Millikelvin Reactive Collisions between Sympathetically Cooled Molecular Ions and Laser-Cooled Atoms in an Ion-Atom Hybrid Trap

Abstract

We report on a study of cold reactive collisions between sympathetically-cooled molecular ions and laser-cooled atoms in an ion-atom hybrid trap. Chemical reactions were studied at average collision energies E coll /k B 20 mK, about two orders of magnitude lower than has been achieved in previous experiments with molecular ions. Choosing N + 2 +Rb as a prototypical system, we find that the reaction rate is independent of the collision energy, but strongly dependent on the internal state of Rb. Highly efficient charge exchange about four times faster than the Langevin rate was observed with Rb in the excited (5p) 2 P 3/2 state. This observation is rationalized in terms of a capture process dominated by the chargequadrupole interaction and a near resonance between the entrance and exit channels of the reaction. Our results provide a test of classical models for reactions of molecular ions at the lowest energies reached thus far.

Beyond universality: Parametrizing ultracold complex-mediated reactions using statistical assumptions

“…In contrast to neutral species, ions can be easily trapped and cooled. The technology of Coulomb crystals in radiofrequency ion traps [6] and the possibility of combining them with traps for neutrals or with slow molecular beams [7,8] promise great progress in the analysis of ion-neutral reactions in the near future.Theoretical simulations employing standard ab initio approaches are not feasible for most of the systems thus far considered. For heavy systems (more convenient experimentally) there are no potential energy surfaces (PESs) accurate enough to describe processes near thresholds.…”

A Dynamic Ion–Atom Hybrid Trap for High‐Resolution Cold‐Collision Studies

“…As already demonstrated in Ref. [18], the present experiments are not limited to atomic ions, but can readily be extended to sympathetically cooled molecular species. In combination with techniques for the state preparation of the molecular ions, see, e.g., Ref.…”

“…Prospective applications include studies of the energy dependence of ion-neutral processes revealing subtle details of intermolecular interactions [18] and possibly collisional quantum features such as scattering resonances [21] both of which have recently been observed in cold collisions between neutral species [4,35,36]. As already demonstrated in Ref.…”

“…Charge exchange processes have been observed in many systems [12][13][14][15][16][17]. Reaction rates were found to increase considerably with the electronic excitation of the collision partners [12,[18][19][20]. It was also established that molecular ions can be formed by radiative association under the cold and dilute conditions of a hybrid trap [12][13][14]21].…”

“…However, in a double-trap experiment there is no straightforward way of tuning the kinetic energies without increasing their spread and thereby compromising the energy resolution. For instance, in previous studies the collision energies were varied by increasing the micromotion energy of the ions, i.e., the energy associated with their motion in the oscillating electric field of the RF trap [13,17,18,26]. However, because of the oscillatory nature of the micromotion, this approach also vastly increased the collision-energy spread thus obscuring subtle details of the underlying collisional dynamics [13,21,27].…”