Session T28: Focus Session: Production and Application of Cold Molecules II

Collision dynamics of molecules and rotational excitons in an ultracold gas confined by an optical lattice

This talk will focus on two problems: (i) inelastic collisions of ultracold molecules confined by optical laser forces; and (ii) dynamics of rotational excitons in an optical lattice of ultracold polar molecules. Optical laser forces can be used to restrict the motion of ultracold particles in one dimension to produce a quasi-2D gas. I will discuss the general features of inelastic scattering and chemical reactions in ultracold quasi-2D gases of molecules. I will demonstrate that the cross sections for inelastic and chemically reactive collisions are suppressed by the confinement forces. This suppression is generally more significant than the effect of the laser confinement on the probability of elastic scattering. The elastic-to-inelastic collision ratios are therefore enhanced in the presence of a laser confinement. Our results thus suggest that applying laser confinement in one dimension may stabilize ultracold systems against reactive collisions. I will show that the threshold energy dependence of cross sections for both elastic and inelastic collisions in quasi-2D gases can be tuned by varying the laser confinement forces and an external magnetic field. Optical laser forces can also be used to generate periodic lattice structures of ultracold molecules. Rotational excitation of molecules in the lattice structures gives rise to rotational excitons. I will show that a combination of an optical confinement and an external dc electric field can be used to induce scattering resonances in collisions of the rotational excitons with lattice impurities. These resonances are analogous to Feshbach resonances in atomic collisions. An external dc electric field can be used to control dynamics of the rotational excitons, leading to localization of excitons by multiple collisions with impurities. The system of ultracold molecules I will describe can thus be used for the detailed study of fundamental physical phenomena such as Anderson localizations.   

Ultracold polar molecules

I will discuss recent experiments at JILA exploring an ultracold gas of fermionic polar molecules. We create a high phase-space-density gas of ground-state KRb molecules and study chemical reactions in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play dominant roles.   

Collisions of ultracold polar molecules in microwave traps

The collisions between linear polar molecules, trapped in a microwave field with a circular polarization, are theoretically analyzed. Here we demonstrate that the microwave trap can provide a successful evaporative cooling for polar molecules not only in their absolute ground state. But the states in which molecules can be safely trapped depend on the frequency and the strength of the AC-field. For $^1\Sigma$ state molecules the collisional dynamics is mostly controlled by two ratios $\nu/B$ and $x=\mu_0E/hB$ ($\nu$ is the microwave frequency, B is the molecular rotational constant, $\mu_0$ is the dipole moment, and $E$ is the electric field strength). We are mostly interested in the lowest energy strong-field-seeking state of the ground vibrational state, $|J=0,M=0>$ and we have found rather large inelastic cross sections for molecules even in this state. But we have found that the nature of this ``inelasticity'' should not cause the loss of molecules from the trap. We conclude that at same cases when the detuning is rather small, the elastic cross section is almost completely defined by the dipole-dipole interaction and it is mostly true for fermionic molecules.From the loading with molecules point of view we suggest that it is ``safer'' to load $|0,0>$ molecules at $\nu/B <2$, $|1, -1>$ at around $\nu/B=3$ , $|2, -2>$ at around $\nu/B =5$ and so on and it would be better to have the parameter $x<1$ which makes the regions of strong mixing smaller. We consider the collisional dynamics of $^2\Pi$ state (like OH) molecules and underline that now ratios $\nu/\Delta$ and $x=\mu_0E/\Delta$ ( $\Delta$ is $\Lambda$- doubling splitting) play as controlling parameters.   

Simple models for ultracold molecular collisions

A simple yet general model of reactive collisions having a quantum defect form is possible based on the separation of the collision dynamics into a long range and a short range part.\footnote{P. S. Julienne, Faraday Disc. 142, 361 (2009).} \footnote{Z. Idziaszek and P. S. Julienne, arXiv:0912.0370.} The long range region governs the quantum transmission of the incident de Broglie wave into the short range region, where reaction occurs. The van der Waals interaction is characterized by a length scale on the order of 5 to 10 nm. Reactive chemical dynamics is characterized by a length scale less than 1 nm. The wave function in the entrance channel in the long range region beyond 1 nm is characterized by two dimensionless quantum defect parameters y and s, which respectively represent the probability of reaction in the short range zone and the phase of the reflected wave from this zone. We find universal collision rate constants in the ``black hole'' limit $y=1$, for which there is a unit probability of loss from the entrance channel in the short range zone and no reflected wave back into the entrance channel. In this limit we get universal low temperature event rate constants depending only on the strength of the van der Waals potential for s wave collisions for bosons or unlike fermions and for p wave collisions of identical fermions. We find good agreement with the recent experimental measurements of the JILA group\footnote{S. Ospelkaus, K. K. Ni, D. Wang, M. H. G. de Miranda, B. Neyenhuis, G. Quéméner, P. S. Julienne, J. L. Bohn, D. S. Jin, and J. Ye, arXiv:0912.3854.} for reactive collision rates for fermionic KRb molecules below 1 microkelvin, as well as for collisions of KRb with K atoms. The model is readily generalized to treat collisions in reduced dimensions and to include the effect of an electric field on polar molecules.