Neutralization of Ions and Ionization of Atoms Near Metal Surfaces

Conditions are examined for the resonance transfer of an electron between a metal and an ion or an atom in close proximity to the surface. The point of view is that of Franck-Condon crossings of potential energy curves of the initial and final states of the system. The effect of ion image forces is found in many cases to cause a significant variation of transition probability with distance. As a result, the conditions for neutralization of a rare gas ion to a metastable state and its subsequent de-excitation with the ejection of a secondary electron is found to be severely restricted. In particular, the theory of Cobas and Lamb for the ejection of electrons by the incidence of ${\mathrm{He}}^{+}$ on a Mo surface are found to be seriously in error. In such cases an Auger type de-excitation is believed to be the mechanism for secondary electron emission.