The absolute cross section for production of atomic Balmer $\alpha$, $\beta$, and $\gamma$ radiation by dissociative electron collisions with molecular hydrogen and molecular deuterium has been measured from threshold to 540 eV. Also measured are some absolute cross sections for the excitation of a few molecular lines. All of the cross sections for dissociation show some structure just above threshold. Below this structure, dissociative excitation is interpreted as proceeding through both predissociation and direct dissociation via a bound state above the dissociation limit of the molecule. Above this structure, dissociation proceeds through production of the unbound doubly excited ${}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ states of ${\mathrm{H}}_2$. It is also argued that excitation of these doubly excited states is the major channel for production of excited atomic states in addition to being the major channel for production of protons by electron collisions with the molecule. Indeed its relative contribution increases with an increase of the principal quantum $n$. This contribution follows an inverse power law of 5.0 ± 0.5. Dissociation via the bound states depends on $n$ with an inverse power law of 6.5 ± 1.0. The cross sections for production of excited deuterium atoms were found to be less than those for production of excited hydrogen atoms in the corresponding states, and the ratio varied from ∼ 0.5 below 30 eV to ∼ 0.82 at high energies, again indicative of three competing processes for dissociative excitation of the molecular target gas.

• Received 8 December 1975

DOI:https://doi.org/10.1103/PhysRevA.13.1989