance neutralized to an excited state and the resulting excited atom is subsequently de-excited in an Auger ejection process. Variation of the electron energy distribution with ion energy has been investigated. Careful measurement for ${\mathrm{Ne}}^{20}$ and ${\mathrm{Ne}}^{22}$ at 200 ev shows ${\ensuremath{\gamma}}_{i}$ to be independent of nuclear mass. Results of ${\ensuremath{\gamma}}_{i}$ and energy distribution measurements for electrons from multiply charged ions up to ${\mathrm{Xe}}^{5+}$ are also reported. A value of $\mathrm{ca}$ 6.3 ev for the energy of the Fermi level above the ground state in the conduction band in tungsten comes out of this work.