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Israel Journal of Chemistry

Playing With a Pair of Ultracold Atoms and Lasers: Towards a Novel Ultracold Photochemistry?

Olivier Dulieu, P. Pillet

Published Oct 1, 2004• 5 citations
Publisher

Abstract

Abstract The use of photoassociation of cold atoms has permitted the first demonstration of ultracold molecules in the microKelvin range. In this article we report the mechanisms and the dynamics studied in the case of the cesium dimer. The first key point is the efficiency of photoassociation for long‐range molecules, which involves a R−3 long‐range potential (R = interatomic distance) due to the dipole‐dipole interaction in the electronically excited potentials. The second one is the existence of the Condon point at intermediate distance to “stabilize” the electronically excited molecules reached by photoassociation after spontaneous decay towards ground state. Several molecular configurations are analyzed such as the so‐called double‐well route or the internal coupling route. We show the possibility to extend these concepts to the case of heteronuclear systems.

DOI
10.1560/rhhr-c4m6-pffn-8a8j

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Cambridge University Press eBooks•Dec 4, 2003•0 citations
Abstract The use of photoassociation of cold atoms has permitted the first demonstration of ultracold molecules in the microKelvin range. In this article we report the mechanisms and the dynamics studied in the case of the cesium dimer. The first key point is the efficiency of photoassociation for long‐range molecules, which involves a R−3 long‐range potential (R = interatomic distance) due to the dipole‐dipole interaction in the electronically excited potentials. The second one is the existence of the Condon point at intermediate distance to “stabilize” the electronically excited molecules reached by photoassociation after spontaneous decay towards ground state. Several molecular configurations are analyzed such as the so‐called double‐well route or the internal coupling route. We show the possibility to extend these concepts to the case of heteronuclear systems.
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.
The paper investigates cold molecules formation in the photoassociation of two cold atoms by a strong laser pulse applied at short interatomic distances, which lead to a molecular dynamics taking place in the light-induced (adiabatic) potentials. A two electronic states model in the cesium dimer is used to analyse the effects of this strong coupling regime and to show specific results: i) acceleration of the ground state population to the inner zone due to a non-impulsive regime of coupling at short and intermediate interatomic distances; ii) formation of cold molecules in strongly bound levels of the ground state, where the population at the end of the pulse is much bigger than the population photoassociated in bound levels of the excited state; iii) the final momentum distribution of the ground state wavepacket keeping the signatures of the maxima in the initial wavefunction continuum. It is shown that the topology of the light-induced potentials plays an important role in dynamics.
We report on the spectra of the long-range attractive molecular states of the Cs/sub 2/ dimer below the dissociation limits 6s+6p/sub 1/2/ and 6s+6p/sub 3/2/ which are accessible by molecular photoassociation of cold Cs atoms. For the states O/sub u//sup +/ and O/sub g//sup -/(6s+6p/sub 3/2/), we have performed trap-loss measurements, which are in good agreement with the theoretical calculations, performed in a perturbative approach. For the O/sub g//sup -/ and 1/sub u/(6s+6p/sub 3/2/) states, we observe, after spontaneous decay of the electronically excited molecules, the formation of translationally cold molecules. A rate of formation of cold molecules of the order of /spl sim/10/sup 6/ molecules per second is obtained in the case of the state O/sub g//sup $/.
This paper investigates the formation of cold molecules in the photoassociation of two cold atoms by a strong laser pulse applied at short interatomic distances, which lead to molecular dynamics taking place in the light-induced (adiabatic) potentials. A two-electronic-state model in the caesium dimer is used to analyse the effects of this strong coupling regime and to show specific results: (i) acceleration of the ground-state population to the inner zone due to a non-impulsive regime of coupling at short and intermediate interatomic distances, (ii) formation of cold molecules in strongly bound levels of the ground state, where the population at the end of the pulse is much bigger than the population photoassociated in bound levels of the excited state, and (iii) the final momentum distribution of the ground-state wavepacket keeping the signatures of the maxima in the initial wavefunction continuum. It is shown that the topology of the light-induced potentials plays an important role in dynamics.
Introduction If, while approaching on an unbound ground-state potential, two atoms absorb a photon and couple to an excited bound molecular state, they are said to undergo photoassociation. Figure 5.1 illustrates the process. At long range electrostatic dispersion forces give rise to the ground-state molecular potential varying as C 6 / R 6 . If the two atoms are homonuclear, then a resonant dipole–dipole interaction sets up ± C 3 / R 3 excited-state repulsive and attractive potentials. Figure 5.2 shows the actual long-range excited potential curves for the sodium dimer, originating from the 2 S ½ + 2 P 3/2 and 2 S ½ + 2 P ½ separated atom states. For cold and ultracold photoassociation processes the long-range attractive potentials play the key role; the repulsive potentials figure importantly in optical shielding and suppression, the subject of Chapter 6. In the presence of a photon with frequency ω p the colliding pair with kinetic energy k B T couples from the ground-state to the attractive molecular state in a free–bound transition near the Condon point R C , the point at which the difference potential just matches ћω p . Scanning the probe laser ω p excites population of vibration–rotation states in the excited bound potential and generates a free–bound spectrum. This general class of measurements is called photoassociative spectroscopy (PAS) and can be observed in several different ways. The observation may consist of bound-state decay by spontaneous emission, most probably as the nuclei move slowly around the outer turning point, to some distribution of continuum states on the ground potential controlled by bound–free nuclear Franck–Condon overlap factors.