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arXiv:physics/0509197 · Physical Review Letters

Atom-Molecule Collisions in an Optically Trapped Gas

Nassim Zahzam, Thibault Vogt, Marcel Carto Constantin Mudrich, D. Comparat, P. Pillet

Published Jan 19, 2006• 123 citations
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Abstract

Cold inelastic collisions between confined cesium (Cs) atoms and ${\mathrm{Cs}}_{2}$ molecules are investigated inside a ${\mathrm{CO}}_{2}$ 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 $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.

DOI
10.1103/physrevlett.96.023202

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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∼2.6\ifmmode×\else\texttimes\fi10\ensuremath−11  cm3 s\ensuremath−1\ensuremath{\sim}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\ensuremath∼2.6\ifmmode×\else\texttimes\fi10\ensuremath−11  cm3 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∼10\ensuremath−11  cm3 s\ensuremath−1\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\ensuremath∼10\ensuremath−11  cm3 s\ensuremath−1.
Ultracold collisions between Cs atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are formed in the triplet ground state by cw photoassociation through the outer well of the ${0}_{g}^{\ensuremath{-}}({P}_{3/2})$ excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the F=3F=3F=3 or F=4F=4F=4 hyperfine ground state, and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules in either highly vibrationally excited states (${v}^{\ensuremath{'}}=32--47$) or in low vibrational states (${v}^{\ensuremath{'}}=4--6$) of the $a\text{ }^{3}\ensuremath{\Sigma}_{u}^{+}$ triplet ground state. The rate coefficients $\ensuremath{\beta}\ensuremath{\sim}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.
We study collisions in an optically trapped, pure sample of ultracold Cs 2 molecules in various internal states. The molecular gas is created by Feshbach association from a near-degenerate atomic gas, with adjustable temperatures in the nanokelvin range. We identify several narrow loss resonances, which point to the coupling to more complex molecular states and may be interpreted as Feshbach resonances in dimerdimer interactions. Moreover, in some molecular states we observe a surprising temperature dependence in collisional loss. This shows that the situation cannot be understood in terms of the usual simple threshold behavior for inelastic two-body collisions. We interpret this observation as further evidence for a more complex molecular structure beyond the well-understood dimer physics.
Abstract Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes in the dark or to laser-free decay pathways. For atom–molecule mixtures, RbCs + Rb and RbCs + Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom–molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.
We investigate collisions of ultracold polar LiCs molecules and ultracold caesium atoms. LiCs molecules are formed in an optical dipole trap by photoassociation of caesium and lithium atoms via the B(1)Π excited state followed by spontaneous emission to the X(1)Σ(+) ground state and the lowest triplet state a(3)Σ(+). The molecules are then stored together with caesium atoms in the same optical trap. Rate coefficients for the loss of molecules induced by collisions with surrounding Cs atoms are measured for molecular ensembles produced via different photoassociation resonances. The results are analyzed in terms of the unitarity limit for the inelastic rates and predictions from the universal model of Idziaszek and Julienne (Phys. Rev. Lett., 2010, 104, 113202).
We have measured the rate of inelastic collisions in a cloud of doubly polarized ground-state cesium atoms ( F=mF=4{F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4F=mF​=4) confined in a magnetic trap for temperatures TTT between 8 and $70\ensuremath{\mu}\mathrm{K}$. We find a two-body rate coefficient varying as ${T}^{\ensuremath{-}0.63}$. At 8 $\ensuremath{\mu}\mathrm{K}$ it reaches $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ which is 3 orders of magnitude larger than predicted, ruling out a Bose-Einstein condensation of Cs in this internal state.