Dissociative Excitation of Molecular Hydrogen by Electron Impact

Abstract

Dissociative excitation of molecular hydrogen can proceed via the process ${\mathrm{H}}_{2}+e\ensuremath{\rightarrow}\mathrm{H}(2s)+\mathrm{H}+e$ and yield metastable $\mathrm{H}(2s)$ atoms that have kinetic energies near 0.3 eV ("slow") or near 4 eV ("fast"). The dissociation process has been studied using a pulsed electron gun with an energy resolution of \ifmmode\pm\else\textpm\fi{} 0.3 eV and using a metastable atom detector capable of viewing $\mathrm{H}(2s)$ atoms with an angular resolution of 1\ifmmode^\circ\else\textdegree\fi{} over a range 60\ifmmode^\circ\else\textdegree\fi{}-120\ifmmode^\circ\else\textdegree\fi{} with respect to the electronbeam direction. The measurement of the angular intensity distribution gives information about the final states that are involved in the dissociation process. (i) For slow $\mathrm{H}(2s)$ atoms, the electron energy threshold for production of the least energetic of the slow metastable atoms is 14.6 \ifmmode\pm\else\textpm\fi{}0.3 eV. The excitation function and the angular distribution of the slow $\mathrm{H}(2s)$ atoms suggest that the ${B}^{\ensuremath{'}}^{1}\ensuremath{\Sigma}_{u}^{+}$, $e^{3}\ensuremath{\Sigma}_{u}^{+}$, $D^{1}\ensuremath{\Pi}_{u}^{+}$, and $d^{3}\ensuremath{\Pi}_{u}^{+}$ excited states are involved in the formation of these metastable fragments. (ii) For fast $\mathrm{H}(2s)$ atoms, the electron energy threshold for production of the least energetic of the fast $\mathrm{H}(2s)$ atoms is near 29 eV. The angular distribution data would indicate that these atoms arise from a ${\ensuremath{\Pi}}_{u}$ state; the form of the excitation function indicates that the parent state has a multiplicity of 1. The change in energy distribution of the fast $\mathrm{H}(2s)$ atoms, measured as a function of electron-gun voltage, supports the view that the $^{1}\ensuremath{\Pi}_{u}$ state is a previously unreported doubly excited state that has an asymptotic energy of 24.9 eV.