The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and helium excitation (11S-21S, 21P) in positron-helium collisions at energies between 31.3 eV and 200 eV. The authors have used a target basis set of the form He(1s, 2s, ks, 2p, kp) and investigated the effect upon excitation due to the addition of the positronium basis set Ps(1s, 2s, 2p). An independent electron model has been combined with a model potential to represent the helium atom. Positronium formation reduces the 21S excitation below 50 eV but the 21P excitation is significantly reduced for all energies up to 150 eV. The present excitation results are in closer agreement with previous close-coupled calculations than with other theoretical methods. The He(n=2) excitation total is in agreement with experimental data below 36 eV. The calculated total positronium formation cross section is in agreement with experiment below 100 eV.

https://iopscience.iop.org/article/10.1088/0953-4075/25/2/022/pdf

He(21S, 21P) excitation and positronium formation in positron-helium collisions at intermediate energies

The total cross section for positronium formation in helium for positrons of energies E from threshold (17.8 eV) to 250 eV has been measured using a gas cell transmission method. The results indicate that the cross section above E=60 eV is satisfactorily described by 0.5e-0.02E pi a02, tending to zero at energies close to 200 eV. The approximate description of the cross section at intermediate energies, falling as E-2.5, is consistent with a number of theoretical predictions but is in conflict with earlier experiments, which show an E-1 behaviour. The implications of these measurements are discussed, with particular reference to the observed merging of the total cross sections for positron- and electron-helium scattering at intermediate energies.

The energy dependence of the positronium formation cross section in helium

A three-body classical trajectory Monte-Carlo calculation is used to evaluate the differential cross sections for the ejected electron, as well as the projectile scattering for 50, 100 and 150 eV e++Ar0 electron removal collisions. In the ionized electron spectra both the electron capture to the continuum (ECC) and binary peak structures are observed. The theoretical ECC peak is diffuse and shifted to lower energies than expected from simple arguments. The calculated double differential cross sections for electron ejection have been compared to experiment. When the experimental apparatus function of Moxom et al. (1992) is used to convolute the theoretical spectra, good agreement with their data is found that confirms a sharp ECC structure is not present in this system. Moreover, comparisons made at non-zero angles show substantial agreement between theory and experiment. The scattered positrons agree well with measurements made at 3 degrees by Kover et al. (1993), but differ at larger angles. In addition, the differential cross section for positronium formation is presented; no evidence of an observable Thomas peak is present.

Projectile and electron spectra resulting from positron-argon collisions

Total ionization cross sections for positron impact on He, Ar and H2 targets from threshold to around 300 eV are presented and compared with values obtained by summing available partial cross section results. Reasonable accord is found with these data. The experimental method and apparatus is described in detail together with the systematic reliability checks.

Ionization of He, Ar and H2 by position impact at intermediate energies

The formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation. In particular the authors analyse the eikonal and continuum-distorted-wave models in both their post and prior forms. Differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV. The results confirm that the total cross section decreases rapidly with increasing energy, as predicted by the simple first Born approximation, and do not agree with experimental evidence of a slower decay. The computed total cross sections for helium targets agree with those of the close-coupling model for energies above 100 eV. Below 100 eV the authors find a large post-prior discrepancy. The calculated differential cross sections show evidence of a Thomas peak at high energies for the CDW models but not for the eikonal models.

https://iopscience.iop.org/article/10.1088/0953-4075/25/22/028/pdf

Positronium formation in positron-atom collisions at intermediate energies

The authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium. Total cross sections for positronium formation are found to be in good agreement with the measurement of Fromme et al. (1986) above 160 eV. This is the energy range in which most theoretical results disagree with the measurements and indeed in previous classical trajectory Monte Carlo (CTMC) calculations it has been suggested that the experiments incorrectly describe large-angle ionization as charge transfer. The authors elaborate on this matter.

D S F Crothers

Papers

Positronium formation in positron-helium collision