Frontiers in Low Temperature Plasma Diagnostics VI in Les Houches (France)

Negative Ions in Rare Gas/Oxygen Discharges

J.A. Wagner, H.M. Katsch, H.F. Döbele
Institut für Laser- und Plasmaphysik, Universität Duisburg-Essen, D-45117 Essen, Germany

In a pulsed inductively excited GEC reactor negative oxygen ions in argon-, krypton- and neon/oxygen mixtures exhibit an unexpected high density in the afterglow due to an unknown generation channel of negative oxygen ions.

The temporal behavior of negative oxygen ions in argon-, krypton- and neon/oxygen mixtures is investigated in a pulsed inductively excited modified GEC reactor. The negative ion densities are measured by laser photodetachment of negative ions in combination with a Langmuir probe as well as by absorption of photons due to photodetachment with a cavity ringdown technique. The main loss mechanism for negative oxygen ions at plasma densities above 1011 cm-3 is mutual neutralization with positive ions. Measurements of the atomic oxygen densities with two-photon laser induced-fluorescence spectroscopy show, that collisions of the atoms with negative ions are a minor loss process for the negative ions. Mass spectrometry measurements of O2--negative ions indicate also, that the loss of negative ions due to collisions with metastable oxygen molecules, O2(a1Δg), are less important.

In the early afterglow an unexpected high density of negative oxygen ions is found at high plasma densities in mixtures with a high content of argon, krypton or neon. These findings are in contrast to the predictions of our global model. It is, therefore, necessary to invoke an additional production channel for the negative ions in the afterglow, in order to explain the observed high negative ion density. It is likely that additional negative ions are generated by dissociative attachment of highly excited oxygen molecules [1], [2]. In fig. 1 the ratio of the negative ion density to the plasma density 50µs after switching off the rf power is shown for various values of the discharge power. A significant discrepancy is found between the measurements and the results of the global model without additional generation reactions of negative ions. If the reaction with metastable states, O2M (A3Σu+, C3Δu, c1Σu-) is included in the global model, see ref. [3], the deviation with respect to the measurement is much less. It is also possible that Rydberg states of the oxygen molecules are excited via collisions with metastable argon atoms [4]. These excited Rydberg molecules may also lead to a generation of negative oxygen ions as mentioned above. Similar findings of the ratio of the negative ion density to the plasma density, as shown in fig 1, are obtained in neon- and krypton/oxygen mixtures. It is unlikely, therefore, that Ryberg states are responsible for the enhanced generation of negative ions by dissociative attachment in the afterglow.

Figure 1
Fig. 1. ratio of the negative ion density to the plasma density in the afterglow 50 μs after switching off, 3.3 Pa, argon/oxygen admixture (70/30), modified GEC-Reference Cell.

[1] Hayashi D and Kadota K 1998 J. Appl. Phys. 83(2) 697-702.
[2] Ding W X, et al. 1999 Plasma Sources Sci. Technol. 8(3) 384-91.
[3] Hayashi D and Kadota 1999 Jpn. J. Appl. Phys. 37(1) 225-230.
[4] Rickey D and Krenos J 1997 J. Chem. Phys. 106(8) 3135-45.

This work was supported by the 'Deutsche Forschungsgemeinschaft' in the frame of the 'Sonderforschungsbereich 591'.