Recombination ofHe+andHe++in the Afterglow of

Abstract

Studies of the time dependence of atomic and molecular light intensities, electron density, atomic metastable densities, and electron temperature have allowed the determination of the He + -electron and He ++electron recombination coefficients as functions of electron density and temperature. The results are in reasonable agreement with the theory of collisional-radiative recombination. The mechanisms controlling metastable densities and heating of the electron gas are discussed. In particular, the disappearance of the 2 3 5 metastable atom seems best explained in terms of collisional de-excitation by electrons.TN earlier experiments, 1 using pulsed microwave A cavities or waveguides, the observed time dependency of the electron density n was used to ascertain the degree to which the loss processes were dominated by electron-ion volume recombination and, simultaneously, to determine the value of the recombination coefficient itself from a plot of \/n{t) versus the time t. It has since been shown 2 that, in the presence of diffusion, linearity of a l/»(/)-versus-$ plot is neither a guarantee of recombination control nor a good measure of the recombination coefficient, unless the region of linearity is at least a factor of approximately 10 in the electron density. Even in cases where this criterion is met, there are other uncertainties. In general, the radial distribution of electron density was surmised rather than measured and was probably time-dependent. This introduces uncertainty into the interpretation of the frequency shift of the cavity in terms of the spacial average of the electron density. There are, moreover, serious limitations to the electron density region over which the cavity technique can be applied. 3 Purity of the gas was an additional problem and, for a while, the measured recombination coefficients became progressively lower as the purity was improved. In all of these early experiments, a dependence of the assumed two-body recombination coefficients on electron density was neither allowed for nor found experimentally. For helium, the theoretical picture was further obscured by the assumption that electron loss proceeded primarily by dissociative recombination of the He 2 + ion, 4 a belief that has persisted until very recently.

Recombination of doubly ionized atoms in the afterglow of a helium plasma produced by laser

“…Experimental data are scarce and the few experimental values published [2,3] do not allow an assertion of the theory.…”