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Physica Scripta

Ultra-Cold Molecules

A. Fioretti, M. Fazzi, Marina Mazzoni, Ticijana Ban, C. Gabbanini

Published Jan 1, 2004• 5 citations
Publisher

Abstract

In this paper we overview recent results on the formation of translationally cold molecules. While there are some methods relying on direct cooling of molecules, we shall concentrate on the photoassociation technique that starts from laser-cooled atoms. The detection methods for the ultra-cold molecules will be presented together with a report on the trapping techniques. The treatment is mainly focused on the results obtained by our group on rubidium dimers.

DOI
10.1238/physica.topical.112a00013

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Physica Scripta•Jan 1, 2003•3 citations
In this paper we overview recent results on the formation of translationally cold molecules. While there are some methods relying on direct cooling of molecules, we shall concentrate on the photoassociation technique that starts from laser-cooled atoms. The detection methods for the ultra-cold molecules will be presented together with a report on the trapping techniques. The treatment is mainly focused on the results obtained by our group on rubidium dimers.
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.
Summary form only given. Although there has been stunning progress in the production of ultracold atoms (defined loosely as atoms with T<1 mK), efforts to produce translationally ultracold molecules were unsuccessful until very recently. Conventional laser cooling of atoms relies on the existence of a cycling transition, generally not an option in molecules because of their rotational and vibrational multiplicity. Various proposals have been offered to overcome this difficulty, including ultracold molecule production by photoassociation (PA) of ultracold atoms, buffer gas cooling, and cooling by a multiple-frequency laser. The first and second of these approaches have now been demonstrated successfully. This talk will describe some of these new experimental results, focusing particularly on our work at the University of Connecticut on ultracold K/sub 2/ molecules.
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.
In recent years several groups of researchers have succeeded in making samples of cold molecules, at temperatures below 1 K, and of ultracold molecules, at temperatures below 1 mK. We concentrate on the formation of Cs2 dimers. Starting with laser-cooled atoms, photoassociation forms molecules in an excited electronic state. Various mechanisms are presented for stabilizing these molecules by spontaneous emission into a bound level of the ground electronic state. The design of new formation schemes requires precise knowledge of molecular potential curves that is well beyond the accuracy of ab initio calculations. We show that analytical long-range potentials can be fitted on photoassociation spectra and yield, through the determination of the C3 long-range coefficient, a value of the radiative lifetime of the 6P3/2 cesium level with an unprecedented accuracy. For short-range potentials it is possible to avoid explicit calculations by using asymptotic methods that generalize to long-range molecules the quantum-defect concepts developed for Rydberg atoms. Considering two coupled channels, we show how generalized Lu–Fano plots can extract precise information from experimental spectra.
In the molecular photoassociation process, two free cold atoms absorb one photon by a resonant way, to create an excited molecule in a well defined ro-vibrational state. The photoassociative spectroscopy of alkaline dimers yields accurate determinations for the long-range parts of molecular potential curves correlated to the first s + p asymptote. 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 also a promising alternative for the formation of cold ground state molecules. Photoassociation of cold cesium atoms has permitted the first observation of translationally cold ground state molecules. The case of the cesium atom is very favorable because the 0 g - and 1 u purely long-range states below the dissociation limit 6s 1/2 +6p 3/2 present Condon points at intermediate distances. Such configurations offer rather efficient channels for the creation of singlet or triplet ground state Cs 2 molecules after spontaneous decay.
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.