Ultra-High-Resolution Optical Absorption Spectroscopy of DC Plasmas at Low Pressure Using a Supercontinuum Laser Combined With a Laser Line Tunable Filter and a Hyperfine Spectrometer

Optical absorption spectroscopy of non-equilibrium plasmas using a supercontinuum laser combined with a laser line tunable filter and a HyperFine spectrometer is examined. Owing to the <2 pm spectral resolution of the HyperFine system, details on the absorption line intensity, position and width can be obtained with very high precision using a broadband light emission source. As an example, absorption spectroscopy measurements were recorded in a reduced-pressure, nominally pure argon DC plasma column and allowed for the determination of the neutral gas temperature and the Ar 1s₂ and Ar 1s₄ (Paschen notation) number densities. At a pressure of 1 Torr, a discharge current of 20 mA, and an absorption length of 18 cm, the analysis of the Ar 2p₃-1s₂ and Ar 2p₈-1s₄ absorption lines at respectively 840.8 nm and 842.5 nm resulted in number densities of n_1s2 = 5.5 x 10¹⁵ m^-3 and n_1s4 = 1.1 x 10¹⁶ m^-3 and a neutral gas temperature of T_g = 340 K. These values are typical of DC discharges operated under similar experimental conditions. While the fractional absorption of both lines decreased with decreasing optical absorption path, the number densities remained the same, as expected. Finally, the number densities increased with increasing discharge current, a result also coherent with the literature.