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

Axial distributions of negative oxygen ions in an asymmetric, capacitively coupled oxygen radiofrequency discharge (of the gaseous electronics confere nce reference cell type) are investigated. The O- ion densities are measure d by detecting the electrons released after photo-detachment with a Langmui r probe. At small power and a pressure of approximately 10 Pa the axial distribution s of both the O- ions and the electrons are Rat. At higher power (character ized by voltage amplitudes in excess of 200 V) both quantities exhibit maxi ma in the central region between the electrodes. These maxima are displaced with increasing pressure (>50 Pa) towards the driven electrode. The absolu te concentration of O- is insensitive to admixtures of argon up to 50%. Thi s can be understood, if one assumes that the dominant loss channel for the negative ions involves metastable O-2(a(1)Delta(g)) molecules. This assumpt ion is confirmed by measurements taken with the discharge operating in a pu lsed mode.