An asymmetric capacitively coupled radio frequency discharge was investigated. The planar RF electrode was made of stainless steel and had a diameter of 120 mm. It was powered at 13.56 MHz by an RF generator (Advanced Energy, Cesar 133) and an automatic matching network (Advanced Energy, Navio). The RF electrode was positioned at a distance of 55 mm beneath the top surface of the grounded reactor vessel, which consisted of aluminum and was cylindrically shaped, with a diameter of 240 mm and a height of 105 mm. The applied RF power was varied between 20 and 100 W.
\n", "procedure": "The reactor outlet was connected to a turbopump system (Welch-Ilmvac, CDK 263) with a throttle valve to constantly adjust the pressure in the chamber.
\n" }, "medium": { "name": "O2", "properties": "Pure O2 was used as a working gas, at a pressure of 0.7 or 1.3 mbar, and a flow rate of 45 or 50 sccm, respectively. Oxygen gas was let into the reactor by a mass flow controller (MFC) (MKS 1179A with control unit MKS PR4000) at a flow rate of 45 or 50 sccm, respectively.
\n" }, "target": [], "diagnostics": { "name": "THz absorption spectroscopy, TALIF", "procedure": "Absolute densities of ground-state atomic oxygen were measured with THz absorption spectroscopy, using the fine structure transition of atomic oxygen at 4.74477749 THz (i.e. 158.268741 cm-1). A tunable THz QCL in continuous-wave mode was used as the radiation source. It was operated in a Stirling cryocooler (Ricor K535) at a temperature of 44.30 K. Temperature fluctuations were minimized to be below 5 mK by an additional temperature controller (Stanford Research Systems, CTC100). Tuning of the laser output was achieved by linearly ramping the input current. The current was supplied by a laser driver (Wavelength Electronics, QCL1000 OEM), which was controlled by a function generator (Tektronix, AFG3022C) to provide sawtooth waveforms with a frequency of 11 Hz. After passing through the plasma reactor, the laser radiation was detected by a helium-cooled bolometer (Infrared Laboratories, 4.2K Bolometer). The detected signals were recorded with a digital sampling oscilloscope (R&S, RTO 1014).
\nAbsolute densities of ground-state atomic oxygen were derived from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements after a calibration using xenon. Photons with a wavelength of approximately 226 nm were provided by a ps laser system that consisted of an Nd:YLF pump laser (EKSPLA, PL3140), a harmonic generator (EKSPLA, APL2100), and an optical parametric generator (EKSLPA, PG411). The fluorescent radiation was detected by a streak camera (HAMAMATSU, C10910-05), which was synchronized with the laser pulses (repetition rate: 5 Hz) by a delay generator (Stanford Research, DG645) and connected to a digital CMOS camera for readout (HAMAMATSU, ORCA-Flash4.0 V3). The streak camera images were recorded using a time range of 200 ns, full gain, and a total acquisition time of 200 s, which corresponds to an accumulation of 1000 laser shots.
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