Charge Exchange Dynamics in Cold Collisions of $^{40}$CaH$^+$ and $^{39}$K

We report the observation of charge-exchange collisions between trapped calcium monohydride molecular ions ($^{40}$CaH$^+$) and ultracold potassium atoms ($^{39}$K) in a hybrid ion-atom trap. The measured charge-exchange rate coefficient is significantly suppressed relative to the Langevin rate constant for the system. We use quantum-chemical calculations to model the (CaH-K)$^+$ system in the ground and excited electronic states and to identify possible charge-exchange mechanisms. Our calculations do not fully explain the measured rate, highlighting the need for a full-dimensional quantum treatment that includes vibrational motion and intermediate complex formation. Our work demonstrates that cold hybrid ion-atom platforms with molecular ions enable access to richer chemical complexity and collisional dynamics inaccessible in purely atomic systems.

💡 Research Summary

In this work the authors investigate charge‑exchange (CE) collisions between trapped calcium monohydride molecular ions ( ⁴⁰CaH⁺ ) and ultracold ³⁹K atoms using a hybrid ion‑atom apparatus that combines a linear Paul trap with a three‑dimensional magneto‑optical trap (MOT) and a time‑of‑flight mass spectrometer. Approximately 120 ⁴⁰Ca⁺ ions are loaded into the Paul trap and partially converted (≈25 %) into ⁴⁰CaH⁺ by a brief H₂ leak; the molecular ions are sympathetically cooled by the remaining Ca⁺ ions. The CaH⁺ ions are assumed to be in their electronic and vibrational ground state, while their rotational population follows a thermal distribution over the first 15 J levels at room‑temperature black‑body radiation. The MOT provides a dense cloud of ³⁹K atoms (≈5 × 10⁸ cm⁻³) with a controllable fraction of atoms in the excited 2P₃/₂ state (average population p ≈ 13.6 %).

The experiment proceeds by overlapping the ion crystal with the K cloud for variable interaction times (0–3 s), then extracting the ions for TOF‑MS analysis. The decay of CaH⁺ and the concomitant rise of K⁺ are fitted to a pseudo‑first‑order rate law, ln