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Comptes Rendus Physique

Photoassociation spectroscopy of ultra-cold long-range molecules

Nicolas Vanhaecke, D. Comparat, A. Crubellier, P. Pillet

Published Mar 1, 2004• 6 citations
Publisher

Abstract

Spectroscopic study of so-called long-range molecules, with average interatomic distances much larger than usual for chemical bonds, is conveniently performed by photoassociation (PA) of ultracold atoms. We present here a review of our results obtained by one and two-photon PA spectroscopy of the Cs 2 molecule, using as intermediate the 0 g − and 1 u potentials correlated to the (6 s +6 p 3/2 ) asymptote. These results have been interpreted using an asymptotic theoretical approach, which was specifically build for such long-range molecules.

DOI
10.1016/j.crhy.2004.01.018

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The Journal of Chemical Physics•Jul 8, 2004•30 citations
Spectroscopic study of so-called long-range molecules, with average interatomic distances much larger than usual for chemical bonds, is conveniently performed by photoassociation (PA) of ultracold atoms. We present here a review of our results obtained by one and two-photon PA spectroscopy of the Cs 2 molecule, using as intermediate the 0 g − and 1 u potentials correlated to the (6 s +6 p 3/2 ) asymptote. These results have been interpreted using an asymptotic theoretical approach, which was specifically build for such long-range molecules.
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.
Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold (<1mK)(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})(<1mK) temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.
The creation of ultracold molecules is currently limited to diatomic species. In this Letter, we present a theoretical description of the photoassociation of ultracold atoms and molecules to create ultracold excited triatomic molecules, thus being a novel example of a light-assisted ultracold chemical reaction. The calculation of the photoassociation rate of an ultracold Cs_{2} molecule in its rovibrational ground state with an ultracold Cs atom at frequencies close to its resonant excitation is reported, based on the solution of the quantum dynamics involving the atom-molecule long-range interactions and assuming a model potential for the short-range physics. The rate for the formation of excited Cs_{3} molecules is predicted to be comparable with currently observed atom-atom photoassociation rates. We formulate an experimental proposal to observe this process relying on the available techniques of optical lattices and standard photoassociation spectroscopy.
Photoassociation (PA) of cold atoms is a key tool widely used for accurate determination of long-range molecular potentials, measurement and tuning of interatomic interactions, as well as creation of desired cold molecules. We report the observation of PA spectra of ultracold ytterbium (Yb) atoms at the 1 S 0 — 3 P 1 inter-combination line by detecting the resonant loss of atoms trapped in a crossed far-off-resonance trap (crossed-FORT). Eight vibrational levels in the 1 0 + u molecular state are observed for PA laser detunings up to 1 GHz with respect to the atomic resonance. The photoassociation resonances have a Lorentzian shape and sub-MHz linewidths due to the ultra low temperature of the atoms, and the resonance positions agree well with that predicted by the semiclassical formula.
We have performed high precision photoassociation spectroscopy of ultracold cesium gas. Using trap-loss fluorescence detection and controlling the background cesium pressure we were able to photoassociate atoms into excited states of ultracold molecules with large detunings up to 56 cm(-1) below the Cs(6S(1/2)) + Cs(6P(1/2)) atomic asymptote. Vibrational progressions are assigned to 0(g)(-), 0(u)(+), and 1(g) long-range states. By fitting the spectral data to the LeRoy-Bernstein expression, the effective coefficients of the leading long-range interactions and the vibrational quantum number at dissociation are obtained. In addition we have observed spectral perturbations between states of the same symmetry belonging to different asymptotes (6P(1/2) and 6P(3/2)). The perturbations are manifested through irregular vibrational level spacings and are especially pronounced in the 0(u)(+) symmetry. Many observed rotational levels indicate d- and higher partial wave contributions to the photoassociation cross section in the presence of trapping laser light, while spectral regions with only weak features suggest nodes in the lower state wave functions corresponding to the two ground state atoms asymptote.