Phase resolved optical emission spectroscopy (PROES) was used to determine the spatio-temporal behavior of excitation rates in rf sheaths of capacitively coupled plasmas at 13.56 MHz. The plasmas were ignited in pure oxygen at pressures from 20 to 100 Pa and rf powers from 10 to 100 W. The spatial and phase resolved excitation rates have shown four characteristic patterns, which differing in their spatial and temporal position.PIC-MCC simulations of the oxygen capacitively coupled radio-frequency discharge were used to get a detailed microscopic description of the dynamic processes in rf plasmas. The PIC-MCC simulations reproduced the excitation patterns observed in experiment quite well and allowed to identify the underlying physics.Three excitation patterns appearing in front of the powered electrode were found to be due to electron impact dissociative excitation of molecular Oxygen, whereas the fourth pattern very close to the powered electrode is attributed to collisions of the positive ions with the background gas.

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The presented work highlights the role of residual weakly-bound surface electrons acting as an effective seed electron reservoir that favors the pre-ionization of diffuse barrier discharges (BDs). A glow-like BD was operated in helium at a pressure of 500 mbar in between two plane electrodes each covered with float glass at a distance of 3 mm. The change in discharge development due to laser photodesorption of surface electrons was studied by electrical measurements and optical emission spectroscopy. Moreover, a 1D numerical fluid model of the diffuse discharge allowed the simulation of the laser photodesorption experiment, the estimation of the released surface electrons, and the understanding of their impact on the reaction kinetics in the volume. The breakdown voltage is clearly reduced when the laser beam at photon energy of 2.33 eV hits the cathodic dielectric that is charged with residual electrons during the discharge pre-phase. According to the adapted simulation, the laser releases only a small amount of surface electrons in the order of 10 pC. Nevertheless, this significantly supports the pre-ionization. Using a lower photon energy of 1.17 eV, the transition from the glow mode to the Townsend mode is induced due to a much higher electron yield up to 1 nC. In this case, both experiment and simulation indicate a retarded stepwise release of surface electrons initiated by the low laser photon energy.

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