Electron-impact Double Ionization Of Helium Research Articles

Absolute total cross sections for electron-impact double ionization of He(1s2s3S) and He−(1s2s2p4P)

We report on the experimental determination of absolute, total cross sections for electron-impact double ionization of helium in its metastable 1s 2s 3 S state and of the helium negative ion He− (1s 2s 2p 4 P). Cross sections are measured for impact energies ranging from threshold to 1000 eV using the animated crossed beams technique. The fast, pure beam of metastable helium required for the experiment is produced by photodetachment of fast He− ions. Results for He(1s 2s 3 S) are compared to those for He(1s 2 1 S), and exhibit similar magnitude and behaviour. The cross section for He− is large compared to other negative ions and indicates the predominance of indirect ionization channels at large impact energies.

Near threshold electron-impact double ionization of helium

We examine the break-up pattern for the electron-impact double ionization of helium using a time-dependent close-coupling approach. This ionization process is one of the simplest examples of the Coulomb four-body problem of three charged particles moving in the Coulomb field of a bare nucleus. Studies of this four-body process in the near-threshold region allow exploration of the break-up dynamics, which have been stimulated by differing predictions of the most likely geometries for break-up in the related processes of triple photoionization of lithium and electron-impact double ionization of helium. While both processes lead to the same final state of three electrons moving in the field of a bare nucleus, the preferred break-up pattern at low energies for the triple photoionization is predicted as a T-shape (Emmanouilidou et al 2008 Phys. Rev. Lett. 100 063002; Colgan et al 2013 Phys. Rev. Lett. 110 063001) and for the electron-impact double ionization as a ‘triangle’ pattern (Ren et al 2008 Phys. Rev. Lett. 101 093201). Our calculations at an excess energy of 5 eV show that a triangle break-up pattern is favored, confirming earlier experimental results (Ren et al 2008 Phys. Rev. Lett. 101 093201).

Asymmetric electron energy sharing in electron-impact double ionization of helium

We present the fully fivefold differential cross sections (FDCSs) for ($e,\phantom{\rule{0.16em}{0ex}}3e$) processes in helium within the first Born approximation. The calculation is performed for a coplanar geometry in which the incident electron is fast ($\ensuremath{\sim}6$ keV), the momentum transfer is small (0.24 a.u.), and for an asymmetric energy sharing between both slow ejected electrons at excess energy of 20 eV. Two cases have been considered: ${E}_{1}=15$ eV, ${E}_{2}=5$ eV and ${E}_{1}=8$ eV, ${E}_{2}=12$ eV. While waiting for new theoretical and experimental results for confrontations, in particular for asymmetric energy sharing, our results clearly demonstrate that, for the same incident energy, the same momentum transfer and the same excess energy, the ($e,\phantom{\rule{0.16em}{0ex}}3e$) process in helium with asymmetric energy sharing between ejected electrons is more likely than the case with symmetric energy sharing. The two- and three-dimensional representation of the FDCSs covering all possible values of the angle of ejections are presented and discussed. The theoretical cross sections are calculated by using a compact-kernel-integral-equation approach associated with the Jacobi matrix method to calculate a three-body wave function and which leads to a full convergence in terms of the basis size.

Pentuple energy and angle differential cross sections for the electron-impact double ionization of helium

A previous time-dependent close-coupling method is modified and extended to calculate pentuple energy and angle differential cross sections for the electron-impact double ionization of helium. At an incident energy of 106 eV, we find reasonably good agreement between theory and scaled crossed-beam reaction microscope measurements for seven different cross sections. Integration of the differential cross sections over all energies and angles yields a total ionization cross section that is also in reasonable agreement with absolute crossed-beam measurements.

Energy and angle differential cross sections for the electron-impact double ionization of helium

Energy and angle differential cross sections for the electron-impact double ionization of helium are calculated using a non-perturbative time-dependent close-coupling method. Collision probabilities are found by the projection of a time-evolved nine-dimensional coordinate space wavefunction onto fully antisymmetric products of spatial and spin functions representing three outgoing Coulomb waves. At an incident energy of 106 eV, we present double energy differential cross sections and pentuple energy and angle differential cross sections. The pentuple energy and angle differential cross sections are found to be in reasonable agreement with the scaled shapes observed in recent (e, 3e) reaction microscope experiments. Integration of the differential cross sections over all energies and angles yields a total ionization cross section that is also in reasonable agreement with absolute crossed-beams experiments.

Momentum correlation of the final-state wavefunction for (e, 3e) collisions on helium

Based on our earlier paper, the momentum correlation of the four bodies in the final state is further considered for (e, 3e) processes on helium. A fivefold differential cross section (FDCS) for electron-impact double ionization of helium is calculated by use of the modified model for high incident energy (1–5.6keV). It has been found that the present results give a better description for the experimental data, as compared with the results of our earlier paper.

Complete experimentsfor the double ionization of He:(e,3e) cross sections at 1 keV impact energyand small momentum transfer

The relative, coplanar angular distributions ofelectrons, produced in an electron-impact doubleionization of helium (e,3e reaction), have beenmeasured at 1.1 keV impact energy. The momentum transferwas 0.45 au and the two `ejected' electrons were detectedwith the same energy of 10 eV. The general features ofthe angular distributions are discussed. The data areanalysed in different angular modes which allows adetailed comparison with state-of-the-art calculations.For high incident energy and small momentum transfer, asin the present case, the (e,3e) cross section canbe related to the single-photon double ionization (PDI).We exploit this fact and compare the present findingswith the PDI and identify the contribution of non-dipoleeffects.

Angular distribution in (e, 3e) collisions on helium

A fivefold differential cross section for electron-impact double ionization of helium is calculated for high incident energy (1-5 keV). The calculation is based on the first and second Born approximation. The results obtained are in reasonably good agreement with the recent (e,3e) experimental data. Some typical situations are investigated in order to identify the mechanisms of double ionization.

(e,3e) electronic momentum spectroscopy: perspectives and advantages

Five-fold and four-fold differential cross sections for electron-impact double ionization of helium are calculated in new proposed experimental situations where the scattered electron and one of the ejected electrons have similar energies. It is shown that only this new electron momentum spectroscopy provides direct information about the electron - electron correlations in a target. Various models of the helium ground-state wavefunction are used to justify this statement.

Angular distributions in the double ionization of helium by electron impact.

Electron-impact double ionization of helium is investigated theoretically for the case of high incident energies (5--10 keV). An {ital ab} {ital initio} calculation is proposed, which includes almost full correlation in the initial state and partial correlations in the final state. The results of this model are compared with those given by other theories in the framework of the wave-function approach. We show that the correlations in the final state must be taken into account for the evaluation of the angular distributions.