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Bulletin of the American Physical Society

Optical collisions of ultracold alkali atoms

H. M. J. M. Boesten, Eite Tiesinga, BOUDEWIJN J. VERHAAR

Published May 1, 1993• 0 citations
OpenAlex

Abstract

We develop an approach for describing ultracold-atom collisions in a laser field, incorporating laser excitation during the collision as well as the subsequent spontaneous emission. To include the latter is essential, because the duration of the collisions is longer than the excited-state lifetime. Existing treatments are based on a density matrix description of internal states, combined with a classical description of translational motion. With the presently achieved temperatures of a few times the recoil temperature in mind, we investigate possibilities to describe also the atomic motion during the collision quantummechanically. In our contribution we present preliminary results for an approach in which only one dressed-state manifold is included in the description. The loss of flux from this original manifold due to spontaneous emission is taken into account, but not its recycling through further laser excitation.

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We develop an approach for describing ultracold-atom collisions in a laser field, incorporating laser excitation during the collision as well as the subsequent spontaneous emission. To include the latter is essential, because the duration of the collisions is longer than the excited-state lifetime. Existing treatments are based on a density matrix description of internal states, combined with a classical description of translational motion. With the presently achieved temperatures of a few times the recoil temperature in mind, we investigate possibilities to describe also the atomic motion during the collision quantummechanically. In our contribution we present preliminary results for an approach in which only one dressed-state manifold is included in the description. The loss of flux from this original manifold due to spontaneous emission is taken into account, but not its recycling through further laser excitation.
We develop an optical-Bloch-equation approach for modeling ultracold-atom collisions in optical traps. The method incorporates a molecular picture of the atomic collision, laser-field dressing of the molecular states participating in the dynamics, and decay of the population and polarization due to spontaneous emission of excited states. The last is important to incorporate because the duration of the cold collisions is longer than the excited-state lifetimes. The relative motion of the atoms during the course of the collision is treated semiclassically with corrections for the time-dependent relative motion of the atoms in the various channels. An application of the method to Cs trap loss due to fine-structure-changing collisions is presented. Good agreement with experiment is obtained.
We give a full quantum treatment of the radiative heating mechanism which arises in atomic clouds at low temperatures due to the collisions in the presence of a light field. The time scale of these collisions is comparable to the time scale of spontaneous decay, hence one needs to combine excitation and decay processes with the quantum motion of the colliding atoms into a single model, which we have done. Our study produces a statistical approximate to the full density matrix by combining wave-packet dynamics with the Monte Carlo state-vector method. We also present a quantitative semiclassical treatment based on the Landau-Zener model, and show that it agrees with the quantal results in certain parameter regimes. However, for low temperatures and strong laser fields the full quantum treatment is needed. We describe our numerical procedures and the statistical aspects of the Monte Carlo state-vector method in detail. Our main result is that the low-energy collisions can produce appreciable heating of the cloud.
Ultracold collisions of neutral atoms can now be studied in the laboratory in the temperature range below 0.001 K. Rate coefficients for ultracold collisions of excited states are decreased by spontaneous emission during the long duration of the collision, but can be dramatically modified by intense laser effects on collision dynamics. Generalized MCQDT is used to develop criteria for the onset of quantum threshold behavior, which generally begins at temperatures above the Doppler cooling limit of light traps. Threshold Penning ionization of He metastable atoms in a light trap is predicted to occur with a rate coefficient larger than 5×10 (−11) cm3/sec.
We report new calculations of two-body associative ionization collisions between sodium atoms confined in a magneto-optic trap at ultracold temperatures. These collisions represent a new kind of {open_quotes}open{close_quotes} or dissipative collision for which the energy of the atom + applied light field need not be conserved due to spontaneous emission coupling to the vacuum modes of the radiation field and require density matrix methods rather than wavefunction methods to describe the collision. In these collisions the Na atoms must absorb a photon when they are in close enough proximity that a significant fraction of the excited population survives relaxation back to the ground state. The Na atoms accelerate towards each other on the excited molecular state and absorbs a second photon to a doubly excited state which then proceeds to associatively ionize at close range. Our calculations incorporate a molecular picture of the atomic collision, laser optical-field dressing of the molecular states participating in the dynamics, and population and coherence decay due to spontaneous emission. The effect of excited state hyperfine structure, which plays a critical role in the dynamics, is simulated by simplified models with a reduced number of states based on the full set of molecular-hyperfine adiabatic potentialmore » curves. Comparison is made with recent measurements of photo-associative ionization. rates as a function of optical field intensity from about 20 to 130 mW cm{sup -2}.« less
In laser-induced collisions between ultracold atoms, the combination of low velocities and long-range interactions results in collision times which can exceed the excited-state lifetime. We use a cooperative effect between two lasers to explicitly observe this time dependence. The first laser, tuned near resonance, excites the atom pair at long range and enhances the collisional flux available for short-range excitation by a second, far-detuned laser. Using pulsed excitation, we find this collisional process to take place on a ${10}^{\ensuremath{-}6}\mathrm{s}$ time scale, in reasonable agreement with trajectory simulations.
Collisions of ultracold atoms can now be investigated in the laboratory at temperatures below 0.001 K. Such collisions are qualitatively different in many ways from collisions at normal energies, where T ≫ 1 K, because of the long time and distance scales associated with such collisions. Spontaneous emission can strongly modify collision dynamics of excited-state species produced by near-resonant optical excitation if the temperature is less than the characteristic temperature TS, where the collision time is comparable with the spontaneous-emission lifetime. We also estimate the temperature TQ that is characteristic of the onset of quantum threshold behavior, where the WKB approximation fails to apply as the de Broglie wavelength becomes large. We find that TQ is generally larger than TD, the Doppler cooling limit, except for collisions controlled by long-range potentials varying as 1/R3.TQ may be larger or smaller than TS. Expressions for estimating the magnitude of rate coefficients in the T → 0 limit are given. As an example, a model for the threshold behavior of Penning ionization of two He 3S1 metastable atoms is given, and the threshold rate coefficient is estimated to be >5 × 10−10 cm3 sec−1.