Magnetically trapped neutral atoms can be cooled with the evaporation technique. This is typically done by using a radiofrequency (rf) field that adiabatically couples trapped and untrapped internal atomic states for atoms with kinetic energies above a value set by the field frequency. The rf-field can also induce nonadiabatic changes of internal atomic spin states (F,M) that lead to heating and enhanced loss of atoms. In this paper we use wave packet simulations to show that the evaporation process can induce these nonadiabatic transitions which change the internal spin state of doubly spin-polarized (2,2) trapped atoms. We also verify the validity of a multistate Landau-Zener model in describing the nonadiabatic dynamics. In addition, we calculate exchange relaxation rate coefficients for collisions between atoms in the (2,M) states of 23-Na atoms. Large exchange relaxation coefficients for 23-Na as compared to 87-Rb F=2 suggest that evaporative cooling of (2,2) Na will be more difficult than for the corresponding state of Rb.
Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields
我们研究了强磁场下塞曼位移的非线性对磁阱中射频场诱导蒸发冷却动力学的影响。我们证明了87-Rb和23-Na的F=2陷阱态的波包模拟如何冷却停止时，射频场频率低于一定的限制（85-Rb的F=2陷阱态的问题不会出现）。我们研究了强场情况下半经典模型的适用性，作为我们以前工作的扩展[Phys.Rev.A58，3983（1998）]。我们的研究结果证实了在最近的$^{87}$ Rb实验中观察到的许多方面[Phys. Rev. A 60，R1759（1999）]。
In recent years, cold atoms could prove their scientific impact not only on ground but in microgravity environments such as the drop tower in Bremen, sounding rockets and parabolic flights. We investigate the preparation of cold atoms in an optical dipole trap, with an emphasis on evaporative cooling under microgravity. Up to $ 1\times10^{6} $ rubidium-87 atoms were optically trapped from a temporarily dark magneto optical trap during free fall in the droptower in Bremen. The efficiency of evaporation is determined to be equal with and without the effect of gravity. This is confirmed using num…
High-precision nonadiabatic calculations of dynamic polarizabilities and hyperpolarizabilities for the lowlying vibrational-rotational states of hydrogen molecular ions
We present a novel optical cooling scheme that relies on hyperfine dark states to enhance loading and cooling atoms inside deep optical dipole traps. We demonstrate a seven-fold increase in the number of atoms loaded in the conservative potential with strongly shifted excited states. In addition, we use the energy selective dark-state to efficiently cool the atoms trapped inside the conservative potential rapidly and without losses. Our findings open the door to optically assisted cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low te…
In the present work, the effect of AC-Stark shift (i.e. light-shift) in optical dipole trap on in-situ absorption probe imaging of the trapped atoms has been investigated. We have calculated the light-shift of various energy levels of $^{87}$Rb atoms relevant for trapping in an optical dipole trap (ODT). The spatial varying intensity of the ODT beam results in position dependent light-shift (i.e. AC-Stark shift). Such modifications in the energy levels of the atom result in modification in the absorption cross-section, which finally modifies the optical density (OD) in in-situ absorption imagi…
We demonstrate a trap that confines polarizable particles around the antinode of a standing-wave microwave field. The trap relies only on the polarizability of the particles far from any resonances, so can trap a wide variety of atoms and molecules in a wide range of internal states, including the ground state. The trap has a volume of about 10 cm$^3$, and a depth approaching 1 K for many polar molecules. We measure the trap properties using $^{7}$Li atoms, showing that when the input microwave power is 610 W, the atoms remain trapped with a $1/e$ lifetime of 1.76(12) s, oscillating with an ax…
