We present a characterization of the ions’ translational energy distribution in a multipole ion trap. A linear mapping between the energy distribution of the trapped ions onto the ions’ time-of-flight (TOF) to a detector is demonstrated. For low ion temperatures, a deviation from linearity is observed and can be attributed to the emergence of multiple potential minima. The potential landscape of the trapped ions is modeled via the finite element method, also accounting for subtleties such as surface-charge accumulation. We demonstrate the validity of our thermometry method by simulating the energy distribution of the ion ensemble thermalized with buffer gas using a Molecular Dynamics (MD) simulation. A comparison between the energy distribution of trapped ions in different multipole trap configurations—i.e., with hyperbolic rods, cylindrical rods, and cylindrical wires—is provided. With these findings, one can map the temperature of the trapped ions down to the Kelvin regime using their TOF distributions. This enables future studies on sympathetic cooling and chemical reactions involving ions in multipole traps.

Associative electronic detachment (AED) between anions and neutral atoms leads to the detachment of the anion’s electron resulting in the formation of a neutral molecule. It plays a key role in chemical reaction networks, like the interstellar medium, the Earth’s ionosphere and biochemical processes. Here, a class of AED involving a closed-shell anion (OH−) and alkali atoms (rubidium) is investigated by precisely controlling the fraction of electronically excited rubidium. Reaction with the ground state atom gives rise to a stable intermediate complex with an electron solely bound via dipolar forces. The stability of the complex is governed by the subtle interplay of diabatic and adiabatic couplings into the autodetachment manifold. The measured rate coefficients are in good agreement with ab initio calculations, revealing pronounced steric effects. For excited state rubidium, however, a lower reaction rate is observed, indicating dynamical stabilization processes suppressing the coupling into the autodetachment region. Our work provides a stringent test of ab initio calculations on anion-neutral collisions and constitutes a generic, conceptual framework for understanding electronic state dependent dynamics in AEDs.

The detachment loss dynamics between rubidium atoms (Rb) and oxygen anions (O−) are studied in a hybrid atom–ion trap. The amount of excited rubidium present in the atomic ensemble is actively controlled, providing a tool to tune the electronic quantum state of the system and, thus, the anion–neutral interaction dynamics. For a ground state Rb interacting with O−, the detachment induced loss rate is consistent with zero, while the excited state Rb yields a significantly higher loss rate. The results are interpreted via ab initio potential energy curves and compared to the previously studied Rb–OH− system, where an associative electronic detachment reactive loss process hinders the sympathetic cooling of the anion. This implies that with the loss channels closed for ground-state Rb and O− anion, this system provides a platform to observe sympathetic cooling of an anion with an ultracold heavy buffer gas.

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