Triatomic Photoassociation in an Ultracold Atom-Molecule Collision

Ultracold collisions of neutral atoms and molecules have been of great interest since experimental advances enabled the cooling and trapping of such species. This study develops a simplified theoretical treatment of a low-energy collision between an alkali atom and a diatomic molecule accompanied by absorption of a photon from an external electromagnetic field. The long-range interaction between the two species is treated, including the atomic spin-orbit interaction. The long-range potential energy curves for the triatomic complex are calculated in realistic detail, while effects of the short-range behavior are mimicked by applying different boundary conditions at the van der Waals length. For neutral colliding species, the leading interaction term is the dipole-dipole interaction. In the case of nonpolar dimers like Cs₂, the second leading term is the quadrupole-quadrupole interaction. However, there is also a strong dipole-quadrupole interaction for dimers with a large permanent dipole moment such as NaCs, making the dipole-quadrupole interaction the second leading term for an atom colliding with a polar dimer. Our applications of the simplified treatment show a higher density of trimer states for a polar dimer compared to the case of a nonpolar dimer like Cs₂. This is a consequence of the strong quadrupole-dipole coupling between the atom and the dimer dipole moment.