Dissociative excitation ofH2, HD, andD2by electron impact

Time-of-flight techniques have been used to investigate the electron-impact dissociation of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$ in order to determine the effect of isotopic mass variation in the target molecule on the dissociative excitation process. At incident electron energies near 100 eV, the time-of-flight spectrum produced from each molecule was found to consist of atoms in the metastable $2s$ state and in high-lying, long-lived Rydberg levels. The individual time-of-flight distributions, kinetic-energy spectra, and relative differential cross sections for these two species resulting from each molecule have been measured. In several instances these results were found to vary significantly between the three isotopic forms. In particular, the kinetic-energy spectrum of the Rydberg atoms produced from dissociative excitation of ${\mathrm{H}}_2$ was notably dissimilar in shape from the corresponding distributions produced from HD and ${\mathrm{D}}_2$. Also the $2s$ and Rydberg production cross sections differed between the three molecules, being less in both cases for HD and ${\mathrm{D}}_2$ than for ${\mathrm{H}}_2$. In the dissociation of the heteronuclear HD molecule, it was found that the ratio of fast $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ atoms to $\mathrm{D}\mathrm{}\left(2s\right)\mathrm{}$ atoms was about 1 to 1, while the same ratio comparing the Rydberg atoms was nearly 2 to 1. These differences indicate the influence of the mass variation on the position of the Franck-Condon region in the production of $2s$ atoms and on the competition between autoionization and dissociation in the formation of Rydberg fragments.