Absorption spectroscopy measurements of resonant and metastable atom densities in atmospheric-pressure discharges using a low-pressure lamp as a spectral-line source and comparison with a collisional-radiative model

The well-known absorption method for measuring metastable and resonant atom densities in reduced-pressure discharges using a low-pressure lamp as a spectral-line source is extended to plasmas sustained at atmospheric pressure where the line profiles are of the Voigt type rather than Gaussian. This technique is further applied to determine the axial distribution of metastable and resonant atom densities in a tubular argon surface-wave (microwave) discharge. At atmospheric pressure, the discharge is radially contracted and determination of the absorption length through lateral probing of the tube becomes a critical issue. Good agreement of the metastable and resonant atom densities calculated from a collisional-radiative model with the measured values supports the validity of the method developed. The model underlines the influence of the molecular ion kinetics on the Ar I 4s level population and reveals an efficient 3-step ionization process involving the 4s and 4p levels for atomic ion formation.