Dmitry V. Fursa

(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).

Positron scattering from argon: total cross sections and the scattering length

We report results from new positron–argon total cross-section (TCS) measurements. Agreement with the corresponding recent data of Jones et al (2011 Phys. Rev. A 83 032701) is found to be very good, except at the lowest energies of common measurement. Excellent qualitative agreement is also found between our measurements and an improved convergent close-coupling (CCC) calculation which was undertaken as a part of this study. This level of accord between our experimental and theoretical TCSs has enabled us to determine an experimental scattering length (a) of a = −4.9 ± 0.7 au for the positron–argon system. That value is in excellent agreement with the relativistic polarized orbital optical potential approach result of Jones et al (a = −4.7 au) and our CCC result of a = −4.3 au.

Multiconfigurational two-centre convergent close-coupling approach to positron scattering on helium

The convergent close-coupling method with two-centre expansions has been developed to calculate positron scattering on helium. The method utilizes a multiconfigurational description of helium wavefunctions. Positronium formation is taken into account explicitly as electron capture into the positronium states. Direct-scattering, positronium-formation and breakup cross sections are calculated at all energies of practical relevance. Convergence in the calculated cross sections is demonstrated by increasing the basis size and orbital quantum number of the included states for each of the centres. Better agreement with experimental data is found when a multiconfigurational description is used for the helium wavefunctions in comparison with the recent frozen-core results.

Low-energy positron interactions with xenon

Low-energy interactions of positrons with xenon have been studied both experimentally and theoretically. The experimental measurements were carried out using a trap-based positron beam with an energy resolution of ?80?meV, while the theoretical calculations were carried out using the convergent close-coupling method and the relativistic optical potential approach. Absolute values of the grand total, positronium formation and grand total minus positronium formation cross sections are presented over the energy range of 1?60?eV. Elastic differential cross sections (DCS), for selected energies, are also presented both below and above the positronium formation threshold. Fine energy-step measurements of the positronium formation cross section over the energy range of 4.4?8.4?eV, and measurements of the elastic DCS at the energies of 5.33 and 6.64?eV, have been carried out to investigate the ionization threshold regions corresponding to the 2P3/2 and 2P1/2 states of the Xe+ ion. The present results are compared with both experimental and theoretical values from the literature where available.

Electron impact ionization of ground-state and metastable Li + ions

Direct ionization of ground-state Li+(1s21S0) and metastable Li+(1s2s 3S1) ions by electron impact has been studied in the energy range from threshold to 1000 eV using state-of-the-art experimental and theoretical approaches. Absolute cross sections obtained by crossing a beam of Li+(1s2) ions with a beam of electrons and results of convergent close coupling (CCC) calculations agree within experimental error bars of approximately 10%. Mixed beams of Li+(1s2) and Li+(1s2s 3S) ions were also prepared and apparent cross sections were measured. Good agreement with CCC theory is obtained assuming suitable fractions of metastable ions in the parent beam.

Complete Solution of Electronic Excitation and Ionization in Electron-Hydrogen Molecule Scattering

The convergent close-coupling method has been used to solve the electron-hydrogen molecule scattering problem in the fixed-nuclei approximation. Excellent agreement with experiment is found for the grand total, elastic, electronic-excitation, and total ionization cross sections from the very low to the very high energies. This shows that for the electronic degrees of freedom the method provides a complete treatment of electron scattering on molecules as it does for atoms.

Benchmark cross sections for electron-impact total single ionization of helium

We establish the total single ionization cross section for electron-impact ionization of helium, to an accuracy of few per cent, from near threshold to 1 keV. This is done fully ab initio using the convergent close-coupling method without the often-used frozen-core approximation. We find excellent agreement with the most recent measurements of Rejoub et al (2002 Phys. Rev. A 65 042713) and Sorokin et al (2004 J. Phys. B: At. Mol. Opt. Phys. 37 3215), which are around 5% less than the previously used standard.

Triple differential cross sections for the electron-impact ionization of helium at 102 eV incident energy.

We examine the time-dependent close-coupling (TDCC) approach to electron-impact single ionization of helium and study the convergence properties of our method. As an example, we compare our calculations to recent measurements of the triple differential cross sections from He after 102 eV electron impact, made for asymmetric electron energies and a variety of electron geometries. We find that our calculations compare well to the measurements and to convergent close-coupling calculations.

Fully differential cross sections for electron-impact ionization of sodium

Unitarity of the close-coupling formalism ensures that the total ionization cross section converges much faster than its individual l-components (Bray 1994 Phys. Rev. Lett. 73 1088). However, convergence in the latter is required when obtaining differential cross sections. This can be computationally difficult whenever the ionization threshold is relatively small, as is the case for sodium. Nevertheless, we present convergent close-coupling calculations of fully differential cross sections for electron-impact ionization of sodium and find very good agreement with the available experiment from near threshold through to high energies.

Differential cross sections for electron impact excitation of the n = 2 states of helium at intermediate energies (80, 100 and 120 eV) measured across the complete angular scattering range (0–180°)

Differential cross sections (DCS) for inelastic electron scattering to the n = 2 states in helium have been measured at incident energies of 80, 100 and 120 eV. These DCS have been determined across the complete angular scattering range (0–180°) using a magnetic angle changer (MAC) with a soft-iron core. The convergent close-coupling (CCC), R-matrix with pseudostates (RMPS), and B-spline R-matrix (BSR) methods have been used to calculate these DCS. Agreement between the experimental data and the predictions from these highly sophisticated theoretical methods is generally good. The remaining discrepancies mainly occur at small and large angles for the triplet states 23S and 23P, whereas excellent agreement is found between 30° and 150°. The small-angle differences are likely due to contamination of the observed experimental signal from the neighbouring 21S and 21P states. The present results demonstrate the effective use of a soft-iron core MAC for DCS measurements at intermediate energies, extending the operational energy range of such devices by a factor of approximately 25.