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Physical Review Letters

Polarization Dependence of Optical Suppression in Photoassociative Ionization Collisions in a Sodium Magneto-optic Trap

Sérgio Carlos Zílio, Luís Gustavo Marcassa, Sérgio Ricardo Muniz, R. J. Horowicz, Vanderlei Salvador Bagnato, Reginaldo de Jesus Napolitano, J. Weiner, Paul S. Julienne

Published Mar 18, 1996• 42 citations
Publisher

Abstract

Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations.

DOI
10.1103/physrevlett.76.2033

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Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations.
Motivated by recent interest in low dimensional arrays of atoms, we experimentally investigated the way cold collisional processes are affected by the geometry of the considered atomic sample. More specifically, we studied the case of photoassociative ionization (PAI) both in a storage ring where collision is more unidirectional in character and in a trap with clear undefinition of collision axis. First, creating a ring shaped trap (atomotron) we investigated two-color PAI dependence with intensity and polarization of a probing laser. The intensity dependence of the PAI rate was also measured in a magneto-optical trap presenting equivalent temperature and density conditions. Indeed, the results show that in the ring trap, the value of the PAI rate constant is much lower and does not show evidences of saturation, unlike in the case of the 3D-MOT. Cold atomic collisions in storage ring may represent new possibilities for study.
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 have developed a full three-dimensional quantum scattering approach to optical suppression of ultracold-collision rates. These calculations are carried out assuming colliding atoms without fine or hyperfine structure, which have a $^{1}\ensuremath{\rightarrow}^{1}$P transition. The three-dimensional model predicts that the optical suppression of ultracold-collision rates saturates with light intensity much more slowly than predicted by two-level curve-crossing models. Circularly polarized light is significantly more effective for optical suppression, and causes less increase in atomic kinetic energy due to excited-state production than linearly polarized light. The suppressor optical field can also cause orders of magnitude increases in ground-state elastic-scattering rates.
\n We report the observation of polarization anisotropy in the optical\nsuppression of photoassociative ionization (PAI) collisions. Binary\ncollisions occur within a tightly collimated and optically cooled velocity\ngroup of a single sodium beam. A laser beam, blue-detuned with respect to\nthe atomic resonance transition, crosses the atomic beam and suppresses the\nPAI rate. We measure this suppression as a function of intensity and\nelectric field vector aligned parallel or perpendicular to the atomic beam\naxis. Quantum close-coupling and multichannel curve-crossing calculations of\nmodel systems yield insight into the nature of the anisotropy and are in\nreasonable agreement with the measurements.\n\n \n