磁光阱中超冷钠-铯原子碰撞的实验研究

研究了磁光阱中异核超冷钠铯原子的碰撞机理, 测量了超冷钠原子的碰撞损失率, 得到了钠-铯原子的碰撞损失系数β_Na-Cs与钠原子俘获光强度之间的关系. 利用多普勒模型计算了不同俘获光强度下的钠原子磁光阱的阱深, 得到了临界光强的理论值, 与实验结果符合得较好.     

Magnetically tunable Feshbach resonances in ultracold Li-Yb mixtures

We investigate the possibility of forming Li+Yb ultracold molecules by magnetoassociation in mixtures of ultracold atoms. We find that magnetically tunable Feshbach resonances exist, but are extremely narrow for even-mass ytterbium isotopes, which all have zero spin. For odd-mass Yb isotopes, however, there is a new mechanism due to hyperfine coupling between the electron spin and the Yb nuclear magnetic moment. This mechanism produces Feshbach resonances for fermionic Yb isotopes that can be more than 2 orders of magnitude larger than for the bosonic counterparts.     

超冷温度下钾和铯原子间弹性散射特性的精确计算

本文分别用量子方法和半经典方法计算了超冷钾和铯原子之间弹性碰撞的s波散射长度,有效力程和p波散射长度等散射参数. 超冷温度下³⁹K-¹³³Cs原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现, 计算发现单重态和三重态截面分别存在显著的g波和d波形状共振.另外,本文应用简并内态近似方法获得了⁴¹K-¹³³Cs 超精细态相互作用时的s波散射长度.     

Metastable feshbach molecules in high rotational states

We experimentally demonstrate Cs2 Feshbach molecules well above the dissociation threshold, which are stable against spontaneous decay on the time scale of 1 s. An optically trapped sample of ultracold dimers is prepared in a high rotational state and magnetically tuned into a region with a negative binding energy. The metastable character of these molecules arises from the large centrifugal barrier in combination with negligible coupling to states with low rotational angular momentum. A sharp onset of dissociation with increasing magnetic field is mediated by a crossing with a lower rotational dimer state and facilitates dissociation on demand with a well-defined energy.     

The prospects of sympathetic cooling of NH molecules with Li atoms

We calculate the quartet potential energy surface for Li+NH and use it to calculate elastic and spin-relaxation cross sections for collisions in magnetically trappable spin-stretched states. The potential is strongly anisotropic but spin-relaxation collisions are still suppressed by centrifugal barriers when both species are in spin-stretched states. In the ultracold regime, both the elastic and inelastic cross sections fluctuate dramatically as the potential is varied because of Feshbach resonances. The potential-dependence is considerably reduced at higher energies. The major effect of using an unconverged basis set in the scattering calculations is to shift the resonances without changing their general behaviour. We have calculated the ratio of elastic and spin-relaxation cross sections, as a function of collision energy and magnetic field, for a variety of potential energy surfaces. Most of the surfaces produce ratios that are favorable for sympathetic cooling, at temperatures below about 20 mK.     

Molecule production via Feshbach resonance in bosonic systems

In this article, we review our recent theoretical works on producing ultracold molecules from ultracold bosonic atoms via magnetically tunable Feshbach resonances. Our analysis relies on a two-channel quantum microscopic model that accounts for many-body effects in the association process. We show that the picture of two-body molecular production depicted by the Landau-Zener model is significantly altered due to many-body effects. We derive an analytic expression for molecular conversion efficiency for the nonadiabatic linearly swept Feshbach resonance, that explains the discrepancy between the prediction of the Landau-Zener formula and the experimental data. With including the thermal dephasing effects in the oscillating magnetic field modulation Feshbach resoance, we reproduce the Lorentzian resonance lineshape and explain the maximum conversion efficiency observed in experiment.     

Ultracold RbSr molecules can be formed by magnetoassociation

We investigate the interactions between ultracold alkali-metal atoms and closed-shell atoms using electronic structure calculations on the prototype system Rb+Sr. There are molecular bound states that can be tuned across atomic thresholds with a magnetic field and previously neglected terms in the collision Hamiltonian that can produce zero-energy Feshbach resonances with significant widths. The largest effect comes from the interaction-induced variation of the Rb hyperfine coupling. The resonances may be used to form paramagnetic polar molecules if the magnetic field can be controlled precisely enough.     

Production and state-selective detection of ultracold RbCs molecules

Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state a (3)Sigma(+) via photoassociation in a laser-cooled mixture of 85Rb and 133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including a (3)Sigma(+), (2) (3)Sigma(+), and (1) (1)Pi. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of 10(6) s(-1).     

超冷极性分子

目前对超冷原子的研究已经从最初的原子分子物理扩展到了物理的很多分支.极性分子可以将电偶极相互作用引入到超冷体系,同时分子又与原子类似,可以灵活地被光和其他电磁场操控,因而很多理论工作都预言了超冷极性分子在超冷化学、量子模拟和量子信息等领域会有重要的应用.但由于超冷基态分子的制备非常困难,如何把超冷物理从原子发展到分子还是一个方兴未艾的课题.过去的10年间,各种分子冷却技术都取得了很大突破,本文回顾了这些进展,并着重介绍了基于异核冷原子的磁缔合结合受激拉曼转移这一技术,该技术在制备高密度的基态碱金属超冷极性分子上取得了较大的成功.本文也总结了超冷极性碱金属分子基本碰撞特性研究的一些实验结果.     

Mg-CO(X1Σ+)体系的冷碰撞动力学

应用量子散射动力学方法, 研究了电场条件下Mg-CO体系的冷碰撞动力学性质, 探索了外电场对碰撞体系低场追索态的弹性和非弹性截面的影响, 为碱土金属Mg原子感应冷却CO分子提供理论预测.     

Observation of optically induced feshbach resonances in collisions of cold atoms

We have observed optically induced Feshbach resonances in a cold ( <1 mK) sodium vapor. The optical coupling of the ground and excited-state potentials changes the scattering properties of an ultracold gas in much the same way as recently observed magnetically induced Feshbach resonances, but allows for some experimental conveniences associated with using lasers. The scattering properties can be varied by changing either the intensity or the detuning of a laser tuned near a photoassociation transition to a molecular state in the dimer. In principle this method allows the scattering length of any atomic species to be altered. A simple model is used to fit the dispersive resonance line shapes.     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

Production of ultracold, polar RbCs* molecules via photoassociation

We have produced ultracold, polar RbCs* molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs* molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound X(1)Sigma(+) ground-state RbCs vibrational levels as high as 5 x 10(5) s(-1) per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.     

Excessive levitation for the efficient loading of large-volume optical dipole traps

We study the excessive levitation effect in the magnetically levitated loading process of ultracold Cs atoms into a large-volume crossed optical dipole trap. We analyze the motion of atoms with a non-zero combined gravito-magnetic force during the loading, where the magnetically levitated force catches up with and surpasses the gravity. We present the theoretical variations of both acceleration and velocity with levitation time and magnetic field gradient. We measure the evolution of the number of trapped atoms with the excessive levitation time at different magnetic field gradients. The dependence of the number of atoms on the magnetic field gradient is also measured for different excessive levitation times.The theoretical analysis shows reasonable agreement with the experimental results. Our investigation illustrates that the excessive levitation can be used to reduce the heating effect of atoms in the magnetically levitated loading process, and to improve the loading rate of a large-volume optical dipole trap.     

High resolution feshbach spectroscopy of cesium

We measure high-resolution Feshbach resonance spectra for ultracold cesium atoms colliding in different hyperfine and magnetic sublevels. More than 25 resonances are observed for magnetic fields up to 230 G and their positions are measured with an accuracy down to 0.03 G. From these spectra several ground-state molecular interaction parameters can be extracted with sufficient accuracy to permit for the first time an unambiguous and accurate determination of cesium's ultracold collision properties [P. J. Leo, C. J. Williams, and P. S. Julienne, following Letter, Phys. Rev. Lett. 85, 2721 (2000)].     

A high phase-space density mixture of 87Rb and 133Cs: towards ultracold heteronuclear molecules

We report the production of a high phase-space density mixture of ⁸⁷Rb and ¹³³Cs atoms in a levitated crossed optical dipole trap as the first step towards the creation of ultracold RbCs molecules via magneto-association. We present a simple and robust experimental setup designed for the sympathetic cooling of ¹³³Cs via interspecies elastic collisions with ⁸⁷Rb. Working with the |F = 1,m _F = +1〉 and the |3, +3〉 states of ⁸⁷Rb and ¹³³Cs respectively, we measure a high interspecies three-body inelastic collision rate ～10^?25?10^?26 cm⁶ ? s^?1 which hinders the sympathetic cooling. Nevertheless by careful tailoring of the evaporation we can produce phase-space densities near quantum degeneracy for both species simultaneously. In addition we report the observation of an interspecies Feshbach resonance at 181.7(5) G and demonstrate the creation of Cs₂ molecules via magneto-association on the 4(g)4 resonance at 19.8 G. These results represent important steps towards the creation of ultracold RbCs molecules in our apparatus.     

Effect of external magnetic field on the shift of resonant frequency in photoassociation of ultracold Cs atoms

We study the influence of external magnetic field on the shift of the resonant frequency in the photoassociation of ultracold Cs atoms, which are captured in a magnetically levitated optical crossed dipole trap. With the increase of the photoassociation laser intensity, the linear variation of the frequency shift is measured by recording the photoassociation spectra of the long-range 0_u~+ state of Cs molecule below the 6S_(1/2)+ 6P_(1/2) dissociation limit at different magnetic fields.The slope of the frequency shift to the intensity of the photoassociation laser exhibits a strong dependence on the external magnetic field. The experimental data is simulated with an analytic theory model, in which a single channel rectangular potential with the tunable well depth is introduced to acquire the influence of the magnetic field on the atomic behavior in the effective range where photoassociation occurs.     

Observation of heteronuclear Feshbach molecules from a 85Rb-87Rb gas

We report on the observation of ultracold heteronuclear Feshbach molecules. Starting with a 87Rb Bose-Einstein condensate and a cold atomic gas of 85Rb, we utilize previously unobserved interspecies Feshbach resonances to create up to 25,000 molecules. Even though the 85Rb gas is nondegenerate, we observe a large molecular conversion efficiency due to the presence of a quantum degenerate 87Rb gas; this represents a key feature of our system. We compare the molecule creation at two different Feshbach resonances with different magnetic-field widths. The two Feshbach resonances are located at 265.44+/-0.15 G and 372.4+/-1.3 G. We also directly measure the small binding energy of the molecules through resonant magnetic-field association.     

Prospects for making polar molecules with microwave fields

We propose a mechanism to produce ultracold polar molecules with microwave fields. It converts trapped ultracold atoms into vibrationally excited molecules by a single microwave transition and entirely depends on the existence of a permanent dipole moment in the molecules. As opposed to production of molecules by photoassociation or magnetic-field Feshbach resonances, our method does not rely on properties of excited states or existence of Feshbach resonances. We determine conditions for optimal creation of polar molecules in vibrationally excited states of the ground-state potential by changing frequency and intensity of the microwave field. We also explore the possibility to produce vibrationally cold molecules by combining the microwave field with an optical Raman transition or by applying a microwave field to Feshbach molecules. The production mechanism is illustrated for KRb and RbCs.     

High resolution photoassociation spectra of an ultracold Cs2 long-range 0u＋ （6S1/2 ＋ 6P1/2） state＊

In this paper, ultracold cesium molecules are formed through photoassociation technology, which is carried out in a magneto-optical trap. High resolution photoassociaion spectra with the rotational progressions up to J = 7 are obtained. Three rovibrational levels of the long-range 0＋ state of Cs2 below the （6S1/2 ＋ 6P1/2） dissociation limit are specifically investigated. By fitting their binding energy intervals to the non-rigid rotational model, the rotational constant of the long- range 0u＋ state is determined. A proportional dependence of the value of the rotational constant on the vibrational quantum number is demonstrated.