Physical Review A

Long-range He2(Σg,u+3) potentials and metastability exchange in He-He collisions

A. P. Hickman, Neal F. Lane

Published Jul 1, 1974• 12 citations

Abstract

Previous semiempirical determinations by Fitzsimmons et al. of the asymptotic form of the potentials ${V}_{g}$ and ${V}_{u}$ for the helium molecular states $^{3}\ensuremath{\Sigma}_{g,u}^{+}$ are refined to include new experimental data. The resulting potentials yield cross sections for elastic scattering, diffusion, and metastability exchange in $\mathrm{He}(2^{3}S_{1})\ensuremath{-}\mathrm{H}\mathrm{e}(1^{1}S_{0})$ collisions in agreement with various experiments from 15 to 500 \ifmmode^\circ\else\textdegree\fi{}K.

DOI: 10.1103/physreva.10.444

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Physical Review A

Long-range He2(Σg,u+3) potentials and metastability exchange in He-He collisions

A. P. Hickman, Neal F. Lane

Published Jul 1, 1974• 12 citations

Publisher

Abstract

Previous semiempirical determinations by Fitzsimmons et al. of the asymptotic form of the potentials

{V}_{g}

and

{V}_{u}

for the helium molecular states

^{3}\ensuremath{\Sigma}_{g,u}^{+}

are refined to include new experimental data. The resulting potentials yield cross sections for elastic scattering, diffusion, and metastability exchange in

\mathrm{He}(2^{3}S_{1})\ensuremath{-}\mathrm{H}\mathrm{e}(1^{1}S_{0})

collisions in agreement with various experiments from 15 to 500 \ifmmode^\circ\else\textdegree\fi{}K.

DOI: 10.1103/physreva.10.444

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Paul J. Leo, V. Venturi, Ian B. Whittingham +1 more

We report scattering lengths for the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+},$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ and ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ adiabatic molecular potentials relevant to collisions of two metastable $2{}^{3}S$ helium atoms as a function of the uncertainty in these potentials. These scattering lengths are used to calculate experimentally observable scattering lengths, elastic cross sections, and inelastic rates for any combination of states of the colliding atoms, at temperatures where the Wigner threshold approximation is valid.We report scattering lengths for the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+},$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ and ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ adiabatic molecular potentials relevant to collisions of two metastable $2{}^{3}S$ helium atoms as a function of the uncertainty in these potentials. These scattering lengths are used to calculate experimentally observable scattering lengths, elastic cross sections, and inelastic rates for any combination of states of the colliding atoms, at temperatures where the Wigner threshold approximation is valid.

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Temperature Dependence of Helium Metastability Exchange Cross Section

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New Journal of PhysicsJun 28, 20107 citations

Elastic and charge-transfer collisions of He + –He at ultralow energies

Syham Lias, Fouzia Bouchelaghem, L. Aissaoui

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Ultracold homonuclear and heteronuclear collisions in metastable helium

Daniel Cocks, Ian B. Whittingham, G. Peach

Scattering and ionizing cross sections and rates are calculated for ultracold collisions between metastable helium atoms using a fully quantum-mechanical close-coupled formalism. Homonuclear collisions of the bosonic $^{4}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}$ and fermionic $^{3}\mathrm{He}^{*}+^{3}\mathrm{He}^{*}$ systems, and heteronuclear collisions of the mixed $^{3}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}$ system, are investigated over the temperature range 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. Carefully constructed Born-Oppenheimer molecular potentials are used to describe the electrostatic interaction between the colliding atoms, and complex optical potentials to represent loss through ionization from the ${}^{1,3}\mathrm{\ensuremath{\Sigma}}$ states. Magnetic spin-dipole-mediated transitions from the ${}^{5}\mathrm{\ensuremath{\Sigma}}$ state are included and results reported for spin-polarized and unpolarized systems. Comparisons are made with experimental results, previous semiclassical models, and a perturbed single-channel model.Scattering and ionizing cross sections and rates are calculated for ultracold collisions between metastable helium atoms using a fully quantum-mechanical close-coupled formalism. Homonuclear collisions of the bosonic $^{4}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}$ and fermionic $^{3}\mathrm{He}^{*}+^{3}\mathrm{He}^{*}$ systems, and heteronuclear collisions of the mixed $^{3}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}$ system, are investigated over the temperature range 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. Carefully constructed Born-Oppenheimer molecular potentials are used to describe the electrostatic interaction between the colliding atoms, and complex optical potentials to represent loss through ionization from the ${}^{1,3}\mathrm{\ensuremath{\Sigma}}$ states. Magnetic spin-dipole-mediated transitions from the ${}^{5}\mathrm{\ensuremath{\Sigma}}$ state are included and results reported for spin-polarized and unpolarized systems. Comparisons are made with experimental results, previous semiclassical models, and a perturbed single-channel model.

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Formation of antiprotonic helium p¯He+ and ionization in low-energy collisions of p¯ with He in the ground 1S1 and metastable 2S3 and 2S1 states

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.

Physical Review AApr 9, 20159 citations

Previous semiempirical determinations by Fitzsimmons et al. of the asymptotic form of the potentials

{V}_{g}

and

{V}_{u}

for the helium molecular states

^{3}\ensuremath{\Sigma}_{g,u}^{+}

are refined to include new experimental data. The resulting potentials yield cross sections for elastic scattering, diffusion, and metastability exchange in

\mathrm{He}(2^{3}S_{1})\ensuremath{-}\mathrm{H}\mathrm{e}(1^{1}S_{0})

collisions in agreement with various experiments from 15 to 500 \ifmmode^\circ\else\textdegree\fi{}K.

2{}^{3}S

helium atoms as a function of the uncertainty in these potentials. These scattering lengths are used to calculate experimentally observable scattering lengths, elastic cross sections, and inelastic rates for any combination of states of the colliding atoms, at temperatures where the Wigner threshold approximation is valid.

The cross section for exchange of metastability in collisions between

{\mathrm{He}}^{3}

atoms in the ground (

1^{1}S_{0}

) state and metastable (

2^{3}S_{1}

) state has been measured as a function of temperature by optical pumping techniques. The exchange cross section decreases from about 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$

{\mathrm{cm}}^{2}

at 4.2\ifmmode^\circ\else\textdegree\fi{}K. These results suggest a thermally activated exchange process in qualitative agreement with the predicted existence of a long-range repulsive interaction between metastable and ground-state helium atoms as proposed by Buckingham and Dalgarno.

^{4}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}

and fermionic

^{3}\mathrm{He}^{*}+^{3}\mathrm{He}^{*}

systems, and heteronuclear collisions of the mixed

^{3}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}

system, are investigated over the temperature range 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. Carefully constructed Born-Oppenheimer molecular potentials are used to describe the electrostatic interaction between the colliding atoms, and complex optical potentials to represent loss through ionization from the ${}^{1,3}\mathrm{\ensuremath{\Sigma}}$ states. Magnetic spin-dipole-mediated transitions from the ${}^{5}\mathrm{\ensuremath{\Sigma}}$ state are included and results reported for spin-polarized and unpolarized systems. Comparisons are made with experimental results, previous semiclassical models, and a perturbed single-channel model.

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.