Influence of N-O chemistry on the radiative emission from a direct current nitrogen plasma jet

In this work we have studied experimentally the influence of N-O chemistry on radiative emissions from a small DC plasma torch at 1 atm. The plasma torch has been designed for the atomizer of an on-line atomic absorption spectrometer, which is used for the detection of vaporized metals in various process gas flows. The plasma torch was burning in pure nitrogen and an oxygen bearing sample gas was introduced into the plasma jet. The temperature of the plasma jet was determined spectroscopically to be around 2000 K. At this temperature the equilibrium UV radiation of excited atoms or molecules is negligible, but molecules and atoms are effectively excited by collisions with metastable nitrogen molecules formed in the plasma. The emission spectra of the plasma jet were measured in the wavelength range 200-400 nm in various gas mixtures. In pure nitrogen the second positive system of N2 was observed, the intensity of which exceeded the estimated radiation at thermal equilibrium even by 1013 times. With a trace amount of oxygen-containing species, such as O2, CO2 or H2O, very intense λ-band radiation of NO was observed. The intensities of the lines were calculated to be around 109 times greater than corresponding equilibrium intensities. High intensities were also observed in atomic resonance lines of several metals. The intensity of the Cu line at 324.75 nm was 5x103 higher than the estimated equilibrium emission at given conditions. Correspondingly, the emission intensity of Pb line at 283.3 nm was around 105 and Cd line at 228.8 nm around 2x107 times more intense than at thermal equilibrium. Our study indicated that the rate of the collisional excitation of metal atoms by metastable nitrogen molecules was roughly the same for all the studied metals. When larger amounts of O2 or H2O were introduced into the plasma jet, the emission intensity of molecular lines and atomic lines of metals decreased drastically. We showed that radiative emissions do not interfere with absorption measurements provided that the percentage of O2 in the plasma jet is above 5 vol% or of water vapour above 2 vol%. (author)