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arXiv:2008.08396 · Physical Review A

Measurement of collisions between laser-cooled cesium atoms and trapped cesium ions

Sourav Dutta, S. A. Rangwala

Published Sep 8, 2020• 4 citations
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Abstract

We report the measurement of collision rate coefficient for collisions between ultracold Cs atoms and low-energy $\mathrm{C}{\mathrm{s}}^{+}$ ions. The experiments are performed in a hybrid trap consisting of a magneto-optical trap (MOT) for Cs atoms and a Paul trap for $\mathrm{C}{\mathrm{s}}^{+}$ ions. The ion-atom collisions impart kinetic energy to the ultracold Cs atoms, resulting in their escape from the shallow MOT and, therefore, in a reduction in the number of Cs atoms in the MOT. By monitoring, using fluorescence measurements, the Cs atom number, and the MOT loading dynamics and then fitting the data to a rate equation model, the ion-atom collision rate is derived. The $\mathrm{Cs}\ensuremath{-}\mathrm{C}{\mathrm{s}}^{+}$ collision rate coefficient $9.3(\ifmmode\pm\else\textpm\fi{}0.4)(\ifmmode\pm\else\textpm\fi{}1.2)(\ifmmode\pm\else\textpm\fi{}3.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, measured for an ion distribution with most probable collision energy of 95 meV $(\ensuremath{\approx}{\mathrm{k}}_{\mathrm{B}}\phantom{\rule{4pt}{0ex}}1100\phantom{\rule{4pt}{0ex}}\mathrm{K})$, is in fair agreement with theoretical calculations. As an intermediate step, we also determine the photoionization cross section of Cs $6{P}_{3/2}$ atoms at 473 nm wavelength to be $2.28(\ifmmode\pm\else\textpm\fi{}0.33)\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}$.

DOI
10.1103/physreva.102.033309

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We report the measurement of collision rate coefficient for collisions between ultracold Cs atoms and low-energy Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ ions. The experiments are performed in a hybrid trap consisting of a magneto-optical trap (MOT) for Cs atoms and a Paul trap for Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ ions. The ion-atom collisions impart kinetic energy to the ultracold Cs atoms, resulting in their escape from the shallow MOT and, therefore, in a reduction in the number of Cs atoms in the MOT. By monitoring, using fluorescence measurements, the Cs atom number, and the MOT loading dynamics and then fitting the data to a rate equation model, the ion-atom collision rate is derived. The Cs\ensuremath−Cs+\mathrm{Cs}\ensuremath{-}\mathrm{C}{\mathrm{s}}^{+}Cs\ensuremath−Cs+ collision rate coefficient 9.3(\ifmmode±\else\textpm\fi0.4)(\ifmmode±\else\textpm\fi1.2)(\ifmmode±\else\textpm\fi3.5)\ifmmode×\else\texttimes\fi10\ensuremath−14m3s\ensuremath−19.3(\ifmmode\pm\else\textpm\fi{}0.4)(\ifmmode\pm\else\textpm\fi{}1.2)(\ifmmode\pm\else\textpm\fi{}3.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}9.3(\ifmmode±\else\textpm\fi0.4)(\ifmmode±\else\textpm\fi1.2)(\ifmmode±\else\textpm\fi3.5)\ifmmode×\else\texttimes\fi10\ensuremath−14m3s\ensuremath−1, measured for an ion distribution with most probable collision energy of 95 meV (\ensuremath≈kB1100K)(\ensuremath{\approx}{\mathrm{k}}_{\mathrm{B}}\phantom{\rule{4pt}{0ex}}1100\phantom{\rule{4pt}{0ex}}\mathrm{K})(\ensuremath≈kB​1100K), is in fair agreement with theoretical calculations. As an intermediate step, we also determine the photoionization cross section of Cs 6P3/26{P}_{3/2}6P3/2​ atoms at 473 nm wavelength to be 2.28(\ifmmode±\else\textpm\fi0.33)\ifmmode×\else\texttimes\fi10\ensuremath−21m22.28(\ifmmode\pm\else\textpm\fi{}0.33)\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}2.28(\ifmmode±\else\textpm\fi0.33)\ifmmode×\else\texttimes\fi10\ensuremath−21m2.
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We measure the collision-rate coefficient between laser-cooled rubidium (Rb) atoms in a magneto-optical trap (MOT) and optically dark Rb+{}^{+}+ ions in an overlapping Paul trap. In such a mixture, the ions are created from the MOT atoms and allowed to accumulate in the ion trap, which results in a significant reduction in the number of steady-state MOT atoms. A theoretical rate-equation model is developed to describe the evolution of the MOT atom number due to ionization and ion-atom collisions, and we derive an expression for the ion-atom collision-rate coefficient. The loss of MOT atoms is studied systematically by sequentially switching on the various mechanisms in the experiment. Combining the measurements with the model allows the direct determination of the ion-atom collision-rate coefficient. Finally, the scope of the experimental technique developed here is discussed.
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 report the measurement of the absolute photoionization cross section for the 6P3/2{6P}_{3/2}6P3/2​ state of cesium. Cross sections were determined by measuring the photoionization rate of cesium atoms confined in a magneto-optical trap. The photoionization rate was measured by monitoring the decay of trap fluorescence during exposure to ionizing laser radiation. The measurements were made in the wavelength range 457.9 to 501.7 nm using several lines from an Ar-ion laser. Our results are compared to other experimental measurements as well as to existing theories.
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We have measured the elastic collision cross section for spin polarized atomic cesium. Neutral cesium atoms are optically cooled, then loaded into a dc magnetic trap. We infer the scattering rate from the rate at which anisotropies in the initial energy distribution are observed to relax. The cross section for F=3, mF{\mathit{m}}_{\mathit{F}}mF​=-3 on F=3, mF{\mathit{m}}_{\mathit{F}}mF​=-3 is 1.5(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ cm2{\mathrm{cm}}^{2}cm2, and is independent of temperature from 30 to 250 \ensuremath{\mu}K. This determination clarifies the technical requirements for attaining Bose-Einstein condensation in a magnetically trapped Cs vapor. We also study heating due to glancing collisions with 300 K background Cs atoms.