Kinematic cooling techniques

In this chapter we will focus on the production and use of cold and ultracold molecules for studies in the field of chemical dynamics of gas phase molecular species. Chemical dynamics is the detailed study of the motion of molecules and atoms on potential energy surfaces in order to learn about the details of the surface as well as the dynamics of their interactions. We want to explore new information, techniques, and insight that can be gained from the use of cold and ultracold molecules. The first step to achieve this requires us to define cold and ultracold in the context of chemical dynamics. We will then discuss the kinematic cooling technique in detail and conclude with several applications of this cooling technique and its potential for guiding and confining kinematically cooled molecules. 

In Sec. 8.2 we discussed the production of cold molecules from a single collision with an atom. It was noted that the cold molecules are necessarily formed at the crossing of the atomic and molecular beams, where the scattering occurs. To date we have put considerable effort to understanding the practical and experimental limits of the crossed molecular beam apparatus for producing cold molecules and what modifications we need and can make in order to produce and confine useful amounts of cold molecules generated from this kinematic cooling technique. 

The kinematic cooling technique we propose is to collide hot molecules with cold atoms in a magneto-optical trap. Since no specific impact angle is required, the molecules can impact the MOT from any angle, thus the molecules can either impinge on the MOT in the form of a molecular beam or as background gas. This technique is illustrated in Fig. 8.16. 

The kinematic cooling technique is not only useful for cooling molecules, but it is easily extended to cooling any atom, since it works on the basis of collisions between particles with equal or near equal masses. It can be applied to cooling atoms with complicated electronic structure which hinder direct laser cooling. 

The importance of the phase space density is to determine if our so called cooling technique , kinematic cooling, actually increases the phase space density or if it is a slowing technique that preserves phase space density. The statistical mechanics approach to this question is to look at the forces acting on the system and to see if that system obeys Liouville s theorem or not. 

Kinematic cooling works for cooling a wide range of molecules and atoms irrespective of the unique properties of the individual molecules and atoms. While this technique is general, it does rely on a favorable mass ratio between the colliding particles, favorable rotational spacings, and favorable differential collision cross-sections. However, as we have demonstrated, even with fixed mass ratios and rotational spacings, the initial velocities and differential cross-sections can be manipulated in order to produce cold molecules. 

This technique can then be generalized to kinematically cool molecules via collisions with atoms in a MOT. The first molecular candidates we propose to scatter from a Rb MOT are DBr (mass 83), RbD (mass 87), Mn02 (mass 87), and HBr (mass 82). While there are several candidates for scattering off rubidium, the NIST chemical handbook lists over 100 molecules that have masses near 85 or 87 amu. 

We will first describe spectroscopy on collimated atomic beams and on kinematically compressed ion beams. Two groups of nonlinear spectroscopic tecliniques will be discussed saturation techniques and two-photon absorption techniques. We will also deal with the optical analogy to the Ramsey fringe technique (Sect. 7.1.2). In a subsequent section (Sect. 9.8) laser cooling and atom- and ion-trap techniques will be discussed. Here, the particles are basically brought to rest, ehminating the Doppler as well as the transit broadening effects. 

---