Chromatic dispersion in the negative glow-cathode fall space transition zone

Abstract

It has been shown that the chromatic dispersion in the boundary zone between the negative glow and cathode fall space of a cold cathode discharge is in many instances correctly predicted qualitatively by the rules that the maximum of intensity of a spectral feature is the nearer the negative glow the lower its excitation potential, and that the width of the intensity versus position curve increases with the excitation potential. The effective excitation potential is however not necessarily the least potential for which the spectrum can be excited by electron impact, but may be a higher one which allows for the excitation cross-section being comparable at higher voltages with its value near threshold. Spectra with excitation potentials approaching the cathode fall must be excluded from the rules because they can only appear relatively close to the negative glow. When more information is required than is furnished by the rules, and it is impracticable to obtain this accurately using a Boltzmann transport theorem, Monte Carlo or equivalent analysis of the discharge, it can sometimes be obtained semi-quantitatively from a simpler approximate model of the fall space. In some discharges allowance may have to be made for changes in number density of the atoms or molecules due to heating of the gas and electrodes, and for the effects of metastable atoms and molecules and resonance radiation.