On the Extraction of Electrons from a Metal Surface by Ions and Metastable Atoms

Abstract

An elementary theory is developed for the process by which a helium ion or a $2{}_{}{}^3{}_{}{}^{}S$ metastable helium atom may extract an electron from a metal surface. The helium ion after collision may become a neutral atom although sometimes an excited or even a metastable atom. In the case of the metastable atom the collision involves an exchange of electrons between the metal and the atom, the excess energy being carried away by the ejected $L$ shell electron. The average distance of transition $\overline{s}$ for ${\mathrm{He}}^{+}$ and for the $2{}_{}{}^3{}_{}{}^{}S$ metastable helium atom is calculated for different velocities. For $v={10}^5$ cm/sec., ${\overline{s}}_{\mathrm{met}}=2\mathrm{}{a}_0$ and ${\overline{s}}_{\mathrm{ion}}=11.5\mathrm{}{a}_0$ where $a_0=0.528\mathrm{}\times 0^{-8\mathrm{}}$ cm. For $v=3\mathrm{}\times {10}^7$ cm/sec., ${\overline{s}}_{\mathrm{met}}$ comes out less than $0.1a_0$ and ${\overline{s}}_{\mathrm{ion}}=6.0\mathrm{}{a}_0$. These values are different from the values obtained by Massey in a similar computation. The theory accounts for some but not all of the experimental evidence. The probable shape of the potential that the metal surface offers to a metastable atom is found.