Studies on ion formation in a glow discharge mass spectrometry ion source

Abstract

The relative populations of singly- and mutiply-charged argon, matrix and argide ions in an Ar-glow discharge have been measured in different matrices using the VG-9000 glow discharge mass spectrometer. The measured relative densitites of multiply-charged argon ions (from +3 to +6) are in good agreement with the Lovett rate model for electron impact ionization/three-body recombination with an electron temperature of about 7 eV (R.J. Lovett, Spectrochim. Acta, 37B, 985 (1982)). An overpopulation of singly- and doubly-charged argon ions relative to this relationship has been observed in all cases. The overpopulation of singly-charged argon ions is caused by a process of quenching of metastables in a doubly excited system. So-called “secondary electrons” with an average energy about 7 eV produced in this reaction can be an agent creating multiply-charged argon ions. Sputtered sample/cathode atoms are ionized mainly by Penning ionization (PI). A dominance of PI has been found in the production of singly- and multiply-charged ionic species for which ionization energies are lower than the energy of argon metastable atoms. The remaining species appear to be ionized by the secondary electrons.