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Physical Review Letters

Optical Suppression of Photoassociative Ionization in a Magneto-Optical Trap

Luís Gustavo Marcassa, Sérgio Ricardo Muniz, E Queiroz, Sérgio Carlos Zílio, Vanderlei Salvador Bagnato, J. Weiner, Paul S. Julienne, Kalle‐Antti Suominen

Published Oct 3, 1994• 61 citations
Publisher

Abstract

We demonstrate suppression of two-color photoassociative ionization in a sodium vapor magneto-optical trap by imposing an auxiliary "suppressor" light beam on the ensemble of colliding trapped atoms. We interpret the experimental results in terms of a simple picture in which the suppressor light intercepts incoming reactant particle flux and reroutes it to long-range repulsive states, effectively preventing further approach of the collision pairs.

DOI
10.1103/physrevlett.73.1911

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Physical Review Letters•May 24, 1993•72 citations
We demonstrate suppression of two-color photoassociative ionization in a sodium vapor magneto-optical trap by imposing an auxiliary "suppressor" light beam on the ensemble of colliding trapped atoms. We interpret the experimental results in terms of a simple picture in which the suppressor light intercepts incoming reactant particle flux and reroutes it to long-range repulsive states, effectively preventing further approach of the collision pairs.
Summary form only given. Collisions with cooled and trapped atoms have been the subject of intense investigation during the past years. Unprecedented details of the intermediate steps in the collision have been revealed. A remarkable feature in the ultracold system is the ability to control entrance-channel scattering flux and thereby control the exit channel of the collisional encounter. Here we present the continuation of our recent work where photoassociative ionizations (PAI) of sodium atoms were suppressed by application of an optical field. The new experiments show that suppression of almost 100% is possible when high intensity is used and a remarkable dependence with the polarization of the suppressor laser field is observed and calculated. This result shows a deviation from the simple picture based on the Landau-Zener avoided crossing model when high intensity is used.
We report the observation of direct, two-color photoassociative ionization (PAI) in collisions between 85Rb^{85}\mathrm{Rb}85Rb atoms optically cooled and confined in a magneto-optical trap. We also measure the resulting R2+{\mathrm{R}}_{2}^{+}R2+​ ion spectrum and compare it to earlier trap-loss fluoresecence spectra. Unlike PAI in Na, which ionizes principally by collisional autoionization, the Rb2∗{\mathrm{Rb}}_{2}^{\mathrm{*}}Rb2∗​ intermediate photoionizes directly to Rb2+{\mathrm{Rb}}_{2}^{+}Rb2+​ in a fixed optical field. These observations confirm PAI as a sensitive and broadly applicable spectroscopic tool.
Introduction Photoassociation uses optical fields to produce bound molecules from free atoms. Optical fields can also prevent atoms from closely approaching, thereby shielding them from shortrange inelastic or reactive interactions and suppressing the rates of these processes. Recently several groups have demonstrated shielding and suppression by shining an optical field on a cold atom sample. Figure 6.1(a) shows how a simple semiclassical picture can be used to interpret the shielding effect as the rerouting of a ground-state entrance channel scattering flux to an excited repulsive curve at an internuclear distance localized around a Condon point. An optical field, blue detuned with respect to the asymptotic atomic transition, resonantly couples the ground and excited states. In the cold and ultracold regime particles approach on the ground state with very little kinetic energy. Excitation to the repulsive state effectively halts their approach in the immediate vicinity of the Condon point, and the scattering flux then exits either on the repulsive excited state or on the ground state. Figure 6.1(b) shows how this picture can be represented as a Landau–Zener (LZ) avoided crossing of field-dressed potentials. As the blue-detuned suppressor laser intensity increases, the avoided crossing gap around the Condon point widens, and the semiclassical particle moves through the optical coupling region adiabatically. The flux effectively enters and exits on the ground state, and the collision becomes elastic.