IONIZATION OF CLUSTERS PRODUCED IN A HOLLOW-CATHODE SOURCE

The competition between neutral, positive and negative metal clusters produced via hollow-cathode discharge, was found to depend on the spee d of the pump used to evacuate the cluster source as well as on the di stance separating the cathode from a grounded grid introduced into the source to intercept the plasma beam. On this evidence it is concluded that the competition between positive and negative cluster ionization depends on the time interval during which clusters and plasma coexist . For coexistence time intervals less than about 1 ms, the relative ab undances remain constant and the ratio of negative to positive cluster intensities ranges between 1.6 +/- 0.3 for Cu and 2.6 +/- 0.3 for Fe. For longer times, the abundance of negative clusters increases and th at of positive decreases to the point of complete extinction. In terms of the cluster ionization mechanisms involved, it is suggested that i nitially several ionizing mechanisms producing negative as well as pos itive clusters proceed with comparable rates. At later time intervals, however, electron attachment becomes the dominant, if not the exclusi ve, particle ionization process. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.