Physical Review A

Scattering of H¯(1s) off metastable helium atom at thermal energies

Prabal K. Sinha, Arnab Ghosh

Published Jun 12, 2006• 2 citations

Abstract

Quantal calculations for scattering of ground-state antihydrogen by metastable $(n=2S)$ helium atoms have been performed using the nonadiabatic, atomic orbital expansion technique at thermal energies. The zero-energy elastic cross sections of the present systems are much greater than the corresponding value for the ground-state helium target. The low-energy elastic cross section for the singlet metastable helium $[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{1}S)]$ target is higher than the corresponding value when the target is in the metastable triplet state $[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{3}S)]$.

DOI: 10.1103/physreva.73.062710

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Physical Review A

Scattering of H¯(1s) off metastable helium atom at thermal energies

Prabal K. Sinha, Arnab Ghosh

Published Jun 12, 2006• 2 citations

Publisher

Abstract

Quantal calculations for scattering of ground-state antihydrogen by metastable

(n=2S)

helium atoms have been performed using the nonadiabatic, atomic orbital expansion technique at thermal energies. The zero-energy elastic cross sections of the present systems are much greater than the corresponding value for the ground-state helium target. The low-energy elastic cross section for the singlet metastable helium

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{1}S)]

target is higher than the corresponding value when the target is in the metastable triplet state

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{3}S)]

DOI: 10.1103/physreva.73.062710

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Kazuhiro Sakimoto

Calculations of low-energy collisions of antiprotons $\overline{p}$ with helium atoms in the ground $(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})$ and metastable $(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}$ and $2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})$ states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the $\overline{p}{\mathrm{He}}^{+}$ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.Calculations of low-energy collisions of antiprotons $\overline{p}$ with helium atoms in the ground $(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})$ and metastable $(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}$ and $2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})$ states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the $\overline{p}{\mathrm{He}}^{+}$ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.

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Quantal calculations for scattering of ground-state antihydrogen by metastable

(n=2S)

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{1}S)]

target is higher than the corresponding value when the target is in the metastable triplet state

[\mathrm{He}(2\phantom{\rule{0.2em}{0ex}}^{3}S)]

We have investigated the

s

-wave elastic scattering of antihydrogen atoms off atomic alkali-metal targets (Li, Na, K, and Rb) at thermal energies $({10}^{\ensuremath{-}16}--{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$ using an atomic orbital expansion technique. The elastic cross sections of these systems at thermal energies are found to be very high compared to $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{H}$ and $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{He}$ systems. The theoretical models employed in this study are so chosen to consider long-range forces dynamically in the calculation. The mechanism of cooling suggests that Li may be considered to be a good candidate as a buffer gas for enhanced cooling of antihydrogen atoms to ultracold temperature.

Low-energy positron scattering from singlet and triplet metastable He(

1s2s

) is investigated using the

R

-matrix propagation method in hyperspherical coordinates. Elastic and positronium-formation cross sections are reported, and near-threshold resonance structures are analyzed in terms of eigenphase sums and the time-delay matrix. For the triplet target, the calculated cross sections are in agreement with available convergent-close-coupling results and display the expected threshold behavior. Beyond the known

S

-wave features, Feshbach resonance series in higher partial waves extending up to highly excited atomic thresholds are systematically identified. The time-delay matrix further uncovers hidden resonances that produce obvious structures in the positronium-formation cross sections.

3\phantom{\rule{0.3em}{0ex}}\mathrm{K}

. Loosely bound metastable antihydrogen-helium states are also investigated, and it is found that a number of relatively long-lived states with up to three units of angular momentum exist.

Calculations of low-energy collisions of antiprotons

\overline{p}

with helium atoms in the ground

(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})

and metastable

(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}

and

2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})

states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the

\overline{p}{\mathrm{He}}^{+}

atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.