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Physical Review Letters

Suppression and Enhancement of Collisions in Optical Lattices

John Lawall, Chad Orzel, S. L. Rolston

Published Jan 19, 1998• 16 citations
Publisher

Abstract

In a three-dimensional optical lattice for metastable xenon we observe the dynamical effects of the optical potential on Penning ionizing collisions between atoms. Enhancement of collisions over that for free atoms is observed at short times after the atoms are loaded into the lattice. After the atoms thermalize and localize into the potential wells, we observe a suppression of the collisions by as much as a factor of 2. From our measurements and a simple model we are able to extract an estimate of the rate at which atoms ``hop'' between wells.

DOI
10.1103/physrevlett.80.480

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Physical Review Letters•Jun 8, 1998•21 citations
In a three-dimensional optical lattice for metastable xenon we observe the dynamical effects of the optical potential on Penning ionizing collisions between atoms. Enhancement of collisions over that for free atoms is observed at short times after the atoms are loaded into the lattice. After the atoms thermalize and localize into the potential wells, we observe a suppression of the collisions by as much as a factor of 2. From our measurements and a simple model we are able to extract an estimate of the rate at which atoms ``hop'' between wells.
Ionizing collisional loss rate of the metastable argon and krypton atoms in an optical lattice is compared with the rate of atoms in free space as a function of lattice laser detuning. Contrary to a simple picture of the isolation of atoms by the lattice potential, our experimental result shows that the collision rate in optical lattice is always larger than that in free space except at a large laser detuning of ∼10 3 transition linewidth. Comparing the experimental result with the theoretical values that are derived from the classical trajectory calculation of the atom, we found that the optical pumping to non-trapped adiabatic potential surface prevents the collision suppression because it keeps the number of non-trapped atoms to a finite value even at low excitation rate.
Near-resonant light is used to modify the collision dynamics of magneto-optically trapped metastable xenon atoms. Enhanced collisional ionization occurs for a "control laser" tuned below resonance, greatly exceeding the predictions of existing models of trap loss very close to resonance. With the trapping laser off, control light tuned above resonance suppresses ionization by a factor of 8. With the trap light on, a suppression factor >30 is observed. Increases in the number and density of trapped atoms and in the trap lifetime attest to the utility of optical control of collisions.
We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results.
Ionizing collisional rate of metastable krypton and argon atoms in an optical lattice klattice{k}_{\mathrm{lattice}}klattice​ is compared with that in free space kfree{k}_{\mathrm{free}}kfree​. Reduction of the collisional rate is observed for krypton atoms. However, the magnitude of reduction is no more than a factor of 2 at the laser detuning of 7 GHz. The ratio klattice/kfree{k}_{\mathrm{lattice}}/{k}_{\mathrm{free}}klattice​/kfree​ is found to change proportional to the square root of the excitation rate of atoms from the 1s5{1s}_{5}1s5​ metastable state to the 2p9{2p}_{9}2p9​ state by the lattice laser. The mechanism that prevents a larger reduction is discussed.
Collisions between laser-cooled metastable xenon atoms in the presence of laser fields provide a good experimental test for ultracold collision theories as the observed ionization can be directly linked to the atomic binary collisions. Hyperfine structure is not present for the even isotopes Xe132{\mathrm{Xe}}^{132}Xe132 and Xe136{\mathrm{Xe}}^{136}Xe136, which simplifies the theoretical work. We present our results and show how these and the results presented in Walhout et al. [Phys. Rev. Lett. 74, 605 (1995)] can be interpreted with suitably adapted two-state models. Our approach suggests one possibility why the strong shielding of collisions predicted by the simple two-state model of Suominen et al. [Phys. Rev. A 51, 1446 (1995)] has not been achieved in the xenon experiment. \textcopyright{} 1996 The American Physical Society.
Using 40 ns laser pulses, we probe the real-time dynamics of ultracold ionizing collisions in metastable xenon. We time resolve both shielding and enhancement effects, and observe the production of Xe2+{\mathrm{Xe}}_{2}^{+}Xe2+​ molecular ions through associative ionization. We estimate the rate of molecule formation in excited-state collisions, and directly measure the role of both flux enhancement and excited state survival in the collisional enhancement process. Conceptually simple theoretical models are used to predict the dynamics of the collisional shielding.