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arXiv:1909.01694 · Physical Review A

Suppression of Penning ionization by orbital angular momentum conservation

Katrin Dulitz, Tobias Sixt, Jiwen Guan, Jonas Grzesiak, Markus Debatin, Frank Stienkemeier

Published Aug 27, 2020• 5 citations
arXivHTMLPDF

Abstract

The efficient suppression of autoionizing collisions is a stringent requirement to achieve quantum degeneracy in metastable rare gases. Here, we report on the suppression of Penning ionization in collisions between metastable He and laser-excited Li atoms. Our results show that the suppression is achieved by the conservation of both the total electron spin and $\mathrm{\ensuremath{\Lambda}}$, i.e., the projection of the total molecular orbital angular momentum along the internuclear axis. Our findings suggest that $\mathrm{\ensuremath{\Lambda}}$ conservation can be used as a more general means of reaction control, in particular, to improve schemes for the simultaneous laser cooling and trapping of metastable He and alkali-metal atoms.

DOI
10.1103/physreva.102.022818

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We demonstrate the control of 4He(23S1)–7Li(22S1/2) chemi-ionization reactions by all-optical electron-spin-state preparation of both atomic species prior to the collision process. Our results demonstrate that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving (spin-conserving) collisions at thermal energies. These findings are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the quasi-molecular states. Small deviations from the model indicate the contribution of the 4Σ+ channel to the reaction rate, which is in violation of spin conservation.
It is demonstrated that in a Penning collision of optically oriented metastable helium atoms with an impurity metal atom such as barium, both the longitudinal and transverse components of spin angular momentum are conserved. This results in a polarization of the ion which may be detected by optical emission in the case of excited ion levels or absorption for the ground states. A series of experiments is described which demonstrates the spin dependence of the Penning-collision process. The magnetic resonance of excited-state ions may be observed directly; from the resonance-linewidth radiative lifetimes may be measured.
We explore coherent control of Penning and associative ionization in cold collisions of metastable He∗(23S){\mathrm{He}}^{*}(2{\phantom{\rule{4pt}{0ex}}}^{3}S)He∗(23S) atoms via the quantum interference between different states of the He2∗{\mathrm{He}}_{2}^{*}He2∗​ collision complex. By tuning the preparation coefficients of the initial atomic spin states, we can benefit from the quantum interference between molecular channels to maximize or minimize the cross sections for Penning and associative ionization. In particular, we find that we can enhance the ionization ratio by 30% in the cold regime. This work is significant for the coherent control of chemical reactions in the cold and ultracold regimes.
In an ultracold, optically trapped mixture of 87Rb^{87}\mathrm{Rb}87Rb and metastable triplet 4He^{4}\mathrm{He}4He atoms we have studied trap loss for different spin-state combinations, for which interspecies Penning ionization is the main two-body loss process. We observe long trapping lifetimes for the purely quartet spin-state combination, indicating strong suppression of Penning-ionization loss by at least two orders of magnitude. For the other spin mixtures we observe short lifetimes that depend linearly on the doublet character of the entrance channel. We compare the extracted loss rate coefficient with recent predictions of multichannel quantum-defect theory for reactive collisions involving a strong exothermic loss channel and find near-universal loss for doublet scattering. Our work demonstrates control of Penning-ionization reactive collisions by internal atomic state preparation.
Transfer of spin polarization from optically pumped metastable helium atoms to the ions of Cd, Sr, and Zn following Penning collisions has been demonstrated indicating that a component of spin angular momentum is conserved. The ion polarization, detected optically, exceeds 10% at ion concentrations on the order of 1010{10}^{10}1010-1011{10}^{11}1011 ${\mathrm{cm}}^{\ensuremath{-}3}$.
We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable (23S1)(2\phantom{\rule{0.2em}{0ex}}^{3}S_{1})(23S1​) helium atoms, considering both the fermionic 3He^{3}\mathrm{He}3He and bosonic 4He^{4}\mathrm{He}4He 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 4He^{4}\mathrm{He}4He 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.