Low-energy quenching of positronium by helium

Abstract

Very-low-energy scattering of orthopositronium by helium has been investigated for the simultaneous study of elastic cross section and pickoff quenching rate using a model exchange potential. The present calculational scheme, while it agrees with the measured cross section of Skalsey et al., reproduces successfully the parameter 1 Z eff , the effective number of electrons per atom in a singlet state relative to the positron. Together with the fact that this model potential also leads to an agreement with measured medium energy cross sections of this system, this study seems to resolve the long-standing discrepancy at low energies among different theoretical calculations and experimental measurements. ͓S1050-2947͑99͒09006-X͔ PACS number͑s͒: 34.10.ϩx, 36.10.DrStudies on positronium-͑Ps-͒ impact scattering have gained momentum these days due to the availability of the ortho-Ps beam in the laboratory and its vast applicational potential coupled with the present inadequate and inconclusive understanding of its interaction dynamics with matter ͓1͔. Ps scattering by neutral targets has posed a challenge to theoreticians on a proper accounting of experimental data as most existing theoretical works disagree with the major experimental trend.The discrepancy figures prominently in the Ps-He system where there are many theoretical and experimental studies. The medium-energy experimental cross section shows a declining trend with decreasing energy ͓2͔ from a peak around 20 eV for Ps-He scattering. A similar trend is also observed in Ps-H 2 and Ps-Ar systems ͓2,3͔. This trend, which is supported by the recent measurement of Skalsey et al. ͓4͔, could not be reproduced in most theoretical predictions ͓5-9͔. At low energies, these theories and experiments ͓2-4,10,11͔ on the Ps-He system are inconsistent with each other and also among themselves. For illustration, the zero-energy cross sections calculated on Ps-He by different authors vary from 3.3 Å 2 ͓12͔ to 16.54 Å 2 ͓13͔ while the measured values range from 2.3Ϯ0.4 Å 2 ͑at 0.915 eV͒ ͓4͔ to 11Ϯ3 Å 2 ͑between 0 and 0.3 eV͒ ͓10͔. Pointing out the very reactive nature of Ps scattering and its associated convergence difficulties, a prescription for the generation of a nonlocal model exchange potential has been advocated recently and applied successfully to different electron-impact ͑targets H, He͒ and Ps-impact ͑targets: H ͓14,15͔, He ͓16͔, H 2 ͓17͔, Ar, Ne ͓18͔͒ scattering problems using static exchange to three-Ps-state models. The three-Ps-state calculation for Ps-He predicts a lower zero-energy cross section of 2.42 Å 2 ͓16͔.In this work we shed light on the above-mentioned discrepancy in the Ps-He system in conjunction with a determination of the parameter 1 Z eff , which denotes the effective number of electrons per atom in a singlet state relative to the positron. The incident ortho-Ps(1s) atom in a triplet state with a lifetime of 142 ns can decay into three photons and is more stable than its para counterpart in a singlet state with a lifetime of 0.125 ns for a two-photo...

S-, P- and D-wave resonances in positronium-sodium and positronium-potassium scattering

“…We suggested [7] a regularized, symmetric, nonlocal electron-exchange model potential and used it in the successful study of Ps scattering by H [8], He [7,[9][10][11], Ne [11], Ar [11], H 2 [12] and Li [13]. Our results were in agreement with experimental total cross sections [1,2], specially at low energies for He, Ne, Ar and H 2 .…”

“…Our results were in agreement with experimental total cross sections [1,2], specially at low energies for He, Ne, Ar and H 2 . Moreover, these studies yielded correct results for resonance and binding energies for the S wave electronic singlet state of Ps-H [4,8] and Ps-Li [13] systems in addition to experimental pick-off quenching rate in Ps-He [10] scattering.…”

“…To summarize, we have performed a three-Ps-state coupled-channel calculation of Ps-Na and Ps-K scattering at low energies using a regularized symmetric nonlocal electron-exchange model potential [7] successfully used [7][8][9][10][11][12][13] previously in different Ps scattering problems.…”

The quenching of ortho ‐positronium

“…Only a few calculations have been made so far for 1 Z eff [12][13][14]. They include H, He, Ne, Ar, Kr, and Xe.…”

Positronium interaction and its Bose‐Einstein condensation

“…dition to cross-sections, results for resonance and binding energies of different Ps-atom systems [24,25] as well as for pickoff quenching rate of the interaction of o-Ps with He [18].…”