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Physical Review A

Close-coupled calculation of collisions of magnetostatically trapped metastable helium atoms

V. Venturi, Ian B. Whittingham, Paul J. Leo, G. Peach

Published Dec 1, 1999• 45 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.60.4635

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arXiv:quant-ph/0502020•Physical Review Letters•Jun 7, 2005•25 citations
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\ensuremathΣg+{}^{5}{\ensuremath{\Sigma}}_{g}^{+}5\ensuremathΣg+​ 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\ensuremathΣg+{}^{1}{\ensuremath{\Sigma}}_{g}^{+}1\ensuremathΣg+​ molecular potential and the autoionization width.
A quantum-mechanical close-coupled calculation of field-free collisions of cold metastable helium atoms has been performed. Theoretical predictions of the rate coefficients for elastic processes and losses due to the Penning and associative ionization processes are reported for temperatures ranging from 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. The sensitivity of the rate constants to uncertainties in the molecular potentials and autoionization widths has been investigated. The total ionization loss rates were found to be sensitive to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization widths, while the total elastic rate constants exhibit greater sensitivity to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential.
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 4He∗+4He∗^{4}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}4He∗+4He∗ and fermionic 3He∗+3He∗^{3}\mathrm{He}^{*}+^{3}\mathrm{He}^{*}3He∗+3He∗ systems, and heteronuclear collisions of the mixed 3He∗+4He∗^{3}\mathrm{He}^{*}+^{4}\mathrm{He}^{*}3He∗+4He∗ 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.
A quantum-reflection model is employed to calculate the rate coefficient for the Penning ionization of rubidium by cold metastable helium atoms in unpolarized collisions. Various results for the p-wave transmission coefficient on a 1/R-6 long-range potential are discussed. The dispersion coefficient for the interaction of these atoms is estimated as 3540 au. The calculated value of the rate coefficient is about four times lower than the recently measured value (Byron et al 2010a Phys. Rev. A 81 013405) for this process when both atoms are held in magneto-optical traps.
We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.
We calculate the rates of elastic and inelastic collisions in a gas of spin-polarized triplet helium (4{(}^{4}(4He∗{\mathrm{He}}^{\mathrm{*}}He∗\ensuremath{\uparrow}) in a wide range of temperatures (0T0.5 K) and magnetic fields (0B100 kG), which includes all values of T and B relevant for trapped 4He∗^{4}\mathrm{He}^{\mathrm{*}}4He∗\ensuremath{\uparrow}. At densities n\ensuremath{\lesssim}1013{10}^{13}1013 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, where three-body recombination is not important, the main inelastic processes are spin relaxation and relaxation-induced ionization, both induced by the spin-dipole interaction in pair collisions. For ultralow temperatures T\ensuremath{\lesssim}10 mK and low magnetic fields B\ensuremath{\lesssim}100 G, the leading decay channel is relaxation-induced ionization. If either T or B is higher, the gas decays through spin relaxation. The ratio of elastic to inelastic rate is found to be large even in the nanokelvin regime, which is promising for evaporative cooling of 4He∗^{4}\mathrm{He}^{\mathrm{*}}4He∗\ensuremath{\uparrow}. \textcopyright{} 1996 The American Physical Society.
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.