Laser absorption spectroscopy measurements of neutral N$$_2$$O in a radio frequency plasma

Infrared laser absorption spectroscopy is tested as a diagnostic for measuring neutral nitrous oxide (N $$_2$$ O) concentrations in a radio frequency (RF) plasma source. Using a quantum cascade laser tuned to an isolated rovibrational mode of N $$_2$$ O, the density of neutral molecules is measured using a Gaussian fit of the absorbance spectrum. The diagnostic is tested with both neutral flows and in the presence of an RF plasma. The absorption spectrometer was found to accurately measure neutral gas density compared to pressure gauge readings. Furthermore, the Gaussian fit method was robust to RF noise, maintaining a signal-to-noise ratio above 7 even while in the presence of plasma. The diagnostic was able to measure concentrations of N $$_2$$ O in two distinct operating modes of the RF plasma, demonstrating that the diagnostic is sensitive to neutral densities above $$5.7 \times 10^{10}$$ cm $$^{-3}$$ .