MULTIPLE IONIZATION OF THE RARE GASES BY SUCCESSIVE ELECTRON IMPACTS (0–250 eV): I. APPEARANCE POTENTIALS AND METASTABLE ION FORMATION

Abstract

Multiple ionization of the rare gases has been examined in a mass spectrometer with a trapped-ion source. Ions with charge multiplicity up to n = 2 (He), n = 5 (Ne), n = 6 (Ar), n = 7 (Kr), and n = 10 (Xe) were observed with electron energies less than 250 eV. For He and Ne the thresholds agree with spectroscopic values of the ionization potentials, indicating a process of the form eN/ee(N + 1), where N represents an ion of charge multiplicity n. For Ar, Kr, and Xe, processes involving metastable states of the ion are also observed, eN/eN^m; eN^m/ee(N + 1). The estimated energies of the metastable levels of Ar⁺, Kr⁺, Xe⁺, Ar²⁺, and Xe²⁺ are in agreement with spectroscopic values. The energies of the metastable states of Ar⁺, Kr⁺, and Xe⁺ are in agreement with measurements of Auger electron emission from metals by metastable ions. The metastable levels estimated for the more highly charged ions (up to n = 5 for Ar, n = 6 for Kr, and n = 8 for Xe) have not been observed previously. The excitation functions of the metastable levels of Ar⁺, Kr⁺, and Xe⁺ are very similar and show a very sharp maximum near threshold.