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Optics Letters

CO_2 laser trap for cesium atoms

T. Takekoshi, R. J. Knize

Published Jan 1, 1996• 108 citations
Publisher

Abstract

We demonstrate a quasi-electrostatic trap for cold Cs atoms, using a focused 20-W CO(2) laser beam (lambda = 10.6 microm). Excluding background gas collisions, we measure a trap loss rate of 0.30(33) atoms/s, which is consistent with the calculated photon-limited loss rate of 3 x 10(-11) atoms/s. We measure a ground-state hyperfine population relaxation lifetime of at least 10 s.

DOI
10.1364/ol.21.000077

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We demonstrate a quasi-electrostatic trap for cold Cs atoms, using a focused 20-W CO(2) laser beam (lambda = 10.6 microm). Excluding background gas collisions, we measure a trap loss rate of 0.30(33) atoms/s, which is consistent with the calculated photon-limited loss rate of 3 x 10(-11) atoms/s. We measure a ground-state hyperfine population relaxation lifetime of at least 10 s.
Cold inelastic collisions between confined cesium (Cs) atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are investigated inside a CO2{\mathrm{CO}}_{2}CO2​ laser dipole trap. Inelastic atom-molecule collisions can be observed and measured with a rate coefficient of $\ensuremath{\sim}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, mainly independent of the molecular rovibrational state populated. Lifetimes of purely atomic and molecular samples are essentially limited by rest gas collisions. The pure molecular trap lifetime ranges 0.3--1 s, 4 times smaller than the atomic one, as is also observed in a pure magnetic trap. We give an estimation of the inelastic molecule-molecule collision rate to be $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.
We have trapped cesium atoms over many minutes in the focus of a CO2{\mathrm{CO}}_{2}CO2​-laser beam employing an extremely simple laser system. Collisional properties of the unpolarized atoms in their electronic ground state are investigated. Inelastic binary collisions changing the hyperfine state lead to trap loss, which is quantitatively analyzed. Elastic collisions result in evaporative cooling of the trapped gas from 25 $\ensuremath{\mu}\mathrm{K}$ to 10 $\ensuremath{\mu}\mathrm{K}$ over a time scale of about 150 s.
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 a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trap with a few trapped Cs atoms first presented in Ueberholz et al ( J. Phys. B: At. Mol. Opt. Phys. 33 (2000) L135 ). The ability to observe individual loss events allows us to identify two-body collisions that lead to the escape of only one of the colliding atoms (up to 10% of all collisional losses). Possible origins of these events are discussed here. We also observed strong modifications of the total loss rate with variations in the repumping laser intensity. This is explained by a simple semiclassical model based on optical suppression of hyperfine-changing collisions between ground-state atoms.