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Using particle-in-cell Monte Carlo collision simulations we have demonstrated an electron heating mode transition from drift-ambipolar (DA) mode to $\alpha$-mode in the capacitively coupled oxygen discharge as the operating pressure [1,2], electrode separation, and driving frequency [3] are increased. Here we explore further the transition as pressure and electrode separation are varied. When operating at low pressure (10 mTorr) the electron heating is a combination of DA- and $\alpha-$mode heating while at higher pressures ($>$ 30 mTorr) electron heating in the sheath regions dominates. At fixed discharge pressure varying the electrode spacing the electron heating is a combination of DA- and $\alpha-$mode heating for small electrode spacing and it transitions to pure $\alpha-$mode heating as the electrode spacing is increased. We relate the transition to increased electronegativity and generation of drift and ambipolar electric field within the electronegative core when the discharge pressure is low or electrode spacing is small. It is important to note that the addition of the single metastable molecules and secondary electron emission to the oxygen discharge model has a significant influence on the discharge properties and in particular to lower the effective electron temperature [3,4].
[1] J. T. Gudmundsson and B. Ventéjou, The pressure dependence of the discharge properties in a capacitively coupled oxygen discharge, Journal of Applied Physics, 118(15) (2015) 153302
[2] H. Hannesdottir and J. T. Gudmundsson, Plasma Sources Science and Technology 25(5) (2016) 055002
[3] J. T. Gudmundsson, D. I. Snorrason and H. Hannesdottir, The frequency dependence of the discharge properties in a capacitively coupled oxygen discharge, Plasma Sources Science and Technology, 27(2) (2018) 025009
[4] A. Proto and J. T. Gudmundsson, The influence of secondary electron emission and electron reflection on a capacitively coupled oxygen discharge, Atoms, 6(4) (2018) 65
