Electron energies and excitation in the helium positive column

Abstract

The results of previous experimental investigations of the intensities of the spectral lines emitted from helium discharges are discussed, and a distribution function for the energies of the electrons is suggested in order to account for these results. The variation of the intensities of the lines with the pressure of the gas is then calculated, and curves are drawn representing the relative intensities of the various lines at different pressures. These curves are compared with those representing the observed variations of the intensities. The general agreement between the two sets of curves supports the conclusion that the distribution of the energies of the electrons in helium is of the form suggested. In this case the distribution may be expressed approximately by the formula dN/N = (A e-3y/2 + e-0.4y2)y1/2 dy, where N is the total number of electrons of mean energy E1, dN is the number with energies lying between E and E + dE, A is a constant of the order of 5 × 10-3, and y is E/E1. It is then concluded that the distribution of the energies of the vast majority of the electrons in the helium positive column is determined by the process of diffusion and the mean loss of energy in elastic collisions with atoms of the gas; but that a small fraction 2 × 10-3 of the total number form a group with their energies distributed according to Maxwell's formula corresponding to the same mean energy. It is pointed out that this small group of electrons is of predominating influence in the helium discharge at high pressures as far as excitation and ionization are concerned.