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Institute of Physics eBooks

Association spectroscopy

Bimalendu Deb

Published Jan 1, 2026• 0 citations
Publisher

Abstract

In contrast to atomic cooling achieved through laser techniques, molecular cooling generally involves not only cooling translational motion but also vibrational and rotational motion. Cooling molecules directly using the laser cooling method is an extremely difficult or even impossible-seeming task in most cases at present. An alternative approach is to associate two colliding cold atoms to form a molecule that is translationally and rotationally cold. The method by which two cold atoms bind together into a molecule in the presence of external fields is called ‘association spectroscopy,’ which is an inelastic collision process afforded by an external field. There are three kinds of association spectroscopy: (1) photoassociation (PA), (2) magnetoassociation, and (3) magneto-optical association. We briefly discussed PA in the previous chapter. This type of spectroscopy is a result of advancements in the field-assisted collision physics of cold atoms. In this chapter, we provide a description of these three types of association spectroscopy.

DOI
10.1088/978-0-7503-3959-9ch6

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In contrast to atomic cooling achieved through laser techniques, molecular cooling generally involves not only cooling translational motion but also vibrational and rotational motion. Cooling molecules directly using the laser cooling method is an extremely difficult or even impossible-seeming task in most cases at present. An alternative approach is to associate two colliding cold atoms to form a molecule that is translationally and rotationally cold. The method by which two cold atoms bind together into a molecule in the presence of external fields is called ‘association spectroscopy,’ which is an inelastic collision process afforded by an external field. There are three kinds of association spectroscopy: (1) photoassociation (PA), (2) magnetoassociation, and (3) magneto-optical association. We briefly discussed PA in the previous chapter. This type of spectroscopy is a result of advancements in the field-assisted collision physics of cold atoms. In this chapter, we provide a description of these three types of association spectroscopy.
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.
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.
Advances in laser cooling of neutral atoms have made possible a new form of high-resolution laser spectroscopy: photoassociation of ultracold atoms. Colliding neutral atoms, confined in a laser trap, are photoassociated to bound excited states of the dimer molecule by absorbing a photon from a tunable laser. The technique can probe long range and “purely long range” molecular states that are difficult or impossible to detect by traditional means and, because of the extremely low energy of the colliding atoms (<1 mK), is capable of high resolution (<0.001 cm-1). The spectra are useful for atomic lifetime measurements, determination of atomic ground-state scattering information, and measurement of curve-crossing probabilities. Theoretical and experimental work in the field, including multiple resonance techniques and photo association line shapes, are reviewed.
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.
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.
Advances in laser cooling of neutral atoms have made possible a new form of high resolution spectoscopy: photoassociation of ultracold atoms. Colliding neutral atoms, confined in a laser trap, are photoassociated to bound excited states of the dimer molecule by absorbing a photon from a tunable laser. The technique can probe long range and {open_quote}purely long range{close_quote} molecular states that are difficult or impossible to detect by traditional means and, because of the extremely low energy of the colliding atoms (1 mK), is capable of high resolution (0.001 cm{minus}1). Recent results include, the first direct (and most precise) measurement of a molecular dissociation limit, the first observation of retardation effects in atom-atom interactions, high precision measurements of atomic lifetimes, and the study of exotic states of alkali dimers.