H. R. J. Walters

Positron scattering by atomic hydrogen

Cross sections for positron scattering by atomic hydrogen are calculated in the energy range 0 - 110 eV using a 33-state approximation which employs the 1s, 2s and 2p eigenstates of both positronium and hydrogen together with 27 hydrogen pseudostates. Cross sections are given for elastic scattering, H(2s) and H(2p) excitation, Ps(1s), Ps(2s), Ps(2p) and total positronium formation, ionization, and total scattering. The degree of agreement with an earlier 18-state calculation of Kernoghan et al, with sophisticated single-centre approximations, and with the available experimental data, strongly suggests that the main cross sections for positron scattering off ground-state atomic hydrogen are now known to quite a high degree of accuracy. Problems with the total cross section highlighted by Kernoghan et al are now resolved by new experimental data of Stein et al which give excellent agreement with theory for both positron and electron scattering. Suggestions are made for further experimental investigations.

Positron collisions with one- and two-electron atoms

Abstract The current status of coupled-state results for positron scattering by atomic hydrogen and the alkali metals is briefly reviewed. The extension of the coupled-state approximation to “two-electron” targets, i.e., He and the alkaline earths, is outlined and new calculations for positron scattering by He are presented.

An 18-state calculation of positron-hydrogen scattering

We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).

Coincidence Studies of Electron and Photon Impact Ionization

Recent Absolute (e,2e) Measurements on Atomic Hydrogen and Helium at Low and Intermediate Energies H. Ehrhardt, J. Roder. Second Born Calculations of (e,2e) Cross Sections at Low Energy Using a Pseudo-State Set F. Rouet, et al. Excitation-Ionization and Excitation-Autoionization of Helium P.J. Marchalant, et al. An Analytical Approach to Resonant and Direct Fragmentation of Many-Body Coulomb Systems J. Berakdar. Ionization Dynamics and Exchange Effects in Pure Three-Body Coulomb Scattering J. Berakdar, J.S. Briggs. Analysis of Integrated Cross Sections and Spin Asymmetries for the Electron-Impact Ionization of One and Two-Electron Atomic Systems J. Berakdar, et al. Double Ionizaton Mechanisms from Triple Coincidence Experiments A. Lahmam-Bennani, et al. Double Ionization of Helium by Electron Impact: A Study of the Two Step Mechanism C. Dal Cappello, et al. Partitioning of Momentum in Electron-Impact Double Ionization J.H. Moore, et al. 5DCS and 4DCS Calculations for Symmetric (e,3e) Impact Ionization Yu.V. Popov, et al. Exact and Approximate Methods of the Rigorous Coulomb Scattering Theory I.V. Farnakeev, et al. Strong and Weak Statements in the Theory of Dipolar (e,2e) Impact Ionization Yu. Popov, et al. Exact and Approximate Methods of the Rigorous Coulomb Scattering Theory I.V. Farnakeev, et al. Strong and Weak Statements in the Theory of Dipolar (e,2e) Impact Ionization Yu. Popov, et al. 25 Additional Articles. Index.

Elastic scattering and excitation of the 1s to 2s and 1s2p transitions in atomic hydrogen by electrons to medium to high energies

Multi-pseudostate close coupling is used to investigate 1s to 1s, 1s to 2s and 1s to 2p scattering at energies from 54.4 to 350 eV. Two sets of pseudostates are considered. The first is that of Fon et al. (1981); the second is an improved set. In constructing the improved pseudostates emphasis is placed upon getting the second Born term right. Target states of angular symmetries not included in the pseudostate bases are taken into account by using the distorted-wave second Born approximation of Kingston and Walters (1980). Results for elastic scattering are satisfactory; it is estimated that the present elastic differential cross sections are accurate to better than 5% at any angle at 200 and 300 eV. For the 2s and 2p excitations the theoretical picture is not so clear. Here there are significant differences between our results and the experiments of Williams (1975) and of Williams and Willis (1975). Some other theoretical calculations are in better accord with these experiments, although the authors believe the present work to be on firmer ground. Cases where the theoretical approximations are unanimously in disagreement with experiment suggest the presence of inaccuracies in the measurements and highlight the need for further experimental investigation. Of special interest are the angular correlation parameters of the 1s to 2p excitation; their results are in reasonable agreement with the main experiment at 54.4 eV in the angular range up to 70 degrees ; beyond 70 degrees there are still problems to be resolved.

The elastic scattering of electrons and positrons by helium and neon: the distorted-wave second Born approximation

It is proposed that the interaction V between an incident electron and an atom may be usefully split into two parts, V1 and V2, in such a way that couplings which are readily amenable to perturbation theory are contained in V2, while V1 corresponds to the remaining couplings which cannot be so effectively handled perturbatively. By making a distorted-wave Born series expansion in powers of V2 it is possible to calculate the scattering amplitude correct to any order in V2 while treating V1 non-perturbatively. Cross sections for the elastic scattering of electrons on helium and neon are calculated in the energy range 100 eV to 2 keV and compared with experiment.

Second-order effects in (e, 2e) excitation-ionization of helium to ( n = 2)

First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to . Results for ionization to the ground-state ion (1s) are also given. The physical content of the approximations is discussed, in particular, the two-step mechanism which appears in the second-order term for excitation-ionization. The second Born term is calculated in the closure approximation using a new numerical method based on prolate spheroidal coordinates. Comparison is made with absolute experimental data from Paris and Rome in very asymmetric coplanar geometry - scattered electron energies of 5500, 1500 and 570 eV and ejected electron energies of 5, 10, 20, 40 and 75 eV. For excitation-ionization the second Born approximation generally gives improved agreement with the experimental data in the recoil region and second-order effects are found to be still significant at 5500 eV. The importance of the second-order term decreases with increasing ejected energy for the cases studied here.

Positron scattering by rubidium and caesium

We report results in the energy range 0.5 to 64 eV for positron scattering by ground state rubidium and caesium in and coupled-state approximations, respectively. The pattern of results is similar to analogous calculations made on potassium by McAlinden et al. Except at the lowest energies, positronium formation is found to be almost entirely into excited states, with there being a dramatic collapse of the Ps(1s) cross section in the case of caesium. Ps(n=2), Ps(n=3) and cross sections are predicted to be of comparable size. formation is estimated from the calculated Ps(n=3) results using the scaling rule. Good agreement in shape and magnitude is obtained with the recent lower bound measurements of total positronium formation in rubidium by Surdutovich et al, although there are differences in detail. Good agreement is also achieved with the total cross section measurements of Parikh et al on rubidium after these have been corrected for discrimination against forward elastic scattering and renormalized upwards by 5%. These total cross section measurements display a pronounced peak near 6 eV which is well reproduced by the present theory but is at variance with the earlier Rb(5s,5p,6s,6p,4d) calculations of McEachran et al which neglect positronium formation. This peak, which also appears in the calculations on potassium and caesium, is primarily associated with the maximum in the total positronium formation cross section. At low energies elastic scattering is dominant, this dominance being directly taken over by the resonance excitation of the atom with increasing impact energy. The next most important cross section is positronium formation, followed by excitation of the lowest atomic d-state. The and transitions in rubidium and the and transitions in caesium are of relatively minor importance.

Positron scattering by potassium

Coupled-state calculations in a K(4s,4p,5s,5p,3d) + Ps(1s,2s,2p,3s,3p,3d) approximation are reported for positron scattering by ground-state potassium in the energy range 0.5 - 60.0 eV. Comparison is made with the earlier work of Hewitt and co-workers in a K(4s,4p,5s,5p) + Ps(1s,2s,2p) approximation. For the first time cross sections for positronium formation in n = 3 states are obtained. We support the conclusion of Hewitt et al that above about 4 eV positronium formation is mainly into excited states, although we disagree significantly on the distribution of the excited state population. Fairly good agreement is obtained with the measurements of Zhou and co-workers for total positronium formation when scaling is used to estimate the amount of positronium in states with . Our cross sections for elastic scattering, 4s - 4p, 4s - 5s and 4s - 5p excitation of potassium are in relatively good agreement with those of Hewitt and co-workers but our calculations also emphasize the importance of the 4s - 3d excitation which is omitted in their approximation. Atom excitation to the n = 5 level is found to be a relatively minor process. Good agreement is obtained with the total cross section measurements of Kwan and co-workers and Parikh and co-workers when these are normalized upwards by 10% and corrections made for near forward elastic scattering. This upward normalization lies within the quoted overall 21% error on the measurements. Below 4 eV the calculated total cross section is dominated by elastic scattering while at high energies 4s - 4p excitation is the main contributor. In the vicinity of 6 eV, and consistent with the measurements of Parikh and co-workers, the total cross section displays a pronounced peak which derives largely, although not totally, from positronium formation. Positronium formation is not important above about 30 eV.

Triple differential cross sections for electron impact ionisation of atomic hydrogen-a coupled pseudostate calculation

A coupled pseudostate approximation for calculated triple differential cross sections for electron ionisation of atomic hydrogen is presented. A partial wave formalism is developed for ionisation from any initial s state. The approximation is best suited to asymmetric geometries where one of the two electrons in the final ionised state has a much smaller energy than the other. Results are presented for ionisation of H(1s) at incident energies of 54.4, 100, 150 and 250 eV and ejected energies of 5, 10 and 14 eV. At 150 and 250 eV comparison is made with the coplanar experimental data of Ehrhardt et al. (1985), Schlemmer and Lohmann et al. (1984) with a generally satisfactory outcome, although there are some discrepancies. The coupled pseudostate approximation substantially confirms the earlier second Born/eikonal-Born series calculations of Byron et al. (1985) for high energy asymmetric coplanar geometries.