The pick-off quenching of orthopositronium in helium

Fraser, P A; Kraidy, M

doi:10.1088/0370-1328/89/3/309

Cited by 47 publications

(36 citation statements)

References 7 publications

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“…It is difficult to reconcile these three experimental results in a theoretical model. Previous static-exchange calculations [7][8][9]13] except those of Ref. [14] all tend to support the largest cross section of Ref.…”

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confidence: 83%

“…At very low energies, these theories and experiments [2][3][4]10,11] 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 3.3Å 2 [12] to 16.54 A 2 [13] while the measured values range from 2.3 ± 0.4 A 2 (at 0.915 eV) [4] to 11 ± 3Å 2 (between 0 to 0.3 eV) [10]. Pointing out the very reactive nature of Ps scattering and its associated convergence difficulties, a prescription for the generation of nonlocal model exchange potential has been advocated recently and applied successfully to different electron-impact (targets: H, He) and Ps-impact (targets: H [15,16], He [14], H 2 [17], Ar, Ne [18]) scatterings problems using static-exchange to three-Ps-state models.…”

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confidence: 99%

“…2 we plot the results for the present calculation of 1 Z eff at different energies. For comparison we also plot the results of previous calculation by Barker and Bransden [7] and by Fraser and Kraidy [13] and the existing three experimental data. Although Drachman and Houston [5] did not calculate 1 Z eff at different energies, their low-energy value of 0.1 is in reasonable agreement with the present result of 0.11 and experiment of Refs.…”

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Low-energy quenching of positronium by helium

1999

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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...

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Low-energy quenching of positronium by helium

1999

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“…However, there are no such experimental data at present. Fraser and Kraidy (1966) and Barker and Bransden (1968) performed calculations taking into account the energy dependence of 'geff. It was found that 'Zefi(T) may deviate considerably from 'pefi( Vo).…”

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confidence: 99%

Brownian local time

Borodin¹

1989

Russ. Math. Surv.

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In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate. It can be considered as a local phase transition in the vicinity of the positron (or positronium). The present article is a review of the present state of the physics of positron and positronium self-trapped states in dense gases and liquids. Basic properties of the positronium bubbles and positron clusters are considered, as well as the conditions under which they exist and their effect on the annihilation rate. Experiments in which the self-trapping was detected are discussed. The positron and positronium self-trapping is considered to be closely related to other similar electronic phenomena, known for dense gases, liquids and other dense media. The main results from the study of positron and positronium interactions with isolated atoms are also presented. The density effects in moderately dense gases anticipating self-trapping are considered.

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“…The p-wave singlet elastic phase shifts calculated by the CCA method are higher by a factor of nearly π/2 than the corresponding static exchange results below 0.439 eV. † Present address: 4285 4286 H Ray and electron-electron exchange (Fraser 1961, 1962, 1968, Fraser and Kraidy 1966, Hara and Fraser 1975 is essential. Inclusion of these features evidently complicates the problem.…”

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Extended Finite Difference Time Domain Analysis of Induced Phase Modulation and Four-Wave Mixing between Two-Color Femtosecond Laser Pulses in a Silica Fiber with Different Initial Delays

Nakamura

Takasawa

Koyamada

et al. 2005

Jpn. J. Appl. Phys.

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The scattering of positronium (Ps) by lithium (Li) is investigated using the staticexchange approximation and a two-state close-coupling approximation (CCA) including exchange. The major effect of very high polarizability of Li, at least 30 times higher than hydrogen and more than four times higher than Ps, has been considered in the present CCA scheme in a dynamic way by using the Li(2p) state of the target Li atom in the basis set. Different scattering parameters, e.g. s-, p-and d-wave phase shifts below the excitation threshold, the partial wave elastic and excitation cross sections for s-, p-and d-waves, the integrated elastic and excitation cross sections, the quenching cross sections, and the ortho-para conversion ratios are presented in the incident energy range from 0.068 E 100 eV. The present study reveals that the long-range dipole correlation potential has an appreciable effect in the cases of Ps and Li scattering in the lower energy region 0.068 E 30 eV. In the incident energy region above 30 eV, both the results almost coincide with the first Born results. A sharp maximum in the elastic cross sections near 3 eV and three sharp peaks in excitation cross sections near the incident energies of 0.425, 13.0 and 19.0 eV appear in the present results, which are consistent with the minima in the conversion ratios. The p-wave singlet elastic phase shifts calculated by the CCA method are higher by a factor of nearly π/2 than the corresponding static exchange results below 0.439 eV. † Present address: 4285 4286 H Ray and electron-electron exchange (Fraser 1961, 1962, 1968, Fraser and Kraidy 1966, Hara and Fraser 1975 is essential. Inclusion of these features evidently complicates the problem.Here an attempt has been made to calculate different scattering parameters using a CCA including the effect of exchange and dipole polarizability for the Ps-Li system. An exact treatment of the exchange interaction is made in all the first Born matrix elements used as input for the CCA. It is to be noted that the mean static interaction in the case of the Ps-atom scattering vanishes due to coincidence of charge and mass centre in Ps. Again the excitation cross section vanishes for the processes in which the parity of Ps changes by an even number, unless exchange is included. There are a few studies dealing with the collision of Ps with one-and two-electron systems, e.g. Ps-H (

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The pick-off quenching of orthopositronium in helium

Cited by 47 publications

References 7 publications

Low-energy quenching of positronium by helium

Low-energy quenching of positronium by helium

Brownian local time

Extended Finite Difference Time Domain Analysis of Induced Phase Modulation and Four-Wave Mixing between Two-Color Femtosecond Laser Pulses in a Silica Fiber with Different Initial Delays

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