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Rethermalizing collisions between magnetically trapped metastable neon atoms

V. P. Mogendorff, BJ Bert Claessens, S. J. M. Kuppens, Ejd Edgar Vredenbregt, Herman C. W. Beijerinck

Published May 1, 2002• 0 citations
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Abstract

The successful Bose-Einstein Condensation (BEC) of alkali atoms has\nstimulated several groups to extend the range to metastable rare gas\natoms. In 2001 this resulted in the achievement of BEC with metastable\nhelium atoms (He^*) [1,2]. The only other rare gas atom suitable for\nachieving BEC is neon. The goal of our group is BEC of metastable neon\n(Ne^*). A condensate of metastable rare gas atoms is of special interest\nbecause their large internal energy enables real time diagnostics of the\nsample during condensation, giving new insight into the dynamics of the\nphase transition. Crucial for achieving BEC is a large (positive)\nscattering length. For He^* a good estimate of the order of magnitude of\nthe scattering length was known. For neon no such theoretical estimate\nexists, therefore, experimental data on the value of the scattering\nlength is necessary to determine whether or not BEC is feasible for\nNe^*. In recent thermalization experiments involving two RF-knives,\nsimilar to the scheme used by Aspect et al. [3], we have observed\nrethermalizing collisions in our magnetic trap. Moreover, preliminary\nmeasurements point in the direction of a large absolute value of the\nscattering length. In this presentation we will discuss these\nthermalization experiments, and hope to present a value of the\nscattering length. [1] A. Robert et al., Science 292, 461 (2001). [2]\nF.P. Dos-Santos et al., Phys. Rev. Lett. 86, 3459 (2001). [3] A.\nBrowaeys et al., Phys. Rev. A. 64, 034703 (2001).

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Herman C. W. Beijerinck, Edgar J. D. Vredenbregt, R. J. W. Stas +2 more

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 $\mathrm{Ne}{(}^{3}{\mathrm{P}}_{2})$ atoms in the fully aligned $|{J=2,m}_{J}=2〉$ 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 ${T=T}_{C}$ reduces the heating a hundredfold. Using a bright beam of laser cooled neon atoms, an initial population of $>~{10}^{10}$ 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.

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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.

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We investigate heteronuclear collisions in isotope mixtures of laser-cooled metastable (${}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2}$) neon. Experiments are performed with spin-polarized atoms in a magnetic trap for all two-isotope combinations of the stable neon isotopes ${}^{20}$Ne, ${}^{21}$Ne, and ${}^{22}$Ne. We determine the rate coefficients for heteronuclear ionizing collisions to ${\ensuremath{\beta}}_{21,20}=(3.9\ifmmode\pm\else\textpm\fi{}2.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm${}^{3}$/s, ${\ensuremath{\beta}}_{22,20}=(2.6\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm${}^{3}$/s, and ${\ensuremath{\beta}}_{21,22}=(3.9\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm${}^{3}$/s. We also study heteronuclear elastic collision processes and give upper bounds for heteronuclear thermal relaxation cross sections. This work significantly extends the limited available experimental data on heteronuclear ionizing collisions for laser-cooled atoms involving one or more rare gas atoms in a metastable state.

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Two groups, in Orsay and in Paris, have recently obtained experimental evidence for Bose-Einstein condensation on He4 atoms in the metastable state 23S1. The condensates which have been obtained are the first ones where atoms are condensed, not in their electronic ground state, but in an excited state, in this case a metastable state about 20 eV above the ground state. In principle, Penning collisions should very rapidly destroy the cloud of cold metastable atoms and it seems hopeless to observe Bose-Einstein condensation in such a system. But, as suggested by Shlyapnikov et al, the conservation of the total spin during the collision reduces the Penning collision cross-sections by several orders of magnitude and this explains why large enough densities of metastable atoms can be obtained to observe the condensation. We briefly describe the first experimental results which have been obtained in Paris. Atoms are optically detected by optical imaging at 1083 nm (wavelength of the transition connecting the metastable state 23S1 to the triplet state 23P2). The evolution of the anisotropy of the images during a ballistic expansion gives clear evidence for the condensation. Orders of magnitude are also obtained for the scattering length describing the collision between 2 metastable atoms. The large value of this scattering length (of the order of 10 nm) allows one to reach the so called hydrodynamic regime where the mean free path between 2 collisions in the thermal cloud becomes smaller than the size of the cloud. Preliminary results showing the transition between the collisionless regime and the hydrodynamic regime are presented and compared to theoretical predictions.

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We present an experimental study of the loading of a magneto-optical trap (MOT) from a brightened and slowed beam of metastable neon atoms. The unprecedented high numbers of $9\ifmmode\times\else\texttimes\fi{}{10}^{920}\mathrm{Ne}$ and $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ ${}^{22}\mathrm{Ne}$ metastable atoms are trapped under unconventional trap conditions as compared to metastable helium traps, such as low intensity and small detuning. These cause the MOT to have an extraordinarily large volume on the order of $1 {\mathrm{cm}}^{3}$ and a typical peak density of ${10}^{10}$ ${\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.$ A simple Doppler model is discussed which explains why the optimum is found under these conditions. The model includes the seventh beam necessary for the last slowing step before loading.

Physical Review A•Jan 15, 2002•21 citations
The successful Bose-Einstein Condensation (BEC) of alkali atoms has\nstimulated several groups to extend the range to metastable rare gas\natoms. In 2001 this resulted in the achievement of BEC with metastable\nhelium atoms (He^) [1,2]. The only other rare gas atom suitable for\nachieving BEC is neon. The goal of our group is BEC of metastable neon\n(Ne^). A condensate of metastable rare gas atoms is of special interest\nbecause their large internal energy enables real time diagnostics of the\nsample during condensation, giving new insight into the dynamics of the\nphase transition. Crucial for achieving BEC is a large (positive)\nscattering length. For He^* a good estimate of the order of magnitude of\nthe scattering length was known. For neon no such theoretical estimate\nexists, therefore, experimental data on the value of the scattering\nlength is necessary to determine whether or not BEC is feasible for\nNe^*. In recent thermalization experiments involving two RF-knives,\nsimilar to the scheme used by Aspect et al. [3], we have observed\nrethermalizing collisions in our magnetic trap. Moreover, preliminary\nmeasurements point in the direction of a large absolute value of the\nscattering length. In this presentation we will discuss these\nthermalization experiments, and hope to present a value of the\nscattering length. [1] A. Robert et al., Science 292, 461 (2001). [2]\nF.P. Dos-Santos et al., Phys. Rev. Lett. 86, 3459 (2001). [3] A.\nBrowaeys et al., Phys. Rev. A. 64, 034703 (2001).
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.
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.
In this thesis we investigate theoretically the cold collision properties of metastable neon (Ne*). The collisions of metastable rare gases (Rg*) exhibit a plethora of interesting phenomena: Due to the high internal excitation energies of the metastable atoms ionization processes are very likely. The produced ions can be detected with single-ion efficiency in the experiment. In experiments with atomic Rg* gases at cold temperatures T~1 mK, this allows for in-situ monitoring of the real-time dynamics of the atomic gas and for example for the study of two-particle correlations. Furthermore, in cold Rg* gases one can examine the prospects of creating a Bose-Einstein condensate or a degenerate Fermi gas of Rg*. Ne* is a promising candidate for Bose-Einstein condensation and in the group of G. Birkl the properties of cold gases of Ne* are investigated experimentally. Two-body loss rate coefficients due to the ionization processes and elastic cross sections have been measured. It is useful to complement the experimental measurements with a theoretical study in order to obtain a better understanding of the cold Ne* gases and the collision physics of Ne*. At the low temperatures of the atomic gases, these gases are dilute with particle number densities of n~10^9 cm^{-3}. For these low densities, the behavior of the gas is determined mainly by two-body collision physics which is solved by quantum scattering theory in terms of the S matrix. The collision physics is given by short-range interactions and long-range interactions of the atoms. The short-range and long-range interaction potentials of Ne* have been calculated in the molecular basis of Ne2. In this thesis we first calculate short-range and long-range interactions of atoms and molecular basis states of diatomic molecules with a simpler electronic structure than Ne* and Ne2 in order to introduce the notation and to discuss the characteristics of the calculated molecular interaction potentials and in order to employ them in the scattering calculations for Ne*. Instead of solving the full scattering equations by taking into account all the molecular interaction potentials, we demonstrate that the collision physics of Ne* can be described in terms of a coupled two-channel model with a single interaction potential only which describes the elastic scattering of Ne*. In this model, ionization in Ne* collisions is described by the transition of the upper elastic interaction channel to the lower channel, representing the loss or ionization channel. We introduce two versions of the two-channel model in this work. In the first version, the two channels are given by square-well potentials. We can solve the scattering equations analytically and for complex wave numbers k and study the solutions in the complex k plane. With an expansion of the S matrix in terms of its poles in the complex k plane we find a parametrization of the two-body loss rate coefficients. We show that these coefficients describe ionizing collisions of Ne* by comparing them to the experimental measurements and to the numerical results obtained by the two-channel model in the second version. In this second version of the two-channel model, the elastic scattering channel is given by a realistic interaction potential of Ne*, consisting of a calculated short-range and long-range molecular potential, and the loss channel by a model ionization potential. For this model we calculate the two-body loss rate coefficients and the elastic cross sections for the isotope mixtures of Ne* which have been measured experimentally and demonstrate that the free potential parameters can be optimized to the experimental data to achieve very good agreement of the numerical results with the experimental measurements. We discuss the validity of the two-channel model by comparing the results to existing models for cold Rg* collisions and find that the two-channel model of this work is a useful extension to these models.
We investigate heteronuclear collisions in isotope mixtures of laser-cooled metastable (3P2{}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2}3P2​) neon. Experiments are performed with spin-polarized atoms in a magnetic trap for all two-isotope combinations of the stable neon isotopes 20{}^{20}20Ne, 21{}^{21}21Ne, and 22{}^{22}22Ne. We determine the rate coefficients for heteronuclear ionizing collisions to ${\ensuremath{\beta}}_{21,20}=(3.9\ifmmode\pm\else\textpm\fi{}2.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s, ${\ensuremath{\beta}}_{22,20}=(2.6\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s, and ${\ensuremath{\beta}}_{21,22}=(3.9\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ cm3{}^{3}3/s. We also study heteronuclear elastic collision processes and give upper bounds for heteronuclear thermal relaxation cross sections. This work significantly extends the limited available experimental data on heteronuclear ionizing collisions for laser-cooled atoms involving one or more rare gas atoms in a metastable state.
Two groups, in Orsay and in Paris, have recently obtained experimental evidence for Bose-Einstein condensation on He4 atoms in the metastable state 23S1. The condensates which have been obtained are the first ones where atoms are condensed, not in their electronic ground state, but in an excited state, in this case a metastable state about 20 eV above the ground state. In principle, Penning collisions should very rapidly destroy the cloud of cold metastable atoms and it seems hopeless to observe Bose-Einstein condensation in such a system. But, as suggested by Shlyapnikov et al, the conservation of the total spin during the collision reduces the Penning collision cross-sections by several orders of magnitude and this explains why large enough densities of metastable atoms can be obtained to observe the condensation. We briefly describe the first experimental results which have been obtained in Paris. Atoms are optically detected by optical imaging at 1083 nm (wavelength of the transition connecting the metastable state 23S1 to the triplet state 23P2). The evolution of the anisotropy of the images during a ballistic expansion gives clear evidence for the condensation. Orders of magnitude are also obtained for the scattering length describing the collision between 2 metastable atoms. The large value of this scattering length (of the order of 10 nm) allows one to reach the so called hydrodynamic regime where the mean free path between 2 collisions in the thermal cloud becomes smaller than the size of the cloud. Preliminary results showing the transition between the collisionless regime and the hydrodynamic regime are presented and compared to theoretical predictions.
We present an experimental study of the loading of a magneto-optical trap (MOT) from a brightened and slowed beam of metastable neon atoms. The unprecedented high numbers of $9\ifmmode\times\else\texttimes\fi{}{10}^{920}\mathrm{Ne}$ and $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ 22Ne{}^{22}\mathrm{Ne}22Ne metastable atoms are trapped under unconventional trap conditions as compared to metastable helium traps, such as low intensity and small detuning. These cause the MOT to have an extraordinarily large volume on the order of 1cm31 {\mathrm{cm}}^{3}1cm3 and a typical peak density of 1010{10}^{10}1010 atoms/cm3.{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.atoms/cm3. A simple Doppler model is discussed which explains why the optimum is found under these conditions. The model includes the seventh beam necessary for the last slowing step before loading.