Photodissociation ofH2+andD2+: Experiment

Abstract

Measurements are reported of the cross sections for photodissociation of ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ at 21 wavelengths ranging from 2472 to 13 613 Å. The measurements are compared with theory using normalized Franck-Condon factors for the vibrational populations of the ions. Deviations are found, which are interpreted as a failure of this latter approximation. A least-squares analysis of the data yields vibrational-state populations different from simple Franck-Condon factors and alleged to be those characteristic of high-energy electron-impact ionization of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. Interpretation of the data using a mechanism requiring that a fraction of the ions formed by electron impact be generated via the autoionization channel can qualitatively lead to the observations of this experiment. However, such an explanation would require about $\frac{2}{3}$ of all ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ to be formed via autoionization—a seemingly unrealistic requirement. Interpretation of the data in terms of a variation with internuclear separation of the electronic matrix element involved in the electron-impact ionization process shows that a matrix element varying as $Q_e\left(\mathcal{r}\right)=1+0.56\mathrm{}{({\mathcal{r}}_{\mathrm{a}.\mathrm{u}.\mathrm{}}-0.99)}^2$ can lead to the experimentally deduced populations of both ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$. This latter interpretation is favored, though one cannot rule out the former or some combination of the two. The data and interpretation are consistent with other related experiments.