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Physica Scripta

Ultra-cold Molecules: Formation, Trapping and Prospects

P. Pillet, Nicolas Vanhaecke, Christian Lisdat, D. Comparat, Olivier Dulieu, A. Crubellier, Françoise Masnou-Seeuws

Published Jan 1, 2003• 3 citations
Publisher

Abstract

Different schemes for the formation of cold molecules via photoassociation are discussed. In a typical magneto-optical trap, the formation rates can reach 0.2 cold molecules per second and per atom. The measured temperatures of the cold molecular samples are in the range 10–200 µK. The possibility for accumulating the so-formed cold molecules inside a trap, or for selecting them in a well-defined level in ground state are analyzed.

DOI
10.1238/physica.topical.105a00007

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Different schemes for the formation of cold molecules via photoassociation are discussed. In a typical magneto-optical trap, the formation rates can reach 0.2 cold molecules per second and per atom. The measured temperatures of the cold molecular samples are in the range 10–200 µK. The possibility for accumulating the so-formed cold molecules inside a trap, or for selecting them in a well-defined level in ground state are analyzed.
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.
We have observed translationally cold electronic ground-state cesium dimers produced directly in a magneto-optical trap and also by photoassociation of cold atoms in this trap. These neutral molecules were detected using photoionization and time-of-flight spectroscopy. Initial experiments indicated a cold molecular formation rate of about $600{\mathrm{s}}^{\mathrm{\ensuremath{-}}1},$ with no photoassociation laser present. The measured translational temperature of the neutral cesium dimers was approximately 100 \ensuremath{\mu}K.
Summary form only given. In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for recycling the population. The molecular photoassociation of cold atoms opens a promising alternative for the formation of cold molecules. The photoassociative spectra are obtained by the usual trap-loss method as well as by pulsed laser photoionization of Cs/sub 2/ molecules into Cs/sub 2//sup +/ ions. Different schemes to obtain formation of cold molecules through photoassociation will be discussed.
We report on the formation of translationally cold Cs2 ground-state molecules through photoassociation of laser-cooled cesium atoms in a magneto-optical trap. The cold molecules are obtained after spontaneous decay of photoassociated molecules, and detected after pulsed-laser photoionization into Cs2+ ions. Photoassociation spectra of cesium attractive molecular states below the 6S1/2+6P3/2 dissociation limits are reported, obtained with both ion detection and trap-loss method. A temperature as low as 20−5+15 has been measured for the molecular cloud.
In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for population recycling. Molecular photoassociation of cold atoms has permitted the first observation of the formation of cold ground state Cs 2 molecules. In this scheme, two free cold atoms absorb one photon, to form an electronically excited molecule in a well-defined ro-vibrational state, which yields translationally cold molecules, after spontaneous decay, in their ground state or in their lowest triplet state.
We report on the observation of translationally cold Cs2 molecules at temperatures in the 300–70 μK range, produced by the photoassociation of cold Cs atoms in a vapor-cell magneto-optical trap. The detection method is based on pulsed-laser photoionization of Cs2 molecules into Cs2+ ions, selectively detected through a time-of-flight mass spectrometer. This technique, together with the usual trap-loss method, enables us to perform the spectroscopy of the long-range attractive molecular states below the 6s 2S1/2+6p 2P3/2 dissociation limit.