Negative ions and the role of metastable molecules in a capacitively coupled radiofrequency excited discharge in oxygen

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Axial distributions of negative oxygen ions in an asymmetric, capacitively coupled oxygen radiofrequency discharge (of the gaseous electronics conference reference cell type) are investigated. The O - ion densities are measured by detecting the electrons released after photo-detachment with a Langmuir probe. At small power and a pressure of approximately 10 Pa the axial distributions of both the O - ions and the electrons are flat. At higher power (characterized by voltage amplitudes in excess of 200 V) both quantities exhibit maxima in the central region between the electrodes. These maxima are displaced with increasing pressure (>50 Pa) towards the driven electrode. The absolute concentration of O - is insensitive to admixtures of argon up to 50%. This can be understood, if one assumes that the dominant loss channel for the negative ions involves metastable O 2 (a 1 Δ g ) molecules. This assumption is confirmed by measurements taken with the discharge operating in a pulsed mode.