Journal of Physics Conference Series

Ultracold collisions in metastable helium

G. Peach, Daniel Cocks, Ian B. Whittingham

Published Feb 1, 2017• 3 citations

Abstract

Photoassociation processes are studied in ultracold collisions between different isotopes of metastable He(2 3 S) and He(2 3 P) atoms; Penning and associative ionization rates for collisions between two He(2 3 S) atoms are also obtained. Comparisons are made with data from existing experiments.

DOI: 10.1088/1742-6596/810/1/012003

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Journal of Physics Conference Series

Ultracold collisions in metastable helium

G. Peach, Daniel Cocks, Ian B. Whittingham

Published Feb 1, 2017• 3 citations

Publisher

Abstract

DOI: 10.1088/1742-6596/810/1/012003

Related papers

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We describe two-photon photoassociation (PA) experiments operated in an ultracold gas of metastable $^{4}\mathrm{He}^{*}$ atoms in the $2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ state. Atom-molecule dark resonances as well as Raman signals provide information on the exotic molecule in the least bound vibrational $J=2$ , $v=14$ state of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ interaction potential of two spin-polarized metastable atoms. The physical origin of the various two-photon PA signals is first discussed. Their linewidths are interpreted in terms of processes limiting the lifetime $\ensuremath{\tau}$ of the exotic molecule. A value of $\ensuremath{\tau}=1.4\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$ is found, which significantly differs from two recent calculations of the Penning ionization rate induced by spin-dipole coupling [T. J. Beams et al., Phys. Rev. A 74, 014702 (2006); M. Portier and G. V. Shlyapnikov (unpublished)]. The limitation of $\ensuremath{\tau}$ by atom-molecule inelastic collisions is investigated and ruled out.We describe two-photon photoassociation (PA) experiments operated in an ultracold gas of metastable $^{4}\mathrm{He}^{*}$ atoms in the $2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ state. Atom-molecule dark resonances as well as Raman signals provide information on the exotic molecule in the least bound vibrational $J=2$, $v=14$ state of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ interaction potential of two spin-polarized metastable atoms. The physical origin of the various two-photon PA signals is first discussed. Their linewidths are interpreted in terms of processes limiting the lifetime $\ensuremath{\tau}$ of the exotic molecule. A value of $\ensuremath{\tau}=1.4\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$ is found, which significantly differs from two recent calculations of the Penning ionization rate induced by spin-dipole coupling [T. J. Beams et al., Phys. Rev. A 74, 014702 (2006); M. Portier and G. V. Shlyapnikov (unpublished)]. The limitation of $\ensuremath{\tau}$ by atom-molecule inelastic collisions is investigated and ruled out.

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Penning ionization reactions in merged beams with precisely controlled collision energies have been shown to accurately probe quantum mechanical effects in reactive collisions. A complete microscopic understanding of the reaction is, however, faced with two major challenges-the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful.Penning ionization reactions in merged beams with precisely controlled collision energies have been shown to accurately probe quantum mechanical effects in reactive collisions. A complete microscopic understanding of the reaction is, however, faced with two major challenges-the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful.

arXiv:1601.00433The Journal of Physical Chemistry AJan 26, 20168 citations

We describe two-photon photoassociation (PA) experiments operated in an ultracold gas of metastable

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

atoms in the

2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}

state. Atom-molecule dark resonances as well as Raman signals provide information on the exotic molecule in the least bound vibrational

J=2

v=14

state of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ interaction potential of two spin-polarized metastable atoms. The physical origin of the various two-photon PA signals is first discussed. Their linewidths are interpreted in terms of processes limiting the lifetime $\ensuremath{\tau}$ of the exotic molecule. A value of $\ensuremath{\tau}=1.4\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$ is found, which significantly differs from two recent calculations of the Penning ionization rate induced by spin-dipole coupling [T. J. Beams et al., Phys. Rev. A 74, 014702 (2006); M. Portier and G. V. Shlyapnikov (unpublished)]. The limitation of $\ensuremath{\tau}$ by atom-molecule inelastic collisions is investigated and ruled out.

^{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.

We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable

(2\phantom{\rule{0.2em}{0ex}}^{3}S_{1})

helium atoms, considering both the fermionic

^{3}\mathrm{He}

and bosonic

^{4}\mathrm{He}

isotopes. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule, and quantum-statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable

^{4}\mathrm{He}

atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples that contain about $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.