Dissociative Electron Attachment Cross Sections for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$

New measurements of the absolute cross sections for dissociative electron attachment (DEA) in molecular hydrogen and deuterium are presented which resolve previous ambiguities and provide a test bed for theory. The experimental methodology is based upon a momentum imaging time-of-flight spectrometer that allowed us to eliminate any contributions due to electronically excited metastable neutrals and ultraviolet light while ensuring detection of all the ions. The isotope effect in the DEA process in the two molecules is found to be considerably larger than previously observed. More importantly, it is found to manifest in the polar dissociation process (also known as ion pair production) as well.

Figure 1

Momentum image of ${\mathrm{H}}^{-}$ from ${\mathrm{H}}_2$ at 11 eV electron energy. This image proves that all the ${\mathrm{H}}^{-}$ ions are transported to the detector. The narrow outer ring is due to the reflection at the glass boundary of the phosphor screen. The image is shifted to the right due to the effect of the magnetic field on the ions.

Figure 2

Absolute cross section for the formation of ${\mathrm{H}}^{-}$ from ${\mathrm{H}}_2$. Circles—from Ref. 7, triangles—from Ref. 8, stars—from Ref. 9, solid line—from Ref. 10, and squares—present results. The arrow indicates the polar dissociation threshold. Also shown are the data in the 3–5 eV range after multiplication by a factor of 10, as indicated in the figure, for clarity.

Figure 3

Comparison of the present cross section for the 4 eV resonance with the theoretical results. Solid line—theoretical data [15] convoluted with a Gaussian electron energy spread of 12 meV and multiplied by a factor 0.5, dotted line—theoretical data [15] convoluted with a Gaussian of FWHM of 0.753 eV and multiplied by a factor of 0.7, dashed line—theoretical data [15] convoluted with a thermionic electron energy spread of 0.840 eV and multiplied by a factor of 0.8, and squares—present results.

Figure 4

Absolute cross section for the formation of ${\mathrm{D}}^{-}$ from ${\mathrm{D}}_2$. Triangles—from Ref. 8 divided by a factor of 2, and circles—present results. Also given is the cross section for ${\mathrm{H}}^{-}$ from ${\mathrm{H}}_2$ (solid line) divided by a factor of 3 for comparison.