Anomalous loss of ionization in argon-hydrogen plasma studied by fast flow glow discharge mass spectrometry

Changes in ion abundance from the plasma of an abnormal dc glow discharge, using a copper cathode, have been studied as a function of (a) gas residence time in the discharge and (b) the amount of H₂ added to the positive column, without disturbing the discharge. The Ar+H₂ spectra show a severe, but selective, quenching of the ionization. All major ions are quenched except for Cu⁺, which increases in abundance. It is shown that this cannot be explained by either ion-molecule reactions or the fast recombination suggested previously [R. F. G. Meulenbroeks et al., Phys. Rev. E 49, 4397 (1994)] for such mixtures. The kinetic behavior is consistent with the heavy involvement of stepwise ionization processes in which high excited metastable states of neutral Ar are precursors for most ions observed in the spectrum. It is proposed that these states are rapidly quenched by H₂ down to the Ar(4s) levels, preventing ionization of most species in the discharge, but significantly boosting the Penning ionization of Cu atoms sputtered from the cathode.