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Physical Review A

Trap loss in a two-species Na-Cs magneto-optical trap: Intramultiplet mixing in heteronuclear ultracold collisions

James P. Shaffer, Witek Chalupczak, N. P. Bigelow

Published Nov 1, 1999• 39 citations
Publisher

Abstract

We describe a comprehensive investigation of trap loss in a two-species Na-Cs magneto-optical trap. Observed losses, due to the interspecies interactions, are attributed to collisions in which a change in the fine-structure state of the Na partner causes the escape of atoms from the trap. Results are described in terms of the heteronuclear pair potentials and the interaction of the colliding pairs with the radiatively active environment.

DOI
10.1103/physreva.60.r3365

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•Jan 1, 1999•28 citations
We describe a comprehensive investigation of trap loss in a two-species Na-Cs magneto-optical trap. Observed losses, due to the interspecies interactions, are attributed to collisions in which a change in the fine-structure state of the Na partner causes the escape of atoms from the trap. Results are described in terms of the heteronuclear pair potentials and the interaction of the colliding pairs with the radiatively active environment.
Trap-loss rates due to hyperfine--structure-changing collisions involving cold Na and K atoms in a mixed species magneto-optical trap have been measured. The investigations were performed in the low intensity regime, where hyperfine--structure-changing collision are believed to be the dominant mechanism for atomic losses. Comparison between homonuclear (Na-Na) and heteronuclear (Na-K) collision rate coefficients reveals a higher loss rate in the case of heteronuclear collisions. Results obtained in the regime of low trap laser intensity are compared with existent loss rates for several alkali metals, showing themselves to be in good agreement with those expected.
The production and research of ultracold heteronuclear molecules have aroused the great interest recently. On the one hand, these molecules are extremely popular in experiments for exploring the collision dynamic behaviors in threshold, photoassociative spectrum and strong dipole-dipole interactions. On the other hand, ultracold polar molecules populated at deeply bound levels in the singlet ground state are the right candidates to investigate quantum memories for quantum simulation, and to study strongly interacting quantum degenerate gases. The precise knowledge of cold collision processes between two different types of alkali atoms is necessary for understanding and utilizing ultracold heteronuclear molecules, sympathetic cooling, and thus formation of two species BEC. The goal of the present investigation is to study the collisions between ultracold sodium atoms and cesium atoms. We systematically demonstrate simultaneously trapping ultracold sodium and cesium atoms in a dual-species magneto-optical trap (MOT). The sodium atom trap loss rate coefficient Na-Cs is measured as a function of Na trapping laser intensity. At low intensities, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities, collisions involving excited atoms are more important. A strong interspecies collision-induced loss for Na atoms in the MOT is observed. In order to obtain the trap loss coefficient Na-Cs, we proceed in two steps. First, the Cs repumping laser is blocked to avoid the formation of ultraold Cs atoms. The loading process of Na atoms is recorded when the Cs trapping laser is on. Second, the loading curves of the Na atoms are obtained as Cs atoms are present with the repumping laser beams. The total losses PNa and PNa' are acquired by fitting the two loading curves of trapped Na atoms. Thus, the trap loss coefficient Na-Cs can be derived from the difference between total losses PNa and PNa' divided by the density of the Cs atoms. The coefficient Na-Cs decreases in a range of 5-10mW/cm2, which originates from the hyperfine-state changing (HFC) collision. A Doppler model is used to calculate the Na atom trap depth, in that the atom trap depth and exoergic energy determine the behavior of the collisional trap loss rate coefficient. The three corresponding calculated critical intensities of Na trapping laser are 7.98, 14.82, 16.2 mW/cm2 respectively in the Na-Cs HFC collision process. The first calculated critical intensity value agrees well with the experimental result. Our work provides a valuable insight into HFC collision between Na and Cs atoms and also paves the way for the production of ultracold NaCs molecules using Photoassociation (PA) technology. Furthermore, the experimental results lay a great basis for the obtainments of high sensitive heteronuclear NaCs molecular PA spectrum and the creation of deeply bound ground state molecules.
We have previously reported experimental results on the trap-loss rate for a mixed species magneto-optical trap involving a pair of Na and Rb atoms [Phys. Rev. A 59, R23 (1999)]. The predicted dependence for the heteronuclear trap-loss rate as a function of laser detuning assuming a colliding pair of ground-state--excited-state atoms fails to explain the experimental results. Further investigation has shown that our observations are consistent if one assumes a loss channel involving an excited state--excited-state colliding pair of atoms. Here we present the model and discuss the physical interpretation applied to our previous experimental results. The present interpretation opens up other ways to understand the trap-loss mechanisms.
Storing and cooling simultaneously lithium and sodium atoms in a two-species magneto-optical trap, rate coefficients for collisions between these two atomic species were determined. The experiments were performed at different trap depths, thus for Li atoms the trap losses due to finestructure changing collisions could be distinguished from losses due to radiative escape. The rate coefficient for collisions between sodium and lithium atoms shows also a step when increasing the trap depth. The formation of cold heteronuclear molecules is claimed.
We investigate collisional properties of lithium and cesium which are simultaneously confined in a combined magneto-optical trap. Trap-loss collisions between the two species are comprehensively studied. Different inelastic collision channels are identified, and inter-species rate coefficients as well as cross sections are determined. It is found that loss rates are independent of the optical excitation of Li, as a consequence of the repulsive Li∗^*∗-Cs interaction. Li and Cs loss by inelastic inter-species collisions can completely be attributed to processes involving optically excited cesium (fine-structure changing collisions and radiative escape). By lowering the trap depth for Li, an additional loss channel of Li is observed which results from ground-state Li-Cs collisions changing the hyperfine state of cesium.