A single-mode continuous-wave QCL (Alpes) with a coverage from 2276 to 2290 cm−1 was used to scan the CO2 absorption transitions. The collimated output of the laser is guided in and out of the discharge chamber through two inserted wedged CaF2 windows to reduce the CO2 absorption outside of the discharge along the beam pass. A combination of a polarizer and a quarter-waveplate is used to adjust the laser intensity to avoid saturation. Two irises help to prevent potential etalon effects in the optical pass by shielding the back reflection from the beam splitter and the CaF2 windows. After traversing the discharge chamber, the laser beam is focused onto a DC coupled photovoltaic detector (Vigo, PVI-3TE-5, bandwidth 1 MHz) by an off-axis parabolic mirror. A narrow bandpass filter (Thorlabs, FB4250-500) and a third iris are mounted in front of the detector to reduce the infrared thermal emission from the discharge. The laser frequency is characterized by a silicon Fabry–Perot interferometer (LightMachinery) with a free spectral range of 0.0176 cm−1 mounted on a flipping mount. (Note: the original entry has been lost and the values here are restored by memory)