Dissociative Recombination in Neon: Spectral Line-Shape Studies

Abstract

Optical interferometric, spectrographic, and microwave techniques have been used to investigate the nature of electron-ion recombination in neon afterglows. Electron-density decay measurements yield a two-body recombination coefficient $\ensuremath{\alpha}\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{3}$/sec, in agreement with earlier studies carried out at electron densities an order of magnitude smaller. The hypothesis that dissociative recombination, $\mathrm{Ne}_{2}^{}{}_{}{}^{+}+e\ensuremath{\rightleftarrows}{(\mathrm{Ne}_{2}^{}{}_{}{}^{*})}_{\mathrm{unstable}}\ensuremath{\rightleftarrows}{\mathrm{Ne}}^{*}+\mathrm{Ne}+\mathrm{kinetic}\mathrm{energy}$, is the process operative is tested by seeking to detect the kinetic energy of dissociation in the excited atoms produced by recombination. Fabry-Perot interferometer studies of the width of the $\ensuremath{\lambda}5852$ $2{p}_{1}\ensuremath{-}1{s}_{2}$ neon line indicate that in the afterglow the line is very much broader than the thermal (300\ifmmode^\circ\else\textdegree\fi{}K) atom width observed in the discharge. This excess width in the afterglow line is found to decrease with increasing neon gas pressure, owing to increased likelihood of excitation transfer from the fast atoms to thermal atoms before radiation. Higher-resolution studies of the spectral-line profiles indicate that the afterglow line consists of a "broad-shouldered," flat-topped component of the form expected for radiation from dissociatively produced excited atoms, surmounted by a narrow, thermal core resulting from radiation from slow-excited atoms produced by excitation transfer. The width of the fast-atom component of the profile yields a value for the dissociation kinetic energy, ${E}_{D}\ensuremath{\simeq}1.2$ eV, leading to a binding energy for the neon molecular ion, $D(\mathrm{Ne}_{2}^{}{}_{}{}^{+})\ensuremath{\simeq}1.4$ eV. The variation with neon pressure of slow-atom to fast-atom component in the line profile yields an excitation transfer cross section between excited $2{p}_{1}$ atoms and normal atoms of ${Q}_{x}\ensuremath{\simeq}(8\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at a relative velocity of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/sec. It is concluded that dissociative recombination is definitely the process responsible for the large electron loss in neon afterglows.