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 de glow discha rge, using a copper cathode, have been studied as a function of (a) ga s residence time in the discharge and (b) the amount of H-2 added to t he positive column, without disturbing the discharge. The Ar+H-2 spect ra show a severe, but selective, quenching of the ionization. All majo r ions are quenched except for Cu+, which increases in abundance. It i s 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 ionizati on processes in which high excited metastable states of neutral Ar are precursors for most ions observed in the spectrum. It is proposed tha t these states are rapidly quenched by H-2 down to the Ar(4s) levels, preventing ionization of most species in the discharge, but significan tly boosting the Penning ionization of Cu atoms spattered from the cat hode.