33rd European Physical Society Conference on Plasma Physics in Rom (Italy)

Investigations of negative oxygen ions in pulsed RF plasmas

J.A. Wagner1, B. Clarenbach2, H.-M. Katsch1 and M. Krämer2
1 Universität Duisburg-Essen, Fachbereich Physik, D-45117 Essen, Germany
2 Ruhr-Universität Bochum, Institut für Experimentalphysik II, D-44780 Bochum, Germany

The spatial and temporal distributions of electrons and ions in an inductively coupled plasma (GEC reference cell) and a large diffusion chamber connected to a helicon plasma source were investigated [1,2,3]. The plasmas differing primarily in their confinement times were operated in noble gas-oxygen mixtures at pressures between 0.5 Pa and 10 Pa. The investiga-tions were focussed on the production and loss processes of negative oxygen ions. Laser-induced photodetachment of the negative oxygen ions using of a frequency-doubled Nd:Yag laser along with a Langmuir probe was applied to diagnose the O- ions. In the early after-glow of the pulsed discharges, strong variations of the electron temperature and the space potential were observed. In this phase, the probe measurements of the detached electrons are expected to be less reliable. Therefore, we applied as alternative method photodetachment in the combination with time-resolved (about 1 μs) measurements of the electron density by means of a 1mm microwave interferometer [4] whose beam was collinearly adjusted to the laser beam within the plasma. The results obtained from the two methods exhibit similar evolutions of the negative ion densities. For both plasmas, the temporal behaviour of the positive and negative charge carriers in the afterglow was compared with model calculations. In the early stage of the afterglow, mutual neutralisation with positive ions was identified as the dominant loss process of the negative ions while, later on, the effect of the oxygen atoms and the metastable O2(a1Δg) molecules becomes increasingly important.

This work was supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich SFB 591 (Project A7).

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[2] Wagner J A and Katsch H-M 2006 Plasma Sources Sci. Technol. 15(1) 156.
[3] Krämer M, Lorenz B and Clarenbach B 2002 Plasma Sources Sci. Technol. 11(3A) A120.
[4] Krämer M, Clarenbach B and Kaiser W 2006 Plasma Sources Sci. Technol. 15(3) 332.