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Chinese Physics Letters

Measurement of Absolute Atomic Collision Cross Section with Helium Using 87 Rb Atoms Confined in Magneto-Optic and Magnetic Traps

Jicheng Wang, Keya Zhou, Yueyuan Wang, Qinghong Liao, Shutian Liu

Published Apr 1, 2011• 0 citations
Publisher

Abstract

We present the measurements and calculations of the absolute total collision cross sections for a room-temperature gas of helium using 87 Rb atoms confined in either a magneto-optic or a magnetic quadrupole trap. The loss rates from the magneto-optic trap and the pure magnetic trap are compared and show significant differences. The collision cross sections as a function of trap depth for helium gas are obtained. These findings are significant for extracting the information about the different cross sections when the trap depth is changed.

DOI
10.1088/0256-307x/28/4/043401

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The implementation of a technique to measure total collision cross sections using laser-cooled rubidium atoms along with the introduction of room-temperature background gases, confined in both magnetooptic and magnetic traps, is proposed. Atom loss from the trap and total collision cross section can be inferred from the knowledge of the density of background gases. The measured cross sections from the magneto-optic and magnetic traps are represented, compared, and found to be significantly different. The measurements using this technique have a very small error in the range of approximately 2%-7%.
The implementation of a technique to measure total collision cross sections using laser-cooled rubidium atoms along with the introduction of room-temperature background gases, confined in both magnetooptic and magnetic traps, is proposed. Atom loss from the trap and total collision cross section can be inferred from the knowledge of the density of background gases. The measured cross sections from the magneto-optic and magnetic traps are represented, compared, and found to be significantly different. The measurements using this technique have a very small error in the range of approximately 2%-7%.
This thesis discusses a new technique for measuring collision cross sections, using a magnetooptical trap (MOT) or magnetic trap as well as theoretical predictions for the resulting measurements based on scattering theory. The initial discussion focuses on a theoretical overview of the principles involved in laser cooling and trapping, magnetic trapping, and scattering theory. Following this, a detailed overview of the experimental instruments and methods describing the vacuum system, laser light, magnetic coils, imaging and control systems will be presented. In addition to an experimental overview, the methods and mechanical formulation involved in predicting cross sections and loss rates based on scattering theory are discussed. Finally, an overview of both experimental and theoretical results including cross sections and loss rates for the case of trapped 87Rb and background argon or helium gas will be given. The results gathered in this thesis show a strong dependence of elastic collision cross section on trap depth, which indicates that the majority of elastic collisions between ultra-cold rubidium and room temperature argon impart energy below 1K. These results suggest that the magnetic trap may be a better tool for measuring total elastic collision cross sections.
We report on measurements of trap-loss collisions between ultracold Rb atoms that are confined in a magneto-optical trap. Both isotopes, 85Rb{}^{85}\mathrm{Rb}85Rb and 87Rb,{}^{87}\mathrm{Rb},87Rb, have been studied over a wide range of trap laser intensities and detunings. The trap-loss collisional rate constant exhibits variations over three orders of magnitude. At low intensities and/or large detunings, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities and small detunings, collisions involving excited atoms are more important. We see significant differences between the isotopes in both regimes. At large detunings, the ability of the trap to recapture products of a hyperfine-changing collision is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room-temperature background gas have also been measured. A surprisingly large increase in this rate is seen when the confining power of the trap is reduced.