Using a multiparameter multicoincidence spectrometer, we have measured the coplanar (e,3e) angular distributions following the double ionization of helium at an incident energy of Ϸ5.6 keV and under a small projectile's scattering angle of 0.45°. The two ejected electrons have been detected with equal energies E b ϭE c ϭ10 eV. The absolute value of the cross section is determined with an accuracy of 25%. The origin of dips and peaks in the spectra is exposed by analyzing the corresponding theoretical calculations. These calculations have been done using a four-body final-state wave function for the three electrons moving in the field of He 2ϩ. The dipolar limit is investigated and the manifestation of the deviation from this limit are pointed out. General features and possible trends for other targets are proposed. ͓S1050-2947͑99͒02805-X͔

The latest, most imponant developments in (e, 2e) studies are reviewed from an metric energy sharing collisions involving double scattering mechanisms, core states ioniratian, (e, 2e) measurements for autoionizing states and for the simultaneous excitation and ionization r.f the target. Double ionizing processes with coincidence detection of two or three elec.rans in the final slate are also reviewed, including (e, &Auger) experiments and the new born ( e , ( 3 -l ) e ) and ( e , 3 e ) domain.

We determine, both experimentally and theoretically, the fully resolved fivefold differential cross section (5DCS) of double ionization of helium by 5.6 keV electron impact. Symmetric energy sharing between the two ejected electrons 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 normalizing the experimental data to the well established single-ionization cross sections. The calculation is performed by using the convergent close-coupling method for the interaction between the two slow ejected electrons, together with the first Born approximation for the interaction of the fast incident electron with the atom. Experimental and theoretical 5DCS tend to agree 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 invites absolute comparison of the present electron-impact double-ionization results 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 momentum transfer also makes the calculated 5DCS nearly invariant with respect to simultaneous inversion of the momenta of the two ejected electrons.