Summary form only given. In a magneto-optical trap (MOT) two main mechanisms contribute to the total trap loss rate. Radiative escape (RE), where the spontaneous emission within the collision duration transforms internal energy into kinetic motion. Fine-structure changing collision (FSC) is an exoergic process where a pair of excited ground state atoms undergoes a change to the lower state of the fine-structure doublet, followed by a release of /spl Delta/E/sub FSC//2 in kinetic energy normally enough to eject the atoms from the trap. In earlier experiments only the total trap loss rate, which is the sum of contributions from RE and FSC, was determined. Distinguishing between these two mechanisms is important because it allows a more precise comparison with the theory, leading to a better understanding of exoergic cold collisions. In this experiment the FSC contribution to the total loss in /sup 85/Rb is measured by direct photoionization of the P/sub 1/2/ fragments.
Measurements of the trap loss rate in ultra-cold magneto-optically trapped
sp6Li and
sp7Li are presented and compared. Clear evidence is presented for two different trap loss mechanisms involving inelastic collisions between one ground state atom and one excited state atom. The fine-structure changing (FSC) mechanism, in which the excited state atom changes fine-structure level during a collision, is seen to dominate the rate at low trap laser intensities. However, as the intensity is increased, the trap becomes sufficiently deep to be able to contain the products of FSC collisions and this mechanism no longer contributes to trap loss. It is a unique aspect of Li that the energy imparted to an atom in a FSC collision is in a range that may be recaptured with experimentally attainable trap parameters. When the products of all FSC collisions remain trapped, only the radiative escape (RE) trap loss mechanism, due to the emission of a less energetic photon than the initial excitation photon, contributes to the trap loss rate. At small detunings, the rate of the RE trap loss mechanism is seen to be over two orders-of-magnitude smaller than the FSC rate. The FSC trap loss rate for
sp6Li and
sp7Li were found to be largest at smaller detunings and of comparable magnitude in
sp6Li and
sp7Li. The RE trap loss rate in
sp6Li was seen to be roughly four times the RE trap loss rate in
sp7Li. To understand better the dynamics of trap loss and magneto-optical traps in general, a sophisticated model of magneto-optical trap kinetics has been developed. This model has demonstrated that most critical factor determining the maximum velocity an atom may have and yet remained trapped is the initial atom's frame detuning,
Delta - k v, where
Delta is the trap laser detuning, k is the propagation vector of the trap beam most nearly anti-parallel to v, the atom's velocity. All else being equal, minimizing the magnitude of this quantity maximizes the velocity that a trap will retain. More detailed results of this model are also presented.
The process of collisional trap loss in magneto-optically trapped 7Li shows a rich dependence on the depth of the trapping potential, ET. When the trap is shallow enough to allow the escape of atoms with kinetic energy equal to one-half the fine-structure splitting, ΔEfs, fine-structure changing collisions [FS] dominate the collisional trap loss rate. However when the trap is deeper than 1 2 Δ E FS, only the radiative escape mechanism [RE] contributes to trap loss. In this regime, Julienne and Vigue predicted that the collisional loss rate will depend on ET as ~ET-17/6.1 We have measured the collisional loss rate, β, of 7Li from a beam-loaded magneto-optical trap.2 The circles in Fig. 1 show the data for four different trap laser detunings, Δ, and a range of trap laser intensities, 1. The solid line in Fig. 1 is the result of a semi-classical, optical Bloch equation calculation of the RE trap loss rate3 using as input ET, calculated from a truly three-dimensional model.4 The trap depth model predicts that ET is anisotropic. The dashed line in Fig. 1 shows the intensity at which the model predicts E T > 1 2 Δ E FS in all directions, for each detuning. The theory predicts the magnitude of the RE trap loss rate accurately near the point at which the fine structure mechanism turns off, where the trap depth is well known. However at larger detunings and intensities the calculations deviate from the measured values. This suggests that the trap depth model overestimates E, at higher intensities and detunings, or that the trap depth dependence of the RE loss rate deviates from ET-17/6.
An exoergic collision converts internal atomic energy to kinetic energy of the colliding species. When there is only one species in the trap (the usual case) this kinetic energy is equally divided between the two partners. If the net gain in kinetic energy exceeds the trapping potential or the ability of the trap to recapture, the atoms escape; and the exoergic collision leads to trap loss. Of the several trapping possibilities described in Chapter 3, by far the most popular choice for collision studies has been the magneto-optical trap (MOT). A MOT uses spatially dependent resonant scattering to cool and confine atoms. If these atoms also absorb the trapping light at the initial stage of a binary collision and approach each other on an excited molecular potential, then during the time of approach the colliding partners can undergo a fine-structure-changing collision (FCC) or relax to the ground state by spontaneously emitting a photon. In either case electronic energy of the quasimolecule converts to nuclear kinetic energy. If both atoms are in their electronic ground states from the beginning to the end of the collision, only elastic and hyperfine changing collisions (HCC) can take place. Elastic collisions (identical scattering entrance and exit states) are not exoergic but figure importantly in the production of Bose–Einstein condensates (BEC). At the very lowest energies only s-waves contribute to the elastic scattering, and in this regime the collisional interaction is characterized by the scattering length.
We have successfully trapped
sp7Li in a vapor cell magneto-optical trap (MOT). The trap is studied in detail and the parameters are measured from the experiment. The trapping mechanism has been understood in terms of the radiation force and a simple one dimensional model is used in interpreting and predicting the experimental results. The cold-cold atom collision rate coefficient is measured in a beam MOT. The two collision mechanisms, fine structure changing and radiative escape, are discussed in a molecular picture. One of the channels, the fine structure changing channel, can be turned on and off by the experimental conditions, namely the laser intensity in this experiment. The rate coefficient for each mechanism has been determined from the experiment.
Collisions between ultra-cold (T << 1K) atoms confined in a magneto- optical trap (MOT) may result in the ejection of atoms from the trap. These collisional loss processes, which involve ground state/excited state collisions, fundamentally limit the density attainable in MOT traps. Although accurate calculations of collision rates between ultra-cold atoms require precise knowledge of the molecular potentials over a long range, the loss rate for Li was recently predicted to be significantly less than for the other alkali metals. Motivated by this result, we are measuring the collisional loss rate of Li in a vapor-cell MOT using a red-detuned laser beam to catalyze the collisions, similar to recent experiments using Rb and Cs. We will report our progress to measure the trap loss rate for {sup 6}Li and {sup 7}Li, which will be compared to investigate an isotopic dependence, as was recently observed for Rb.
The authors have designed and assembled an experiment to determine the rate of fine structure changing collisions between ultra-cold ({approximately} 1 mK) laser cooled Li atoms. The atoms are confined by a magneto-optical trap which consists of six polarized orthogonal laser beams tuned slightly below the 2S{sub 1/2}-2P{sub 3/2} resonance frequency of lithium. Measurements show that about 2 x 10{sup 7} atoms are confined to a roughly spherical region of about 1 mm in diameter. Fine structure changing collisions occur when an atom in the 2S{sub 1/2} state and an atom in the 2P{sub 3/2} state collide, and yield an atom in the 2S{sub 1/2} state and an atom in the 2P{sub 1/2} state, with an energy release corresponding to about 10 GHz. This energy adds kinetic energy to the atoms in the trap, and knocks atoms out of the trap. The authors have devised a method to measure the rate of this collisional loss mechanism. The method uses a laser diode and a dye laser to selectively photo-ionize the 2P{sub 1/2} atoms, and a channeltron particle multipiler measures the rate of ion formation. We will report the progress of this experiment.