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arXiv:1304.2491 · Physical Review A

Measurement of collisions between rubidium atoms and optically dark rubidium ions in trapped mixtures

Seunghyun Lee, K. Ravi, S. A. Rangwala

Published May 2, 2013• 36 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.87.052701

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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.
An ion–neutral hybrid trap is built to study low-energy ion–atom collisions. The ion–neutral hybrid trap is combined with two spatially concentric traps: a linear Paul trap for the ionic species and a magneto-optical trap (MOT) for the neutral species. The total ion–atom collision rate coefficient between 87 Rb atoms and optically dark 87 Rb + ions is measured by monitoring the reduction of the steady-state MOT atoms by sequentially introducing various mechanisms, namely photoionization and ion–atom collisions. In contrast to other experiments, a concise experimental procedure is devised to obtain the loss rates of the MOT atoms due to photoionization and ion–atom collisions in one experimental cycle, and then the collision rate of cold 87 Rb atoms with 87 Rb + ions is deduced to m 3 / s with K measured by the time of flight of the ion signal. The measurements show good agreement with the collision rate derived from the Langevin model.
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 $\mathrm{Cs}\ensuremath{-}\mathrm{C}{\mathrm{s}}^{+}$ 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 6P3/26{P}_{3/2}6P3/2​ atoms at 473 nm wavelength to be $2.28(\ifmmode\pm\else\textpm\fi{}0.33)\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}$.
We present measurements of the total elastic and resonant charge-exchange ion-atom collision rate coefficient kia{k}_{\mathrm{ia}}kia​ of cold sodium (Na) with optically dark low-energy Na+{\mathrm{Na}}^{+}Na+ ions in a hybrid ion-neutral trap. To determine kia{k}_{\mathrm{ia}}kia​, we measured the trap loading and loss rates from both a Na magneto-optical trap (MOT) and a linear radio-frequency quadrupole Paul trap. We found the total rate coefficient to be $7.4\ifmmode\pm\else\textpm\fi{}1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}/\mathrm{s}$ for the type-I Na MOT immersed within an $\ensuremath{\approx}140\ensuremath{-}\mathrm{K}$ ion cloud and $1.10\ifmmode\pm\else\textpm\fi{}0.25\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}/\mathrm{s}$ for the type-II Na MOT within an $\ensuremath{\approx}1070\ensuremath{-}\mathrm{K}$ ion cloud. Our measurements show excellent agreement with previously reported theoretical fully quantal ab initio calculations. In the process of determining the total rate coefficient, we demonstrate that a MOT can be used to probe an optically dark ion cloud's spatial distribution within a hybrid trap.
The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.
We realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.
We demonstrate the simultaneous magneto-optical trapping (MOT) of Rb and Sr and examine the characteristic loss of Rb in the MOT due to photoionization by the cooling laser for Sr. The photoionization cross section of Rb in the 5P3/25P_{3/2}5P3/2​ state at 461 nm is determined to be 1.2×10−171.2\times10^{-17}1.2×10−17 cm2^22. It is important to consider this loss rate to realize a sufficiently large number of trapped Rb atoms to achieve a quantum degenerate mixture of Rb and Sr.