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arXiv:physics/9806018 · Physical Review A

Limit on suppression of ionization in metastable neon traps due to long-range anisotropy

M. R. Doery, Edgar J. D. Vredenbregt, S. S. Op de Beek, Herman C. W. Beijerinck, BOUDEWIJN J. VERHAAR

Published Nov 1, 1998• 23 citations
arXivHTMLPDF

Abstract

This paper investigates the possibility of suppressing the ionization rate in a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms. Suppression of the ionization is critical for the possibility of reaching Bose-Einstein condensation with such atoms. We estimate the relevant long-range interactions for the system, consisting of electric quadrupole-quadrupole and dipole-induced dipole terms, and develop short-range potentials based on the ${\mathrm{Na}}_{2}$ singlet and triplet potentials. The autoionization widths of the system are also calculated. With these ingredients we calculate the ionization rate for spin-polarized and for spin-isotropic samples, caused by anisotropy of the long-range interactions. We find that spin polarization may allow for four orders of magnitude suppression of the ionization rate for Ne. The results depend sensitively on a precise knowledge of the interaction potentials, however, pointing out the need for experimental input. The same model gives a suppression ratio close to unity for metastable xenon in accordance with experimental results, due to a much increased anisotropy in this case.

DOI
10.1103/physreva.58.3673

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arXiv:quant-ph/0502020•Physical Review Letters•Jun 7, 2005•25 citations
This paper investigates the possibility of suppressing the ionization rate in a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms. Suppression of the ionization is critical for the possibility of reaching Bose-Einstein condensation with such atoms. We estimate the relevant long-range interactions for the system, consisting of electric quadrupole-quadrupole and dipole-induced dipole terms, and develop short-range potentials based on the Na2{\mathrm{Na}}_{2}Na2​ singlet and triplet potentials. The autoionization widths of the system are also calculated. With these ingredients we calculate the ionization rate for spin-polarized and for spin-isotropic samples, caused by anisotropy of the long-range interactions. We find that spin polarization may allow for four orders of magnitude suppression of the ionization rate for Ne. The results depend sensitively on a precise knowledge of the interaction potentials, however, pointing out the need for experimental input. The same model gives a suppression ratio close to unity for metastable xenon in accordance with experimental results, due to a much increased anisotropy in this case.
The calculated upper limit ${\mathcal{K}}_{i}^{\mathrm{pol}}<~{10}^{\ensuremath{-}14}{\mathrm{cm}}^{3}/\mathrm{s}$ of the rate constant for suppressed ionization in a gas of metastable Ne(3P2)\mathrm{Ne}{(}^{3}{\mathrm{P}}_{2})Ne(3P2​) atoms in the fully aligned ∣J=2,mJ=2〉|{J=2,m}_{J}=2〉∣J=2,mJ​=2〉 state is used as input to investigate the prospects for achieving Bose-Einstein condensation (BEC). The heating rate of the trap population by secondary collisions of the hot products of the process of ionization---i.e., ground-state atoms, ions, and dimer-ions---with cold trapped metastable atoms is discussed in terms of a semiclassical model. An important step lies in limiting the depth of the magnetic trap to a value of a few millikelvin, to limit the range of small-angle scattering that contributes to heating. Also, a tight radial confinement reduces the probability for secondary collisions. At a trap depth of 10 mK, a radial dimension of $3\ensuremath{\mu}\mathrm{m},$ and a density of $2\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$ the heating rate is $1.4\ensuremath{\mu}\mathrm{K}/\mathrm{s},$ which should be compared to the transition temperature to BEC of $0.6\ensuremath{\mu}\mathrm{K}.$ The collisional heating is dominated by ion---metastable-atom collisions, due to their long-range charge-induced dipole interaction. Keeping the evaporative cooling switched on at T=TC{T=T}_{C}T=TC​ reduces the heating a hundredfold. Using a bright beam of laser cooled neon atoms, an initial population of > 1010>~{10}^{10}> 1010 atoms can be loaded into a magneto-optical trap in one second. Tight magnetic traps are easy to achieve for metastable neon atoms, due to their magnetic moment of $3{\ensuremath{\mu}}_{B}.$ We conclude that achieving BEC is feasible for metastable neon. This also holds for triplet metastable helium, once the loading rate of traps has been improved. Finally, the semiclassical model used for calculating the heating rate is applicable to a wide range of inelastic collisions in trapped alkali gases and/or collisions with background gas.
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.
Summary form only given. The understanding of the interaction of ultracold atoms will be substantially extended by the experimental study of ultracold non-alkaline systems. Due to the special internal structure and the high internal energy, metastable noble gases will play a crucial role. The objective of our work is to attain a detailed study of the atomic structure and the interactions in a gas of ultracold metastable neon atoms. Ensembles of high phase space density and, if possible, Bose condensates shall be achieved. The elementary pair interaction as well as the collective interactions are to be characterized. Of special interest is Penning ionization as it is the main loss process in dense metastable atom traps. The degree of its suppression in a spin polarized gas will strongly affect the feasibility of a quantum-degenerate system of metastable atoms. In addition, metastable noble gas atoms will allow the implementation of novel diagnostics for the investigation of Bose condensates due to their high detectability. We want to make use of this advantage in order to investigate higher order correlation functions and study the laser-like behavior of Bose condensates and atom-laser outputs. We have achieved loading of 4/spl times/10/sup 8/ atoms in a magneto-optical trap (MOT) and after efficient spin polarization of 2.5 /spl times/ 10/sup 8/ atoms in a Ioffe-Pritchard-type magnetic trap. We present a precision measurement of the lifetime of the 3s[3/2]/sub 2/-metastable state of neon which we could determine to 14.70(13)s. We report on the status of the determination of the elastic scattering rate, the rates of Penning ionization for spin-polarized and non-spin-polarized atom samples, and the implementation of evaporative cooling.
Spin-dipole mediated interactions between tightly confined metastable helium atoms couple the spin-polarized quintet ${^{5}\ensuremath{\sum}}_{g}^{+}$ state to the singlet ${^{1}\ensuremath{\sum}}_{g}^{+}$ state from which autoionization is highly probable, resulting in finite lifetimes for the trap eigenstates. We extend our earlier study on spherically symmetric harmonic traps to the interesting cases of axially symmetric anisotropic harmonic traps and report results for the lowest 10 states in ``cigarlike'' and ``pancakelike'' traps with average frequencies of 100 kHz and 1 MHz. We find that there is a significant suppression of ionization, and subsequent increase in lifetimes, at trap aspect ratios A=p/qA=p/qA=p/q, where ppp and qqq are integers, for those states that are degenerate in the absence of collisions or spin-dipole interactions.
We consider the decay kinetics of a trapped spin-polarized gas of metastable triplet helium 4He(23S^{4}\mathrm{He}(2^{3}S4He(23S at ultralow temperatures. The sample lifetime is found to be determined by spin relaxation and Penning ionization, both induced by spin-dipole interaction in pair collisions. The rates of these processes are calculated. The Penning ionization proves to be 5 orders of magnitude slower than in the unpolarized case. The results indicate that spin-polarized triplet helium is a promising candidate for Bose-Einstein condensation.
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.