Formation of ultracold molecules

An experiment by the scheme illustrated in Fig. 8.7 was carried out with molecules by Nikolov et al. (1999) using a magnetooptical trap. In this experiment, the trapped laser-cooled potassium atoms were photoassociated to u = 191 ofthe 

The spontaneous decay of the excited state proceeds by two channels, one leading to a bound state and the other to a free state  

Most decays are bound-free transitions, but a small proportion (0.15%) are bound-bound ones, giving rise to a distribution of highly vibrationally excited levels, with Pranck-Condon factors suggesting that the most populated vibrational state is u = 36. 

The most interesting implementations and applications of laser-induced photoassociation of ultracold atoms have emerged in experiments with quantum gases (BECs and Fermi-degenerate gases). These experiments made it possible to obtain and investigate molecular quantum gases. They are briefly discussed in Section 8.5. 

The photoassociation of ultracold atoms in a trap by means of a tunable laser makes possible the spectroscopy of the energy states of the molecules formed near the dissociation limit, with a very high spectral resolution. To this end, it is necessary to detect the electronically excited molecules being formed. Photoionization of the excited molecules with an additional laser has become the standard technique. Such detection methods were developed earlier for atoms and molecules at room temperatures (Letokhov 1987) (see Chapters 9 and 10), but thanks to their exceptional detection sensitivity (up to single atoms and molecules), they have proved fairly effective in experiments with a small number of excited ultracold molecules confined in traps. Photoassociation spectroscopy uses two approaches (1) spectroscopy of the excited molecular states near the dissociation limit, with variation of the radiation frequency of the first laser, used for the purpose of photoassociation, and (2) spectroscopy of the ionized bound or free states, with variation of the radiation frequency of the second laser, used for the photoionization of the excited molecules. 

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A. Vardi, D. Abrashkevich, E. Frishman, M. Shapiro, Theory of radiative recombination with strong laser pulses and the formation of ultracold molecules via stimulated photorecombination of cold atoms, J. Chem. Phys. 107 (1997) 6166. 

This chapter concentrates on understanding molecules that can be made by combining two cold atoms using either magnetically tunable Feshbach resonance states [1] or optically tunable photoassociation (PA) resonance states [2]. Such resonances provide a mechanism for the formation of ultracold molecules from already cold atoms. In addition, magnetically tunable resonances have been used very successfully... 

Vatasescu, M. and Masnou-Seeuws, F., Time-dependent analysis of tunneling effect in the formation of ultracold molecules via photoassociation of laser-cooled atoms, Eur. Phys. J. D, 21, 191-204, 2002. 

The formation of ultracold molecules starting with readily produced ultracold atoms is an attractive and relatively simple method for obtaining such molecules. There are two widely employed methods for such formation photoassociation (discussed in this section) and magnetoassociation by tuning a magnetic field to convert a Feshbach resonance between two colliding atoms into a very weakly bound molecular level (discussed in the next section). 

Fig. 8.8 Illustration of the two-color formation of ultracold molecules in the electronic ground state by way of (a) two-step resonant excitation and (b) Raman photoassociation.

Fig. 8.9 Illustration of the physics of the magnetic Feshbach resonance (a) molecular potentials involved in a Feshbach resonance (b) formation of ultracold molecules B from ultracold atoms A using a magnetically tunable Feshbach resonance.

Masnou-Seeuws, F., and Pillet, P. (2001). Formation of ultracold molecules (T < 200 uK) via photoassociation in a gas of laser-cooled atoms. Advances in Atomic, Molecular and Optical Physics, 47, 54-127. 

Optical-Optical double resonance (PPOODR), and the lambda technique could also be called pump-dump optical-optical double resonance (PDOODR), or a stimulated Raman process. The lambda approach is the focus of Chapters 6, 7, and 8, because formation of ultracold X- and a-state molecules is the goal. The critical difference compared to ordinary OODR is that the initial state is a continuum state (albeit with an ultracold kinetic energy). 

Pellegrini, R, Gacesa, M., and Cote, R., Giant formation rates of ultracold molecules via Feshbach optimized photoassociation, Phys. Rev. Lett., 101, 053201, 2008. 

Azizi, S., Aymar, M., and Dulieu, O., Prospects for the formation of ultracold ground state polar molecules from mixed alkali atom pairs, Eur. Phys. J. D, 31, 195, 2004. 

Dion, C.M., Drag, C., Duheu, O., Laburthe Toha, B., Masnou-Seeuws, F., and Fillet, R, Resonant coupling in the formation of ultracold ground state molecules via photoassociation, Phys. Rev. Lett., 86, 2253, 2001. 

T Prospects for Control of Ultracold Molecule Formation via Photoassociation with Chirped Laser Pulses... 

When a chirped laser pulse is used for the PA process, a coherent wavepacket is formed in the excited state, and has components in all the vibrational levels within the resonance window. After the pulse, this wavepacket propagates toward short distances. Because population transfer back to the initial state with a second (dump) pulse is a coherent process, it is convenient to use this property to optimize the formation of ultracold stable molecules. For instance, in the chosen example of Cs2 0 (65 -f 6P3/2), the time-dependent Franck-Condon overlap with the bound levels in the lower electronic state can be optimized by achieving a focused wavepacket. 

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