Emission spectroscopic study of an analytical glow discharge with plane and hollow cathodes: Titanium and iron in argon discharge

Abstract

An extensive analysis of Ti and Fe emission spectra from both plane and hollow cathode glow discharges in argon in Grimm-type geometry, measured by high resolution Fourier transform spectrometer has allowed a novel comparison of excitation mechanisms. Deviations from simple models of laboratory plasmas assuming local thermodynamic equilibrium are observed. Analyte ionization in both discharge modes is dominated by charge transfer (CT) between Ar+ ions and neutral analyte atoms, while this process is more pronounced in a plane cathode than hollow cathode discharge. Radiative decay of the CT-excited levels is found to cause overpopulation of long-lived ground term and metastable analyte ions, relative to the equilibrium number densities that would be expected from the Saha-Boltzmann distribution. This overpopulation is found to spread into high-energy levels of neutral atoms by collisional 3-body recombination with electrons. Lower atomic levels of the analytes are excited by collisions with electrons, the number density of which is not high enough to support conditions close to local thermodynamic equilibrium. The analysis of the spectra is based on experimental Boltzmann plots, transition rate diagrams and intensity ratios of emission lines measured in both discharge configurations. Uncertainty propagation in the data processing procedures is analyzed and described.