A Lahmam-Bennani

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 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.

A multidetection, multicoincidence spectrometer for (e,2e) and (e,3e) electron impact ionization measurements

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.

Double ionization of helium by fast electrons: theoretical investigations of the fourfold differential cross section

Recent electron-impact double ionization (e, 3 - 1e) experiments on helium are investigated theoretically. An ab initio calculation has been performed on the basis of the first Born approximation using different correlated wavefunctions for the initial bound-state and a correlated double-continuum wavefunction which satisfies the exact asymptotic boundary conditions for the final state with two slow electrons. The results of this calculation are compared with those of other theories.

Complete experiments for the double ionization of He: (e,3e) cross sections at 1 keV impact energy and small momentum transfer

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.

Electron impact ionisation of helium: convergence of triple differential cross sections to the Born approximation limit

A set of measured triple differential cross sections with symmetric kinematics is shown to converge for increasing incident energy to a finite limit identical to the first Born approximation. Experiments on helium for momentum transfer 0.5 au are compared with each other as a function of electron impact energies between 150 and 8000 eV. For all impact energies, for which deviations from the first Born approximation appear, the most prominent correction term is the post-collision interaction.

Is the Optical Limit Approached in Coplanar (e,3e) Experiments on He at High and Intermediate Energies?

It is well established that the electron impact ionisation should converge towards the photon impact case in the limit of infinite incident energy and vanishingly small momentum transfer. Recent (e,3e) experiments on helium at high impact energy are re-analysed in this sense, together with new ones at intermediate energy. Surprisingly large deviations from the corresponding photo double ionisation ones are evidentiated. One of the most significant results of this analysis is that it is shown that the optical limit is reached differently for different escape angles of the emitted electrons while the momentum transfer vector is being fixed.