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Physical Review A

Suppression of Penning ionization by spin polarization of cold He(23S) atoms

Norbert Herschbach, Paul J. J. Tol, W. Hogervorst, W. Vassen

Published Apr 13, 2000• 27 citations
Publisher

Abstract

We measure the suppression of Penning ionization in an ultracold $\mathrm{He}(2{}^{3}{\mathrm{S}}_{1})$ atomic cloud after spin-polarizing the atoms by optical pumping in a small magnetic field. An upper limit of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} {\mathrm{cm}}^{3}/\mathrm{s}$ for the ionization rate constant is deduced, at least a factor 20 lower than measured in an unpolarized gas.

DOI
10.1103/physreva.61.050702

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We measure the suppression of Penning ionization in an ultracold He(23S1)\mathrm{He}(2{}^{3}{\mathrm{S}}_{1})He(23S1​) atomic cloud after spin-polarizing the atoms by optical pumping in a small magnetic field. An upper limit of 6\ifmmode×\else\texttimes\fi10\ensuremath−12cm3/s6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} {\mathrm{cm}}^{3}/\mathrm{s}6\ifmmode×\else\texttimes\fi10\ensuremath−12cm3/s for the ionization rate constant is deduced, at least a factor 20 lower than measured in an unpolarized gas.
This paper presents the first study of the collision dynamics of an ultra-cold spin-polarized mixture of rubidium and metastable helium (He * ) atoms. Our experiment monitors ion production from the mixture for both magnetically polarized and unpolarized cases. In the unpolarized case, we observe an increase in our background ion rate. However, in the completely polarized sample the ion production is below the sensitivity of our experiment. Nonetheless, we determine an upper limit of 5 × 10 −12 cm 3 s −1 for the polarized rate constant (β Rb-He * ), which is two orders of magnitude below the unpolarized rate constant. Such a suppression of the He * – 87 Rb polarized rate was not apparent a priori and opens the intriguing possibility of creating a dual Bose–Einstein condensate comprising an alkali ground-state atom and an excited-state noble-gas atom.
We measure the rates of elastic and inelastic two-body collisions of cold spin-polarized neon atoms in the metastable 3P2^{3}P_{2}3P2​ state for 20Ne^{20}\mathrm{Ne}20Ne and 22Ne^{22}\mathrm{Ne}22Ne in a magnetic trap. From particle loss, we determine the loss parameter of inelastic collisions $\ensuremath{\beta}=6.5(18)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ for 20Ne^{20}\mathrm{Ne}20Ne and $\ensuremath{\beta}=1.2(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ for 22Ne^{22}\mathrm{Ne}22Ne. These losses are caused by ionizing (i.e., Penning) collisions and occur less frequently than for unpolarized atoms. This proves the suppression of Penning ionization due to spin polarization. From cross-dimensional relaxation measurements, we obtain elastic scattering lengths of $a=\ensuremath{-}180(40){a}_{0}$ for 20Ne^{20}\mathrm{Ne}20Ne and $a=+{150}_{\ensuremath{-}50}^{+80}{a}_{0}$ for 22Ne^{22}\mathrm{Ne}22Ne, where a0=0.0529  nm{a}_{0}=0.0529\text{ }\text{ }\mathrm{nm}a0​=0.0529  nm.
We have measured ultracold ionizing collision rates for three bosonic (132Xe,134Xe,{(}^{132}\mathrm{Xe},{}^{134}\mathrm{Xe},(132Xe,134Xe, and 136Xe){}^{136}\mathrm{Xe})136Xe) and two fermionic (129Xe{(}^{129}\mathrm{Xe}(129Xe and 131Xe){}^{131}\mathrm{Xe})131Xe) isotopes of xenon in the 6s[3/2]26s[3/2{]}_{2}6s[3/2]2​ metastable state, for both spin-polarized and unpolarized samples. For unpolarized samples at temperatures above the p-wave centrifugal barrier $(\ensuremath{\sim}39 \ensuremath{\mu}\mathrm{K}),$ we find that collision rates for all isotopes are identical. Quantum-statistical effects forbid s-wave collisions for spin-polarized fermions, giving rise to significant differences between bosonic and fermionic isotopes below the p-wave barrier. We present a technique for measuring collision rates at temperatures below $1 \ensuremath{\mu}\mathrm{K},$ and find that the ratio of polarized to unpolarized collision rates for fermions decreases by a factor of 2 at low temperatures, while the ratio for bosons increases by 5050%.50 We find no evidence of an overall reduction in the collision rate for spin-polarized samples, as has been observed in metastable helium. These results are explained using a simple theoretical model of transmission and quantum reflection off long-range interatomic potentials.
By confining 3He{}^{3}\mathrm{He}3He and 4He{}^{4}\mathrm{He}4He in their respective magneto-optical traps, ionization rate coefficients for Penning collisions between two cold He(2s3S1)\mathrm{He}(2s^{3}S_{1})He(2s3S1​) atoms have been measured. At 0.5 mK, the rate coefficients obtained are $(3.8\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}$ for 4He{}^{4}\mathrm{He}4He- 4He{}^{4}\mathrm{He}4He collisions and $(1.1\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ for 3He{}^{3}\mathrm{He}3He- 3He{}^{3}\mathrm{He}3He, a difference of a factor of 3. Theoretical calculations have shown that such a large difference arises only when the lowest order partial wave alone contributes to the ionization, and that it is mainly due to the difference in the quantum statistical symmetry between 3He{}^{3}\mathrm{He}3He and 4He{}^{4}\mathrm{He}4He.
A quantum-mechanical close-coupled calculation for collisions of spin-polarized triplet helium atoms in a magnetostatic trap is presented in which the Penning and associative ionization processes are represented by a complex optical potential. The rate coefficients for the spin relaxation and relaxation-induced Penning ionization processes for a range of magnetic fields and temperatures, spanning the cold and ultracold temperature regimes, have been calculated and the sensitivity of these results to uncertainties in the molecular potentials and autoionization width investigated. The rate constants for both elastic and inelastic processes were found to be particularly sensitive to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and, to a lesser extent, the rate constants for relaxation-induced Penning ionization displayed sensitivity to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization width.