Dissociative Excitation of Molecular Hydrogen by Electron Impact

Abstract

Dissociative excitation of molecular hydrogen can proceed via the process ${\mathrm{H}}_2+e\to \mathrm{H}\mathrm{}\left(2s\right)+\mathrm{H}+e$ and yield metastable $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ 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 ± 0.3 eV and using a metastable atom detector capable of viewing $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms with an angular resolution of 1° over a range 60°-120° 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 $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms, the electron energy threshold for production of the least energetic of the slow metastable atoms is 14.6 ±0.3 eV. The excitation function and the angular distribution of the slow $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms suggest that the $B^{\prime }{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, $e{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, $D{}_{}{}^1{}_{}{}^{}\Pi _u^{+}{}_{}{}^{}$, and $d{}_{}{}^3{}_{}{}^{}\Pi _u^{+}{}_{}{}^{}$ excited states are involved in the formation of these metastable fragments. (ii) For fast $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms, the electron energy threshold for production of the least energetic of the fast $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms is near 29 eV. The angular distribution data would indicate that these atoms arise from a ${\Pi }_u$ 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 $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms, measured as a function of electron-gun voltage, supports the view that the ${}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state is a previously unreported doubly excited state that has an asymptotic energy of 24.9 eV.