Near-Threshold Ionization of Argon by Positron Impact

Abstract

The direct single-ionization cross section for Ar by positron impact has been measured in the region above the first ionization threshold. These measurements are compared to semiclassical calculations which give rise to a power law variation of the cross section in the threshold region. The experimental results appear to be in disagreement with extensions to the Wannier theory applied to positron impact ionization, with a smaller exponent than that calculated by most previous works. In fact, in this work, we see no difference in threshold behavior between the positron and electron cases. Possible reasons for this discrepancy are discussed.

Near-Threshold Cross Sections for Electron and Positron Impact Ionization of Atomic Hydrogen

“…In conclusion, using the atomic hydrogen target we have demonstrated that the CCC calculations lend support to the threshold laws of E 1.127 for electron-impact ionization cross sections to within ∼2 eV of threshold where the corresponding spin asymmetries are constant, and E 2.65 for the positron impact to within ∼5 eV where the Ps-formation cross section is constant. Consequently, the CCC calculations do not support the recent measurements or the suggestion of Babij et al [4] that the electron-impact and positron-impact near-threshold ionization cross section behavior is the same.…”

“…This coincides with the Ps-formation cross section, a competing ionization process, being constant over this energy range (note the Ps-formation threshold is E ¼ −6.8 eV). Apart from the data of Babij et al [4] agreement with the other measurements is mostly very good over a broad energy range.…”

“…Thus, the cross section behavior is predicted to be much more suppressed near threshold in comparison to the electron case. Yet recently Babij et al [4] performed near-threshold measurements and suggested that the positron-impact behavior is much the same as for electron impact. It is particularly challenging extracting threshold behavior from experiments, as also discussed by Ashley et al [5], but the rather large variation between the measured and the predicted power law is what motivates the present study.…”

A Precis of Threshold Laws for Positron vs. Electron Impact Ionization of Atoms

“…We conclude by noting that a recent numerical calculation of the positron-atom impact cross section at very low energies by Bray et al [8] gives results that are consistent with Klar's prediction. On the other hand, an experiment on positron-argon ionization [9], with an energy resolution not fine enough to test the modulation of the CD theory, found linear dependence, which is consistent with the CD theory [1,2] in the first approximation.…”

Energy and angular distributions in 250 eV electron and positron collisions with argon atom

“…During the past years the cross sections with electron and positron impact have been studied extensively both experimentally [1][2][3][4][5] and using various models and methods such as applying the R-matrix approach [6], various version of the distorted-wave Born approximation (DWBA) [6][7][8][9][10][11] and the convergent close-coupling method [12]. Here, not aiming for completeness, we mention only a few representative studies.…”