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Annual Meeting Optical Society of America

Collision dyamics of ultracold atoms

J. Weiner

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Recent advances in optical stopping and trapping of particles now permit the investigation of atom collision dynamics at temperatures below 1 mK. We discuss three specific examples of inelastic processes appropriate for study in this new energy domain: radiative association; collisional heating; and associative ionization. Theoretical study of radiative association and collisional heating demonstrate new quantal features apparent in the total inelastic cross section due to the small number of partial waves contributing to the nuclear motion. We present the first experimental observation of an inelastic process, associative ionization between excited sodium atoms, and report a measure of the absolute cross section ~3 orders of magnitude larger than the cross section measured in conventional conditions.

DOI
10.1364/oam.1988.fa3

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Recent advances in optical stopping and trapping of particles now permit the investigation of atom collision dynamics at temperatures below 1 mK. We discuss three specific examples of inelastic processes appropriate for study in this new energy domain: radiative association; collisional heating; and associative ionization. Theoretical study of radiative association and collisional heating demonstrate new quantal features apparent in the total inelastic cross section due to the small number of partial waves contributing to the nuclear motion. We present the first experimental observation of an inelastic process, associative ionization between excited sodium atoms, and report a measure of the absolute cross section ~3 orders of magnitude larger than the cross section measured in conventional conditions.
We have observed associative ionization of laser-cooled and -trapped sodium atoms. The measured rate coefficient for the process at a temperature of 0.75\ifmmode\pm\else\textpm\fi{}0.25 mK is (1.1${\ifmmode\pm\else\textpm\fi{}}_{0.5}^{1.3}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which implies a cross section of 8.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ cm2{\mathrm{cm}}^{2}cm2. This is 3 orders of magnitude larger than the cross section measured in previous experiments at higher temperatures. The measurement involves the use of a new kind of laser trap.
Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗-Na interaction initiates inward motion that is completed as Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].
We present the results from two experiments investigating collisions that differ in time scale by three orders of magnitude. The first experiment enables the determination of absolute total collision cross sections using a technique that measures a change in the loss rate of trapped atoms from a magneto optical trap (MOT). We also investigate light assisted collision processes between cold metastable neon atoms in the 3 P 2 metastable state within the MOT. A catalysis laser is scanned in frequency across the 3 P 2 – 3 D 3 cooling transition and the ionization rate was observed. Ionization spectra are obtained which demonstrate a dependence on the magnetic sublevels of the transition that the catalysis laser is exciting.