Spectroscopic investigations of a cathode fall region of the Grimm-type glow discharge

Abstract

This paper presents the results of the spectroscopic study of the cathode fall region of a plane cathode Grimm-type glow discharge in pure hydrogen and in argon with small admixtures of hydrogen. In contrast with the discharge in an argon-hydrogen mixture, the volt-ampere characteristics of the pure hydrogen discharge show a maximum typical for an abnormal glow-to-arc transition. This maximum in the V-A curve is explained here as being due to the increasing role of self-sputtering of the cathode material in sustaining the discharge at higher currents. For the measurements of the electric fields in the cathode fall region, Stark spectroscopy of the hydrogen Balmer lines is employed. Consistent results were obtained from H β and H γ recordings in a pure hydrogen discharge. Some of the difficulties in applying Stark spectroscopy for the diagnostics of a spatially inhomogeneous electric field inherent to Grimm glow discharges are discussed in detail. The experimental results are used to test the theoretical predictions of the electric field distribution in the cathode fall region. Reasonable agreement between theories and experiment is reported. Doppler spectroscopy of the same Balmer lines is used to determine the energies of the excited hydrogen atoms in the discharge. In the cathode fall region of a pure hydrogen discharge, two groups of excited atoms are detected: “slow”, in the range 3.4–8.2 eV, and “fast”, in the range 80–190 eV. The relative concentrations of “slow” and “fast” excited hydrogen atoms in the cathode fall region are determined. In addition, the relative concentration of hydrogen atoms with temperatures around 0.1 eV, excited in the plasma of the negative glow region, is also determined. The origin of these “slow” and “fast” hydrogen atoms is related to the presence of H + and H 3 + ions, respectively. In the cathode fall region of an argon-hydrogen mixture discharge, only excited hydrogen neutrals with energies of 32–43 eV are detected. Their origin is related to the dominant role of H 3 + ions in this discharge. For both gases, in the negative glow region, an increase in the temperature of excited hydrogen atoms is detected, and is explained by the additional excitation of energetic neutrals in collisions with electrons. The axial intensity distributions of the hydrogen Balmer lines, in comparison with other atomic and ionic lines, show different shapes with maxima in the vicinity of the cathode surface. These shapes are explained by the excitation of reflected high-energy neutral atoms in collisions with the matrix gas.