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.
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, the study of cold gases of neon atoms in different metastable states is described. It contains measurements of the collisional parameters for both the 3s[3/2]2 and the 3s'[1/2]0 metastable state and the dependence of the inelastic loss on external fields. Furthermore, the investigation of frequency dependent laser-induced collisions, and the possibility to excite photoassociation resonances is presented. Based on previous measurements we have selected 22Ne for evaporative cooling. Although, we can experimentally achieve an increase in phase-space density with evaporative cooling, the relatively high inelastic collision parameters prevent the realization of a Bose-Einstein condensate in neon. For the measurements described here, neon atoms have been confined in a magneto-optical trap, in a magnetostatic trap, or in an optical dipole trap, respectively. By laser cooling inside the magnetic trap, atomic samples with more than 95 percent occupation of the magnetic substate m=+2 could be prepared. They have a typical temperature of 0.5 mK, central densities up to 1e11 cm^-3, and a central phase-space density of up to 2.2e-7. After loading the optical dipole trap from the magnetic trap, 2.5e6 atoms with typical temperatures of 0.1 mK, and central densities up to 5e10 cm^-3 were trapped. By evaporative cooling of the atoms in the magnetic trap we could increase the phase-space density by a factor of 200 to 5e-5. Also simulations of optimized evaporation for our experimental parameters show clearly that we are limited to a phase-space density on the order of 1e-5. From these simulations it became clear that a 5-fold increase in the good-to-bad ratio for evaporative cooling suffices to reach the quantum degenerate regime. Investigating the frequency dependence of laser-induced collisions did not reveal an experimental signature for the excitation of photoassociation resonances. For the 3D3 line a frequency dependence of laser enhanced Penning ionization was observed, which is interesting in itself. The absence of the collisional enhancement effect by laser light for the transition to the 3D2 line is intriguing and for an explanation calculations are required. Measurement of the two-body loss coefficient as function of the magnetic field showed a field dependence of the inelastic loss. These losses increase towards both small and large offset fields. In the magnetic trap, we are limited to offset fields < 50 G. In this range of fields, the two-body losses are too large to achieve a Bose-Einstein condensate of magnetically trapped metastable neon. The implementation of an optical dipole trap allowed us to trap the 3P0 metastable state. From the trap loss measurements we determined the two-body loss coefficient of the 3P0 metastable state for both bosonic isotopes 20Ne and 22Ne. For 20Ne we obtained s=6(+5,-4)e-10 cm^3/s and for 22Ne s=11(+7,-6)e-10 cm^3/s. These large two-body losses make it extremely unlikely to reach degeneracy with this metastable state. Nevertheless it is important that the 3P0 metastable state can be trapped to investigate other interesting physical effects. For example, it is essential to apply the STIRAP technique for trapped atoms and to realize the proposed coherent control of collisions. There is also a large interest in a precise determination of the lifetime of this metastable state, which is of importance for the verification of QED. We can also trap neon atoms in their energetically lowest magnetic substate 3P2(m=-2) with the perspective of reducing inelastic collisions in the energetically lowest state.
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) atoms in the fully aligned ∣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 reduces the heating a hundredfold. Using a bright beam of laser cooled neon atoms, an initial population of >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.
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.
This thesis presents investigations into the collision dynamics of cold metastable neon atoms in a magneto-optical trap (MOT). At very cold temperatures (approx. 1 mK) the atomic interaction occurs at long range and cannot be treated classically as the interaction between two nuclear point particles. In addition, the collision dynamics can be considerably altered in the presence of an optical eld. In this thesis results are presented for collisions that are altered by the presence of an optical eld and also collisions where the optical eld can be neglected. Controlled optical collisions of cold, metastable Ne atoms are performed by using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The control laser modies the rate of ionizing collisions by promoting a pair of colliding atoms onto an excited state potential. In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the eect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate. However, the measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules as predicted by Doery et al [1]. Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory [2]. Collision cross sections are measured for neon in the (3s)3P2 metastable state with ground state, thermal atoms and molecules. The technique is based on the measurement of the loss rate due to collisions with a background gas. Generally, these type of collisions are considered undesirable, however, it is possible to infer information about the collision cross section if the rate of collisions is accurately known. This novel technique is described and collision cross section measurements are presented for metastable neon in the (3s)3P2 state with He, Ne, Ar, H2, O2, and N2. The average thermal energy of the collision ranges from 11 meV to 27 meV and the measurements have relatively small errors, of the order of 10%.
We measure the rates of elastic and inelastic two-body collisions of cold spin-polarized neon atoms in the metastable 3P2 state for 20Ne and 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 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. 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 and $a=+{150}_{\ensuremath{-}50}^{+80}{a}_{0}$ for 22Ne, where a0=0.0529nm.