E M Staicu Casagrande

(e,2e) ionization of helium and the hydrogen molecule: signature of two-centre interference effects

Relative (e,2e) triply differential cross sections (TDCS) are measured for the ionization of the helium atom and the hydrogen molecule in coplanar asymmetric geometry at a scattered electron energy of 500 eV and ejected electron energies of 205, 74 and 37 eV. The He experimental results are found to be in very good agreement with convergent close-coupling calculations (CCC). The H2 experimental results are compared with two state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. Both models yield an overall good agreement with experiments, except for some intensity deviations in the recoil region. Similar (e,2e) works were recently published on H2 with contrasted conclusions to the hypothesis that the two H nuclei could give rise to an interference pattern in the TDCS structure. Murray (2005 J. Phys. B: At. Mol. Opt. Phys. 38 1999) found no evidence for such an effect, whereas Milne-Brownlie et al (2006 Phys. Rev. Lett. 96 233201) reported its indirect observation. In this work, based on a direct comparison between experimental results for He and H2, we observe an oscillatory pattern due to these interference effects, and for the first time the destructive or constructive character of the interference is observed, depending on the de Broglie wavelength of the ejected electron wave. The experimental finding is in good agreement with the theoretical prediction by Stia et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L257).

Dynamics of electron impact ionization of the outer and inner valence (1t2 and 2a1) molecular orbitals of CH4 at intermediate and large ion recoil momentum

The triply differential cross section has been measured for electron-impact ionization of the outer valence 1t2 and the inner valence 2a1 orbitals of methane using the (e,2e) technique with coplanar asymmetric kinematics. The measurements are performed at scattered electron energy of 500 eV, ejected electron energy of 12, 37 and 74 eV and for scattering angle of the fast outgoing electron of 6?. This kinematics is characterized by a target ion recoil momentum ranging from moderate (0.25 au) to very large (3.2 au) values. The results are compared with theoretical cross sections calculated using the 1CW and the BBK models recently extended to molecules. The experimental cross sections exhibit a very large recoil scattering, especially for the inner 2a1 molecular orbital, which is not predicted by the theory. The differences between experiment and theory are attributed to the very strong scattering from the ion, not properly accounted for by theory. This indicates the need for further theoretical developments as well as experimental investigations in order to correctly model the process of molecular ionization.

DWBA-G calculations of electron impact ionization of noble gas atoms

We perform calculations of electron impact ionization of noble gas atoms within the distorted wave Born approximation (DWBA) corrected by the Gamow factor (G) to account for the post-collision interaction. We make an extensive comparison with experimental data on He 1s2, Ne 2s2, 2p6 and Ar 3p6 under kinematics characterized by large energy transfer and close to minimum momentum transfer from the projectile to the target. For all atoms, good agreement between theory and experiment is achieved. In the case of Ar, the disagreement of experimental data with theory reported earlier by Catoire et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 2827) is reconciled.

Comparison of the electron impact experimental and theoretical fourfold differential cross sections for Ne and CH4

New measurements of the electron impact (e,3-1e) fourfold differential cross section for double ionization of isoelectronic atomic and molecular targets, namely Ne and CH4, are reported. The measurements are performed at intermediate incident energy and a different energy range for the pair of ejected electrons. We compare the experimental results with the predictions of a recently developed kinematical model (Lahmam-Bennani et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 105201), which can explain the appearance of the structures related to the second-order mechanism (such as TS2). Some of the data are compared with the predictions of the first and second Born models, showing mixed agreement. Moreover, the targets are isoelectronic hence the difference between the behaviour of the cross sections is qualitatively discussed in terms of the molecular structure influence over the ionization process.

(e, 2e) triple differential cross-sections for ionization beyond helium: the neon case at large energy transfer

We report new coplanar (e, 2e) results for ionization of the He 1s and Ne 2p and 2s subshells under kinematics which have remained rather unexplored to date and characterized by large energy transfer and close to minimum momentum transfer from the projectile to the target. The experimental results obtained in two laboratories are used as a sensitive test of a number of state-of-the-art available theoretical models for multi-electron atoms. The He results are in excellent agreement with convergent close-coupling predictions as well as with distorted-wave + Gamow and hybrid distorted-wave + R-matrix models. In the Ne case, an overall satisfactory agreement with experiments is obtained for all considered models. The importance of post-collisional interaction effects is demonstrated whereas second-order effects in the projectile-target interaction at small distances are shown to be negligibly small. Remaining discrepancies between theories and experiments are discussed.

An energetic (e, 2e) reaction away from the Bethe ridge: recoil versus binary

We analyse the recoil-to-binary (RB) peak intensity ratio in an energetic (e, 2e) reaction performed on the valence ns sub-shell of noble gas atoms away from the Bethe ridge condition. A qualitative change in the RB ratio dependence on the ejected electron energy from He to Ar can be explained by the variation of reflectivity of the short-range Hartree–Fock potential. The reflectivity increases profoundly from lighter (He) to heavier (Ne and Ar) noble gas atoms because of modification of the scattering phases due to occupation of the target p orbitals (Levinson–Seaton theorem). This effect is further modified due to strong inter-shell correlations in Ar. These theoretical predictions are confirmed experimentally.

Triple differential cross sections for the ionization of the valence states of NH3 by electron impact

We report new experimental and theoretical triple differential cross sections for the electron impact ionization of the three valence states of ammonia in an intermediate energy regime. Measurements are performed in an asymmetric coplanar geometry under kinematics which have been unexplored to date. The data are compared to predictions from the first order approaches and BBK model. The experimental cross sections exhibit a very large recoil scattering, which is not predicted by the theory.

Coincidence angular correlation in electron impact single or double ionisation of atoms and molecules

Experimental results obtained with our multi parameter multi-coincidence spectrometer are presented for the (e,3e) double ionisation of Ar and (e,2e) single ionisation of small molecules. The (e,3e) measurements are discussed in terms of competition between the two double ionisation processes present under the chosen kinematics, and qualitative conclusions are given. The results for the ionisation of H2 and the outer orbital of N2 are compared with the predictions of the most elaborate available theoretical models for description of the molecular ionisation process. Overall reasonable agreement is observed and tentative interpretations for the discrepancies are discussed.

Electron impact experimental and theoretical fourfold differential cross section for CH4

New development of the 3C approximated model describing the fourfold differential cross section for double ionization of CH4 will be reported. A corrective factor called Ward- Macek will take into account the repulsion between the two ejected electrons instead of the usual Gamow factor. To validate the developed model, we performed the coplanar (e, 3-1e) experiments for the double ionization of the methane target by electron impact at intermediate incident energy and different energy range for the pair of ejected electrons. Some agreement is found for the forward part of the angular distributions. The differences are discussed in terms of double ionization mechanisms.

Dynamics of single or double ionization of small systems from coincidence electron impact experiments

In this brief review, we illustrate the potentialities and the power of electron-electron coincidence studies to investigate the dynamics of single ionization SI [(e,2e) case] or double ionization DI [(e,3e) case] processes. An example for (e,2e) SI of rare gas atoms is presented with a new insight into the behaviour of the recoil versus the binary intensity. A second example for (e,2e) SI of molecules is used to illustrate the observation of a purely molecular effect, namely the signature of interference effects due to the two-center nature of the H2 molecule. The third example discusses an unprecedented triple coincidence study involving the scattered – ejected – Auger electrons emitted from an argon target. It is shown that the method allows to disentangle the contribution of various DI mechanisms.