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Physical Review Letters

Observation of associative ionization of ultracold laser-trapped sodium atoms

P. L. Gould, Paul D. Lett, Paul S. Julienne, William D. Phillips, H. R. Thorsheim, J. Weiner

Published Feb 29, 1988• 129 citations
Publisher

Abstract

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{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, which implies a cross section of 8.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$. 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.

DOI
10.1103/physrevlett.60.788

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Annual Meeting Optical Society of America•Jan 1, 1988•0 citations
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±\else\textpm\fi0.51.3{\ifmmode\pm\else\textpm\fi{}}_{0.5}^{1.3}\ifmmode±\else\textpm\fi0.51.3​)\ifmmode\times\else\texttimes\fi{}10\ensuremath−11{10}^{\mathrm{\ensuremath{-}}11}10\ensuremath−11 cm3{\mathrm{cm}}^{3}cm3 s\ensuremath−1{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}s\ensuremath−1, which implies a cross section of 8.6\ifmmode\times\else\texttimes\fi{}10\ensuremath−14{10}^{\mathrm{\ensuremath{-}}14}10\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.
Recent advances in laser cooling techniques have made possible a new type of spectroscopy. In the first experiments of this kind alkali atoms are laser cooled to temperatures around 1 mK and trapped, using laser fields, at densities around 1011 cm−3. Atoms colliding under such ultracold conditions can be resonantly photoassociated to bound states of the dimer molecules. These molecules can in turn be photoionized, providing a sensitive detection of the photoassociation spectrum and revealing in extremely high resolution the structure of the dimer molecules. The particular advantages of this technique include high resolution and signal‐to‐noise and the ability to reach extremely long range states of the molecule, accessible only through such slow collisions.
Intense laser fields in optical neutral-atom traps can have a profound effect on collision cross sections at ultracold temperatures, $T<{10}^{\ensuremath{-}3}$ K. A simple model is developed for the long-range interatomic potentials of two field-dressed atoms. Field dressing ensures that the cross section for associative ionization of two ultracold Na atoms is large in an optical trap, but the cross section is predicted to be about 4 orders of magnitude smaller in optical molasses.
The rate coefficient for the laser-induced associative-ionization reaction of ultracold Na in an optical trap is calculated as a function of the trap laser detuning from the Na(2{(}^{2}(2S1/2{\mathit{S}}_{1/2}S1/2​,F=2)\ensuremath{\rightarrow}Na(2{(}^{2}(2P3/2{\mathit{P}}_{3/2}P3/2​,F=3) resonance. Aided by a knowledge of the excited-state potential-energy curves and of the nature of the free-atom optical-pumping process, we propose the following mechanism, which we call photoassociative ionization (PAI): 2Na(3{(}^{3}(3${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$)${\ensuremath{\rightarrow}}^{\mathrm{\ensuremath{\Elzxh}}\mathrm{\ensuremath{\omega}}}$Na2∗{\mathrm{Na}}_{2}^{\mathrm{*}}Na2∗​(${0}_{\mathit{g}}^{\mathrm{\ensuremath{-}}}$ and 1g{1}_{\mathit{g}}1g​)${\ensuremath{\rightarrow}}^{\mathrm{\ensuremath{\Elzxh}}\mathrm{\ensuremath{\omega}}}$Na2∗∗{\mathrm{Na}}_{2}^{\mathrm{*}\mathrm{*}}Na2∗∗​(1u{1}_{\mathit{u}}1u​)\ensuremath{\rightarrow}Na2+{\mathrm{Na}}_{2}^{+}Na2+​+${\mathit{e}}^{\mathrm{\ensuremath{-}}}$. Maxima in the calculated PAI rate coefficient occur at detunings that are simultaneously one-photon resonant with bound levels of the long-range ${0}_{\mathit{g}}^{\mathrm{\ensuremath{-}}}$ state and two-photon resonant with the lower rotational levels of a vibrational level lying 9 GHz below the dissociation threshold of the autoionizing 1u{1}_{\mathit{u}}1u​ state. The calculated PAI spectrum (PAI rate coefficient versus trap-laser detuning) displays a series of broad peaks between -0.5 and -4 GHz detuning and a cutoff at -5 GHz detuning, as does the experimentally measured spectrum of Lett et al. [Phys. Rev. Lett. 67, 2139 (1991)].The broad widths of the peaks in the PAI spectrum is due in part to the orientation averaging of the collision vector with respect to the electric-field vector and to the optical pumping of the Na2{\mathrm{Na}}_{2}Na2​\ensuremath{\rightarrow}Na2∗{\mathrm{Na}}_{2}^{\mathrm{*}}Na2∗​ rovibronic transition. The calculated PAI rate coefficient at -0.6 GHz detuning is a factor of 4 higher than the experimental value. Fine-structure-changing transitions play a role in the doubly excited states, because of the 1u{1}_{\mathit{u}}1u​ avoided crossing, but not in the intermediate states, because they have gerade symmetry and the only states that have been shown to undergo fine-structure-changing transitions with large probability have ungerade symmetry. Several predictions based on the proposed model and suggestions for future experiments are discussed.
The results of an experiment on the associative ionization collision between sodium and cesium excited atoms are reported. The atoms are excited by two resonant cw lasers to the 3P3/2{\mathit{P}}_{3/2}P3/2​ and 6P3/2{\mathit{P}}_{3/2}P3/2​ levels, respectively, and the formation of the NaCs+{\mathrm{NaCs}}^{+}NaCs+ molecular ion is observed. The rate constant of such process has been measured relative to the known rate constant of sodium associative ionization and its value resulted in K=(5.4\ifmmode\pm\else\textpm\fi{}3.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.
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.