Kinematic cooling

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... 

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. 

The first realization of kinematic cooling was during the performance of inelastic collision studies in a Crossed Atomic and Molecular Beam (C AMB)... 

In our first experiments on the kinematic cooling process, we chose to scatter a molecular beam of NO from an atomic beam of argon. The energetics were selected to provide a velocity vector cancellation that results in the the post collision NO7 5 (NO in the j = 7.5 rotational state) being stationary in the laboratory frame. We will show below why this quantum state of NO is essentially stationary in the laboratory reference frame,... 

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. 

Before we look at the evolution of the phase space density for kinematic cooling, we are going to look at a system where it is accepted that cooling occurs, ID atomic laser cooling. 

In kinematic cooling, the force that an atom or molecule feels is an impulse from the collision with an atom. The change in velocity Av is given by vp — Vo, where vp is determined from Eq. (8.11) vp = Eo — ) + Enot] The force is then F = m Av/dt. For simplicity we will neglect the rotational energy and focus on the translational energy, simphfying... 

There are two key side effects of the velocity dependent force. First, kinematic cooling results in real cooling, not just a rotation of position-momentum phase space, yielding an increased phase space for the cold molecules. Second, since there is dissipation, if the collisions occur in a region containing a trap for the molecules, the trap can be continuously loaded without the worry of how to load pre-cooled molecules into a conservative potential well. 

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. 

Fig. 8.10. Cut through the laboratory origin for the j = 1 state of H Br after kinematic cooling from colliding with 5% Kr doped in He. The image fits to 10 m/s spread, which gives a ID temperature of 500 mK.

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. 

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