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Europhysics Letters (EPL)

Controlling the formation of cold molecules via a Feshbach resonance

B. Laburthe-Tolra, Nathalie Hoang, B. T'Jampens, Nicolas Vanhaecke, Cyril Drag, A. Crubellier, D. Comparat, P. Pillet

Published Oct 1, 2003• 28 citations
Publisher

Abstract

Ultracold molecules are formed via photoassociation in a polarized sample of cold cesium atoms. The rate of molecule formation is controlled by tuning a magnetic field. The radial dependence of the colliding wave function is probed by the photoassociation spectroscopy technique. We have in this way observed the drastic influence of the magnetic field in a broad Feshbach resonance for f = 3, m = 3 colliding atoms and for magnetic fields up to 30 G.

DOI
10.1209/epl/i2003-00284-x

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Ultracold molecules are formed via photoassociation in a polarized sample of cold cesium atoms. The rate of molecule formation is controlled by tuning a magnetic field. The radial dependence of the colliding wave function is probed by the photoassociation spectroscopy technique. We have in this way observed the drastic influence of the magnetic field in a broad Feshbach resonance for f = 3, m = 3 colliding atoms and for magnetic fields up to 30 G.
Ultracold molecules offer a broad variety of applications, ranging from metrology to quantum computing. However, forming "real" ultracold molecules, i.e., in deeply bound levels, is a very difficult proposition. Here, we show how photoassociation in the vicinity of a Feshbach resonance enhances molecular formation rates by several orders of magnitude. We illustrate this effect in heteronuclear systems, and find giant rate coefficients even in deeply bound levels. We also give a simple analytical expression for the photoassociation rate and discuss future applications of the Feshbach-optimized photoassociation technique.
Several different experiments involving the making of cold and ultracold molecules are performed and proposed. Ultracold cesium molecules are produced in a magneto-optical trap (MOT) by photoassociation near the 6P j, 7Pj, 9S 1/2, 6D3/2, 7D3/2 and doubly excited asymptotes. Perturbation effects related to resonant channel coupling were observed in photoassociation spectra below the 6 P1/2 asymptote. Such effects are used to illustrate a ground state ultracold molecule formation mechanism, which can also be formed in one step photoassociation to the double minimum 31S+m state. In addition, a new technique of making cold molecules of cesium hydride is proposed, using a cold hydrogen beam source and a cesium MOT. Due to reaction energetics, the formed CsH molecules should be in their lowest rovibrational states. ^
We investigate theoretically the influence of magnetically tunable Feshbach collision resonances on the photoassociation spectra of ultracold atoms. As an example we consider recently predicted resonances for 85Rb{}^{85}\mathrm{Rb}85Rb atoms. For excitation to the ${}^{85}{\mathrm{Rb}}_{2}{\mathrm{ }0}_{g}^{\ensuremath{-}}{(S}_{1/2}{+P}_{1/2})$ electronic state, we predict that the photoassociation rate is resonantly enhanced by at least two orders of magnitude. We find that photoassociation could serve as a useful probe with which to study Feshbach resonances in ultracold collisions. In turn, these resonances should be very important to Bose-condensed gases, and more generally, to coherent atom optics.
The cold-molecule field concerns the physics and the applications of molecular systems with translational temperature well below the 1 К range. The possibility of controlling all the motion as well as the internal quantum state of a sample of molecules is a long-term goal that opens the possibility for many new experiments and measurements ranging from fundamental constants to quantum chemistry and quantum computation. Although, direct laser cooling of the translational degree of freedom of molecules is still waiting for further technological or theoretical breakthroughs and ideas, many different techniques have proven to be successful in producing different types of cold as well as ultracold (i.e. T < 1 mK) molecules. In this chapter, we will concentrate on the description of one of these techniques: the photoassociation of laser cooled atoms. We will report on the status of the art of this technique for the case of cesium atoms, describing all the main experimental findings. In particular, we will illustrate the different photoassociation schemes for molecule formation, the detection schemes through photoionization, the molecule trapping in a magnetic or dipolar trap, the vibrational cooling into a single vibrational state and finally the present prospects for rotational cooling.
This talk reviews some of the basic concepts in cold collisions, and gives examples of calculations directed towards understanding the following areas: (1) collisions of Na atoms trapped in Zeeman components of the F=1 hyperfine sublevel, and their control by magnetically-induced Feshbach resonances; (2) elastic and inelastic collisions of Cs atoms in the double spin-polarized F=4, M=4 level; and (3) stimulated Raman photoassociation of two colliding atoms in a Bose-Einstein condensate (BEC) to make translationally cold molecules in a selected target level.
We probe sss-wave collisions of laser-cooled ${}^{85}\mathrm{Rb}({f\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2,m}_{f}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}2)$ atoms with Zeeman-resolved photoassociation spectroscopy. We observe that these collisions exhibit a magnetically tunable Feshbach resonance, and determine that this resonance tunes to zero energy at a magnetic field of $164\ifmmode\pm\else\textpm\fi{}7\mathrm{G}$. This result indicates that the self-interaction energy of an 85Rb{}^{85}\mathrm{Rb}85Rb Bose-Einstein condensate can be magnetically tuned. We also demonstrate that Zeeman-resolved photoassociation spectroscopy provides a useful new tool for the study of ultracold atomic collisions.