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 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.
Using a new multiparameter multicoincidence spectrometer, we measure coplanar (e, 3e) angular distributions from the double ionization of argon at ∼ 5.5 keV impact energy, under a small scattering angle, 0.45 • , and equal energy sharing among the two 'ejected' electrons, E b = E c = 10 eV. The absolute scale for the cross sections is determined within a factor of 2. General features and possible trends of the distributions are outlined. The data are discussed in terms of double ionization mechanisms and it is proposed here, using qualitative arguments, that the angular distributions are dominated by a double hit process (the so-called 'two-step 2' mechanism). Such predominance is shown to be due to the chosen kinematical conditions, characterized by both an energy selectivity (equal ejected electrons energies) and a spatial selectivity (ejected electrons observed in opposite half-planes).
A new spectrometer for the coincidence study of (e,2e) single ionization, and e,(3-1)e or (e,3e) double ionization by electron impact is described. The system is based on two double toroidal analyzers for the ejected electrons and a cylindrical electrostatic analyzer for the scattered electrons. It allows angular and energy distributions of the ejected electrons to be measured over almost the totality of the collision plane, using dual two-dimensional position sensitive detectors. The design and operation of the spectrometer is described with a special emphasis on the imaging properties of the system. The performance (energy and angular resolutions,…) and the calibration of the spectrometer are discussed and sample results for (e,2e), e,(3-1)e and (e,3e) experiments on the rare gases are presented.
Using a new multi-parameter multi-coincidence spectrometer, we have measured relative, coplanar (e, 3e) angular distributions for the double ionization of neon at ∼ 5.5 keV impact energy, under a small scattering angle, 0.45 • , and equal energy sharing among the two 'ejected' electrons, E b = E c = 10 eV. The general features of the distributions are outlined and compared with similar measurements on argon (El Marji et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 3677). It is found that the distributions are dominated by the simultaneous backward emission of both ejected electrons. On the basis of qualitative arguments, this may be attributed, as in argon, to a double-hit process (a so-called 'two step 2' mechanism). However, additional structures are found for neon, which are not present for argon, namely a forward emission of both ejected electrons. Though their origin could not be explained in terms of intuitive doubleionization mechanisms, they clearly indicate that the recoil ion actively participates during the collision.
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