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

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.

Angular distributions of the two ejected electrons following electron impact double ionization of helium have been measured using a multicoincidence multiangle ͑e, 3e͒ spectrometer at an incident energy ϳ5.5 keV and equal outgoing energies E b E c 10 eV. The measured distributions are similar to photodouble ionization ͑hn, 2e͒ ones, but with evidence of additional nondipolar contributions which tend to fill up the characteristic node at the mutual angle u bc 180 ± . They are also in satisfactory agreement with recent ͑e, 3e͒ calculations. [S0031-9007(98)07708-4] PACS numbers: 34.80.DpThe study of the double ionization (DI) process under photon or charged particle impact has undergone a recent revival due to the spectacular advances in coincidence detection techniques. In these measurements the energy transfer to the target, the energies and momenta of the final electrons are determined. Such a detailed differential investigation is fundamental to unravel the role of the dynamical electron-electron correlation during the collision. The understanding of correlation in multielectron transitions, where the Coulomb force plays a paramount role, is one of the basic unsolved problems of modern atomic physics. Measurement of the coincidence angular distributions of the DI products should provide an essential insight into this problem. The ideal target for such a study is helium, the simplest two-electron system that yields a pure three-body or four-body problem in the final state under photon or electron impact, respectively. In the last few years ͑hn, 2e͒ photodouble ionization (PDI) experiments [1] have provided a wealth of new results in several kinematics and energy sharing conditions from near zero up to 50 eV above the threshold of the process [2]. In contrast, electron impact ͑e, 3e͒ experiments [3] have proved much more difficult to perform. Detailed ͑e, 3e͒ experiments have been recently reported for the outer-shell DI of argon [4] and neon [5], but were unsuccessful for helium. Moreover, because of the low triple coincidence counting rate, the modest energy resolution did not allow us to distinguish among different final states of the doubly charged ion [4,5]. In that sense, these were not kinematically completely determined experiments.In this paper we report the ͑e, 3e͒ cross sections for DI of He. For the first time, a kinematically completely determined experiment (apart from electron spins variables) has been performed, since the He 21 ion is a bare nucleus with no relevant internal structure. These experiments were performed in the so-called dipole scattering regime, characterized by high incident energy and small momentum transfer to the target (i.e., small scattering angle), where the electron impact cross section is expected to converge on the photoionization cross section [6].A complete description of the electron impact ͑e, 3e͒ process in a coplanar geometry is given by the variables represented in the vector diagram in Fig. 1. E j and k j ͑j 0, a, b, or c͒ are the electron energies and momenta. The ...

The relative, coplanar angular distributions of electrons, produced in an electron-impact double ionization of helium (e, 3e reaction), have been measured at 1.1 keV impact energy. The momentum transfer was 0.45 au and the two 'ejected' electrons were detected with the same energy of 10 eV. The general features of the angular distributions are discussed. The data are analysed in different angular modes which allows a detailed comparison with state-ofthe-art calculations. For high incident energy and small momentum transfer, as in the present case, the (e, 3e) cross section can be related to the single-photon double ionization (PDI). We exploit this fact and compare the present findings with the PDI and identify the contribution of non-dipole effects.