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Physical Review A

Intensity dependence of optical suppression in photoassociative ionization collisions in a sodium magneto-optic trap

Luís Gustavo Marcassa, R. J. Horowicz, Sérgio Carlos Zílio, Vanderlei Salvador Bagnato, J. Weiner

Published Aug 1, 1995• 14 citations
Publisher

Abstract

We report the intensity dependence of photoassociative ionization collisions in a sodium magneto-optic trap (MOT). A suppressor beam with frequency tuned 600 MHz to the blue of the MOT trapping transition reduces the rate constant of the inelastic process by as much as a factor of 20 at an intensity of 8 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. A comparison with theory shows good agreement.

DOI
10.1103/physreva.52.r913

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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.

Quantum Electronics and Laser Science Conference•Jun 7, 1996•0 citations
We report the intensity dependence of photoassociative ionization collisions in a sodium magneto-optic trap (MOT). A suppressor beam with frequency tuned 600 MHz to the blue of the MOT trapping transition reduces the rate constant of the inelastic process by as much as a factor of 20 at an intensity of 8 W cm\ensuremath−2{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}cm\ensuremath−2. A comparison with theory shows good agreement.
We report here measurements of two-body associative-ionization collisions between sodium atoms confined in a magneto-optic trap. These collisions represent a kind of ``open'' or dissipative collision for which the energy of the atom plus applied light field subsystem need not be conserved due to spontaneous-emission coupling to the vacuum modes of the radiation field. The experiment measures the photoassociative-ionization-rate constant as a function of the optical field intensity from about 40 to 260 mW ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. These results are in reasonable agreement with the predictions of an optical-Bloch-equation theory [Y. B. Band and P. S. Julienne, Phys. Rev. A 46, 330 (1992)], but differ strongly from the predictions of a local-equilibrium theory [A. Gallagher, Phys. Rev. A 44, 4249 (1991)].
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.
We present results for controlled optical collisions of cold, metastable Ne atoms in a magneto-optical trap. The modification of the ionizing collision rate is demonstrated using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules connected to the omega = 5 excited state potential as predicted by Doery et al (1998 Phys. Rev. A 57 3603-20). Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory (Gallagher and Pritchard 1989 Phys. Rev. Lett. 63 957). In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the effect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate.
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.