Direct observation of ultracold atom-ion excitation exchange

Abstract

Ultracold atom-ion collisions are an emerging field of research that can ultimately lead to their precise quantum control. In collisions in which the ion is prepared in an excited state, previous studies showed that the dominant reaction pathway was charge exchange. Here, we explored the outcome products and the energy released from a single ultracold collision between a single 88 Sr + ion and a single 87 Rb atom prepared in excited metastable and ground electronic states, respectively, with control over their relative spins. We found that the ion's long-lived D 5/2 and D 3/2 states quench after roughly three collisions, acquiring immense kinetic energy in the process. By performing single-shot thermometry on the ion after the collision, we identified two dominant reaction pathways: electronic excitation exchange and spin-orbit change. In contrast to previous experiments, we observed no charge-exchange events. These processes are theoretically understood to occur through Landau-Zener avoided crossings leading to the observed reaction pathways. We also found that spin orientation has almost no effect on the reaction pathways, due to strong Coriolis-spin mixing. Our results provide a deeper understanding of ultracold atom-ion inelastic collisions and offer additional quantum control tools for the cold chemistry field.

High-energy-resolution measurement of ultracold atom-ion collisional cross section

“…To demonstrate our method, we measured the energy dependence of a non-adiabatic quench of a metastable electronically excited level of the ion during a collision with a ground-state atom . In previous work [36], we found that the excited long-lived 4d 2 D 5/2 and 4d 2 D 3/2 states of the 88 Sr + ion, quench after roughly three Langevin collisions with ground-state 87 Rb atoms, and that the excitation energy is transformed into kinetic energy of the colliding particles.…”

“…In Ref. [36] we identified two types of collisional quenching. One is the EEE where the ion relaxes to the ground S state and the atom is excited to the P state followed by energy release of ∼ 3000 K•k B .…”

“…After the atoms passed through the ion, we performed a single-shot Doppler thermometry [41] on the ion to detect the quenched (hotter than ∼10's K•k B ) events from non-quenched events. Due to the large energy separation between the SOC (400 K•k B ) and EEE (3000 K•k B ) energy release, these events are easily separated in the single-shot thermometry [36]. As a control experiment we tested whether quench events are detected in the absence of atoms in the optical lattice.…”

“…We demonstrated our method by measuring the energy dependence of the inelastic collisions cross sections of the Electronic-Excitation Exchange (EEE) and Spin-Orbit Change (SOC), channels that occur when a 88 Sr + ion, optically excited to the 4d 2 D 5/2 meta-stable state, collides with ground-state 87 Rb atoms. These processes were shown [36] to occur through a non-adiabatic Landau-Zener crossing and their energy dependence was only theoretically discussed up to now [14,37], for the same collisional energy range. We measured the energy dependence of the inelastic collision cross section of the EEE channel and the SOC channels separately.…”

Interaction potentials and ultracold scattering cross sections for the Li+7–Li7 ion-atom system

“…For energy E k B × 1 mK, an ion-atom collision involves many partial waves , due to the strongly attractive long-range nature of their interaction [17], allowing a semiclassical description of the collision. Despite continuous progress regarding the precise control of the trapped ion motion, reaching the ultralow relative energy regime (E /k B ≈ 1 μK or lower) for ion-atom collisions is still challenging experimentally [4,6,[18][19][20]. At these energies, quantum effects emerge, as few partial waves contribute to the collision.…”