Structures in the electron transmission spectrum ofH2near theH2+threshold

The energy dependence near the ionization threshold of the electron-scattering cross section of ${\mathrm{H}}_2$ has been investigated. Two series of structures which proceed across the ${\mathrm{H}}_2^{+}$ threshold have been observed which can be correlated with structures observed previously in the ionization efficiency function. These results suggest that in the threshold region a significant portion of the ionization process by electron collision proceeds via the various decay modes of temporary ${\mathrm{H}}_2^{-}$ states. Furthermore, these structures can also be correlated with some structures in the photoionization cross section. Those ${\mathrm{H}}_2^{-}$ states that decay to autoionizing levels of ${\mathrm{H}}_2$ and then to ${\mathrm{H}}_2^{+}$ may account for the correlation with some of the photoionization data. To differentiate experimentally between one-and two-electron decay modes of ${\mathrm{H}}_2^{-}$ leading to ${\mathrm{H}}_2^{+}$ formation, coincidence experiments between ${\mathrm{H}}_2^{+}$ ions and electrons with specific energy losses will be necessary.