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Physical Review A

Secondary scattering on the intensity dependence of the capture velocity in a magneto-optical trap

Marcio Rodrigo Loos, S. B. Massardo, R. Zanon, A.L. de Oliveira

Published Aug 22, 2005• 0 citations
Publisher

Abstract

In this work, we consider a three-dimensional model to simulate the capture velocity behavior in a sample of cold-trapped sodium atoms as a function of the trapping laser intensity. We expand on previous work [V. S. Bagnato, L. G. Marcassa, S. G. Miranda, S. R. Muniz, and A. L. de Oliveira, Phys. Rev. A 62, 013404 (2000)] by calculating the capture velocity over a broad range of light intensities considering the secondary scattering in a magneto-optical trap. Our calculations are in a good agreement with recent measured values [S. R. Muniz et al., Phys. Rev. A 65, 015402 (2001)].

DOI
10.1103/physreva.72.025403

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In this work, we consider a three-dimensional model to simulate the capture velocity behavior in a sample of cold-trapped sodium atoms as a function of the trapping laser intensity. We expand on previous work [V. S. Bagnato, L. G. Marcassa, S. G. Miranda, S. R. Muniz, and A. L. de Oliveira, Phys. Rev. A 62, 013404 (2000)] by calculating the capture velocity over a broad range of light intensities considering the secondary scattering in a magneto-optical trap. Our calculations are in a good agreement with recent measured values [S. R. Muniz et al., Phys. Rev. A 65, 015402 (2001)].
Using the dark-spot Zeeman-tuned slowing technique, we have measured the capture velocity of a magneto-optical trap as a function of the trap laser intensity. The values obtained are in reasonable agreement with calculations based on a simple model. Nevertheless, the dependence of capture velocity in the low-intensity regime seems to be slightly different from the predictions. The measurement of this type of trap parameter is of interest in the analysis of exoergic cold collisions, where the knowledge of such values is important for the interpretation of experimental results.
We use a dark-spot Zeeman-tuned slowing technique to measure the capture velocity in a sodium magneto-optical trap as a function of the trapping laser intensity. We expand on previous work by measuring the capture velocity over a broad range of light intensities. We observe that the capture velocity reaches a maximum value and then decreases with increasing light intensities, which might imply a minimum in the trap-loss rate. This observation supports a recently published explanation of the dependence of the trap-loss rates, at low intensities, based mainly on the escape velocity.
We have measured the collisional loss rate for cold strontium atoms held in a magneto-optical trap as a function of light intensity in the regime of low intensity (2-6 mW/cm²). The results confirm our recently proposed model, where we showed that the sudden increase of loss rate at low intensities does not depend on hyperfine structure changing collision only. The model, which is based on radiative escape mechanism and a light intensity dependent escape velocity, is able to reproduce quite well the behavior of the experimental observations. The data here reported may be of importance for a recent proposed application for cold trapped 88Sr atoms, as well as other trappable alkaline earth metal atoms such as Ca.
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)].
In this work, we have explored ab initio the capture process in a magneto-optical trap by theory, simulation and experiment. We measured the capture velocity vc of a cesium vapor cell magneto-optical trap (VCMOT) from its capture rate R and developed an exact model for the capture rate of a VCMOT in terms of its capture velocity, background density and trap laser beam diameter. We measured the capture velocity of a cesium VCMOT for various trap laser intensities and magnetic field gradients. We observed that the capture velocity is a damping force as well as a restoring force phenomenon. We supported our findings by performing simulations for single atom trajectories in a 1D cesium MOT. Finally, we concluded that two MOTs can have the same capture velocities but very different capture rates, thereby revealing that these are two fundamentally different characteristics of the MOT.