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

Abstract

Axial distributions of negative oxygen ions in an asymmetric,capacitively coupled oxygen radiofrequency discharge (of the gaseouselectronics conference reference cell type) are investigated. The O- ion densities are measured by detecting the electrons released afterphoto-detachment with a Langmuir probe.At small power and a pressure of approximately 10 Pa the axialdistributions of both the O- ions and the electrons are flat. Athigher power (characterized by voltage amplitudes in excess of 200 V) bothquantities 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- isinsensitive to admixtures of argon up to 50%. This can be understood, ifone assumes that the dominant loss channel for the negative ions involvesmetastable O2(a 1Δg) molecules. This assumption isconfirmed by measurements taken with the discharge operating in a pulsedmode.