Spectroscopic diagnostics of glow discharge plasmas with non-Maxwellian electron energy distributions

Electron energy distribution functions in glow discharges (I=5-50 mA, p=200-1000 Pa) were derived from the intensities of Ar and He spectral lines and N2 molecular bands with different excitation thresholds. The relevant rate coefficients represent weighted integrals in the range from 12 to 24 eV. In molecular gases, structures in the low-energy region was inferred from theoretical calculations. An analytical formula with one experimentally adjusted parameter was used to describe the energy distribution. The total electron density was calculated from the electrical conductivity. The mean electron energies from three spectroscopic criteria, as well as those from an ionization-diffusion model of the discharge, are in close agreement for values of the fitting parameter characteristic for the particular plasma conditions. The high-energy part of the distribution functions is not far from a Druyvesteyn shape both in pure nitrogen and in noble gases. The electron densities in nitrogen and mixture plasmas were also measured from N2+ molecular band intensities. Because of the rapid decrease in electron population at higher energies, the ionization of neutral N2 into the upper N2+ level can be ignored and these bands reflect the N2+ ion ground state population. The excitation rate coefficients were calibrated in pure nitrogen and then applied to mixture plasmas, where the N2+ ion fraction was calculated including charge exchange processes. Agreement with ne results from electrical conductivity is very good over the entire parameter range investigated.