Controlling the quantum stereodynamics of ultracold bimolecular reactions

Molecular collisions in the quantum regime represent a new opportunity to explore chemical reactions. Recently, atom-exchangereactions were observed in a trapped ultracold gas of KRb molecules. In an external electric field, these polar molecules can easily be oriented and the exothermic and barrierless bimolecular reactions, KRb+KRb→K 2 +Rb 2 , occur at a rate that rises steeply with increasing dipole moment. Here we demonstrate the suppression of the bimolecular chemical reaction rate by nearly two orders of magnitude when we use an optical lattice trap to confine the fermionic polar molecules in a quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along the tight confinement direction. With the combination of sufficiently tight confinement and Fermi statistics of the molecules, two polar molecules can approach each other only in a ‘side-by-side’ collision under repulsive dipole–dipole interactions. The suppression of chemical reactions is a prerequisite for the realization of new molecule-based quantum systems. The investigation of how chemical reactions depend on molecular orientation has a long history. In particular, the spatial anisotropy of the dipole–dipole interaction between polar molecules leads to a dependence of stereodynamics of collisions on long-range interactions. A study with ultracold molecules, where all internal and external states of the molecules can be controlled, now extends such studies into the quantum regime.