A comparative experimental and theoretical investigation of the electron-impact double ionization of He in the keV regime

We determine, both experimentally and theoretically, the fully resolved fiv efold differential cross section (5DCS) of double ionization of helium by 5 .6 keV electron impact. Symmetric energy sharing between the two ejected el ectrons is investigated at the excess energy of 8 and 20 eV with 0.22 and 0 .24 au momentum transfer, respectively. Absolute 5DCS are determined by nor malizing the experimental data to the well established single-ionization cr oss sections. The calculation is performed by using the convergent close-co upling method for the interaction between the two slow ejected electrons, t ogether with the first Born approximation for the interaction of the fast i ncident electron with the atom. Experimental and theoretical 5DCS tend to a gree in shape but disagree in magnitude by factors of three and 14 for the 20 and 8 eV excess energies, respectively. The small momentum transfer invi tes absolute comparison of the present electron-impact double-ionization re sults with the corresponding double-photoionization experiment and theory. Theoretically, the momentum transfer is sufficiently close to zero to show the scaling between the two scattering processes. This smallness of the mom entum transfer also makes the calculated 5DCS nearly invariant with respect to simultaneous inversion of the moments of the two ejected electrons.