Studies of the Mechanism of Electron-Ion Recombination. II

Abstract

Microwave, optical spectrometric, and interferometric techniques have been used to investigate the nature of the electron-ion recombination process in low-pressure, pure helium afterglows at 300 and 77°K. To determine whether dissociative capture of electrons by helium molecular ions, i.e., ${{\mathrm{He}}_2}^{+}+e\to {\mathrm{He}}^{*}+\mathrm{He}+\left(\mathrm{KE}\right)$, is the recombination reaction taking place, we have attempted to detect the kinetic energy of dissociation of the excited atoms produced in the recombination process by a study of the widths of the emitted afterglow lines. A model of the low-pressure helium afterglow which reasonably accounts for our electron density, line intensity, and linewidth observations involves principally the creation of ${\mathrm{He}}^{+}$ ions and electrons by the collisions of pairs of helium metastable atoms, the three-body conversion of ${\mathrm{He}}^{+}$ ions to ${\mathrm{He}}_2^{+}$ ions, the ambipolar diffusion of ${\mathrm{He}}^{+}$, ${\mathrm{He}}_2^{+}$, and electrons, and the diffusion of helium metastable atoms to the walls. The $\lambda 5876$ radiation ($3{}_{}{}^3{}_{}{}^{}D-2\mathrm{}{}_{}{}^3{}_{}{}^{}P$ transition) from the later part of the afterglow, when recombination of ${\mathrm{He}}_2^{+}$ and electrons is expected to be the source of the excited atoms, exhibits a substantial broadening over the approximately thermal width observed during the discharge and in the early afterglow, when the radiation is expected to originate from electron impact excitation of helium atoms. It is concluded that the dissociative recombination process is the likely source of the observed afterglow line broadening.