J. Winter and A. Leukens*

Institut fur Experimentalphysik II
Ruhr-Universitut Bochum, D-44780 Bochum, Germany
jw@plasma.ep2.ruhr-uni-bochum.de

*Institut fur Plasmaphysik, Forschungszentrum Julich GmbH, D 52425 Julich, Germany

To complement earlier studies on large positive ions [1], we investigated the formation of heavy negative ions with masses up to 600 amu in a pulsed low pressure DC glow discharge in methane and other hydrocarbon gases. The cylindrical UHV discharge vessel has a diameter and length of 0.6 m with a central anode, the system constituting a hollow cathode arrangement. Typical pressures and current densities were in the range 1-10 Pa and 10Acm-2, respectively. We applied plasma ion mass spectroscopy as the main diagnostics. Pulsing of the discharge at acoustic frequencies was used to measure the negative ions in the afterglow phase.

Dominant negative ions are C2H- , C4H- with decreasing intensities of the higher hydrocarbon species. Very little C1Hx- and C3H- is observed. The evolution of the different anionic species as a function of the "plasma on" duration with the "plasma off" time held constant clearly shows that the negative ions are produced during the plasma phase and not in the afterglow. Different kinetics of the species was observed: a decrease of low-mass precursors and an increase of the high-mass species. The measurements are interpreted in terms of successive ion-molecule reactions involving C2Hx species as the main precursor. An iterative rate equation model describes well the time evolution of the different species. These data will be presented and discussed.

[1] J. Winter and A. Leukens, Proc. XXIII ICPIG (Toulouse, France), 17-22 July 1997, V-54