We use the one-dimensional object-oriented particle-in-cell Monte Carlocollision code oopd1 to explore the charged particle densities, theelectronegativity, the electron energy probability function (EEPF), and theelectron heating mechanism in a single frequency capacitively coupled oxygendischarge when the applied voltage amplitude is varied. We explore dischargesoperated at 10 mTorr, where electron heating within the plasma bulk (theelectronegative core) dominates, and at 50 mTorr where sheath heatingdominates. At 10 mTorr the discharge is operated in combined drift-ambipolar(DA) and $\alpha$-mode and at 50 mTorr it is operated in pure $\alpha$-mode. At10 mTorr the effective electron temperature is high and increases withincreased driving voltage amplitude, while at 50 mTorr the effective electrontemperature is much lower, in particular within the electronegative core, whereit is roughly 0.2 - 0.3 eV, and varies only a little with the voltageamplitude.
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