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).

Absolute 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.