Cooling of molecules

The optical cooling techniques discussed so far are restricted to true two-level systems because the cooling cycle of induced absorption and spontaneous emission has to be performed many times before the atoms come to rest. In molecules the fluorescence from the upper excited level generally ends in many rotational-vibrational levels in the electronic ground state that differ Irom the initial level. Therefore most of the molecules cannot be excited again with the same laser. They are lost for further cooling cycles. 

There have been several proposals how molecules might be cooled in spite of the above-mentioned difficulties [1141-1143]. An optical version of these proposals is based on a frequency comb laser, which oscillates on many frequencies, matching the relevant frequencies of the transitions from the upper to the lower levels with the highest transition probabilities [1143]. In this case, the moleeules can be repumped into the upper level from many lower levels, thus allowing at least several pumping cycles. 

A very interesting optical cooling technique starts with the selective excitation of a collision pair of cold atoms into a bound level in an upper electronic state (Fig. 9.17). While this excitation occurs at the outer turning point of the upper-state potential, a second laser dumps the excited molecule down into a low vibrational level of the electronic ground state by stimulated emission pumping (photo-induced association). In favorable cases the level u = 0 can be reached. If the colliding atoms 

A promising nonoptical technique relies on cooling of molecules by collisions with cold atoms. If the gas mixture of atoms and molecules can be trapped in a sufficiently small volume long enough to achieve thermal equilibrium between atoms and molecules, the optically cooled atoms act as a heat sink for the molecules, which will approach the same temperature as the atoms (sympathetic cooling) [1145]. 

An interesting proposai that could be realized uses a cold supersonic molecular beam with flow velocity u, which expands through a rotating nozzle (Fig. 9.18). We saw in Chap. 4 that in supersonic beams the velocity spread around the flow veloc- 

An elegant technique has been developed in several laboratories, where cold helium clusters moving through a gas cell of atoms or molecules pick up these molecules, which then can diffuse into the interior of the helium cluster. The molecules then aquire the low temperature of the cluster. The binding energy is taken away by evaporation of He atoms from the cluster surface [14.40,14.41]. 

Fig 14 16. Rotating nozzle for producing a beam of slow molecules 

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There are several aspects of laser spectroscopy performed with molecular beams that have contributed to the success of these combined techniques. First, the spectral resolution of absorption and fluorescence spectra can be increased by using collimated molecular beams with reduced transverse velocity components (Sect. 4.1). Second, the internal cooling of molecules during the adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This greatly reduces the number of absorbing levels and results in a drastic simplification of the absorption spectrum (Sect. 4.2). 

Another method is based on the sympathetic cooling of molecules in a cold atomic gas which does not react with the molecules but cools their translational energy down to the atomic temperature. 

Bahns, J.T., Stwalley, W.C., and Gould, P.L. Laser cooling of molecules a sequential scheme for rotation, translation and vibration, J. Chem. Phys., 104, 9689, 1996. 

The chapter by Strecker and Chandler is called Kinematic cooling of molecules. Their method relies on single, well-defined collisions in which just the correct amount of momentum is transferred for one of the particles to... 

Fig. 8.16. Schematic of a magneto-optical trap (MOT) used for kinematic cooling of molecules. A MOT is formed from 6 counter propagating lasers and a set of anti-Helmhotz coils. The MOT keeps an atomic sample near ImK while molecules impinge on the trapped atoms, some fraction of the molecules are kinematically cooled.

Kinematic cooling of molecules via collisions with Magneto-Optically trapped atoms provides a straightforward, yet undemonstrated, method for producing cold molecules in environments where they can undergo thermal-izing collisions with cold atoms and potentially be further sympathetically cooled into the ultracold regime. 

From the general quantum-statistical standpoint, Bose-Einstein condensation can be achieved with both atoms and molecules. However the realization of BEC with a molecular gas is a much more formidable task because of the difficulties involved in the deep cooling of molecules. 

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