Photoassociation experiment

Photoassociation experiments involving alkali-metal systems have stimulated considerable interest in chemical reactivity in alkali-metal dimer-alkali-metal atom collisions at ultracold temperatures [14]. Rearrangement collisions in identical particle alkali-metal trimer systems occur without energy barrier, and recent studies have... 

To interiDret tliis experiment. Julienne and Headier [45] proposed a mechanism that has become tlie standard picture for cold and ultracold photoassociative ionization. Figure Cl.4.14 details tlie model. 

The construction of such a TDMEP was in harmony with femtosecond experiments of "transition state spectroscopy" that had been done in the late 1980s [58, 59]. It is also relevant to phenomena in current ultracold collision physics that go under the name "photoassociation" [60]. 

For the photoassociation of the collision pair Na + Kr a crossed beam experiment was performed (Fig. 8.37). The Na-atoms were excited by a laser into the 3p state. These excited atoms collide with Kr-atoms. The collision pair was further excited by a second laser anticollinear with the first laser into a bound Rydberg state of the NaKr dimer which could decay by fluorescence into a bound A TJ state of the NaKr molecule [1099]. 

The KRb experiments take place in a dual-species vapor-cell MOT [45]. Diode lasers at 767 and 780 nm are used to cool and trap K and Rb atoms, respectively, in the dark-spot MOT configuration. Atomic densities of 3 x lO cm" for K and 1 x 10 cm for Rb are realized, with corresponding temperatures of 300 and 100 tiK. Photoassociation is induced by illuminating the overlapping cold atomic clouds with light from a tunable cw titanium-sapphire laser (Coherent 899-29). This laser typically provides 500 mW with a linewidth of 1 MHz and its output is focused down to approximately match the size ( 300 p,m diameter) of the cold atom clouds. 

Photoassociation of like atoms to form homonuclear molecules, emphasizing the widely studied alkali metals, is treated first. The simple one-color experiments are described in detail, including the variety of techniques used for detection trap loss (decrease in atomic fluoresence), direct detection of excited molecule ionization, detection of fragments by resonance-enhanced multiphoton ionization, and detection of ground or metastable molecules (formed by decay of the upper photoassociation level) by resonance-enhanced multiphoton ionization. 

It is also evident from the material expounded in this volume that the research field of cold molecules has been marching ahead with big strides. What seemed unfeasible less than ten years ago (e.g., the creation of ultracold polar molecules in the absolute ground state by photoassociation, magnetic trapping of dense molecular gases, and the collision experiments with molecules at ultracold temperatures, among others) has now been realized by multiple research groups. 

To interpret the experiment of Lett et Julienne and Heather proposed a mechanism which has become the standard picture for cold and ultracold photoassociative ionization. Fig. 10.17 details the model. Two colliding atoms approach on the molecular ground state potential. During the molasses cycle with the optical fields detuned only about one line width to the red of atomic resonance, the initial excitation occurs at very long range, around a Condon point at 1800 oq. A second Condon point... 

Fig. 10.30. Experimental setup for FVeiburg experiment searching for coherent control and optimization of photoassociation in Rb2.

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 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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