Observation of trap-assisted bound states in ultracold atom-ion collisions

Classically, a collision between two freely moving bodies cannot lead to the creation of a bound state. In many ultracold experiments, however, atoms and ions collide while being trapped. In principle, the presence of the trap, the relative and center-of-mass motion are no longer uncoupled. In atom-ion collisions, in which the ion is deeply trapped in a Paul trap, and an ultracold atom collides with it, energy can be transferred from the relative frame of reference to that of the center-of-mass. This energy transfer can leave the atom bound to the ion for a few 100’s nsec during which multiple short-range collisions occur while this weakly-bound molecule is oscillating in the trap. Here, we report on the detection of these bound molecular states in a linear Paul trap by observing the electronic spin-exchange rates between a Sr⁺ ion and Rb atoms at various magnetic fields. We estimate the bond energy and lifetime of these bound states from our results. We compare these results with numerical simulations and find good agreement.