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

TL;DR

This work reports the first observation of charge-exchange collisions between trapped $^{40}$CaH$^+$ molecular ions and ultracold $^{39}$K atoms in a hybrid trap, finding rates significantly suppressed relative to the Langevin limit. Using high-level ab initio calculations (MRCISD and CCSD(T)) and the IOS approximation, the authors map potential energy surfaces and evaluate radiative and non-radiative CE pathways, concluding that direct single-surface transfer is unlikely and that intermediate complex formation and vibrational dynamics likely govern the observed rates. Experimentally, the CE rate shows little or weak dependence on the K MOT excited-state population, with extracted rates $k_S = 0.29(27) imes10^{-9}$ cm$^3$ s$^{-1}$ and $k_P = 1.99(81) imes10^{-9}$ cm$^3$ s$^{-1}$, compared to Langevin rates $k_L(S_{1/2})=3.44 imes10^{-9}$ cm$^3$ s$^{-1}$ and $k_L(P_{3/2})=4.99 imes10^{-9}$ cm$^3$ s$^{-1}$. The findings reveal rich molecular-collision dynamics in cold hybrid systems and underscore the need for full-dimensional quantum treatments that include vibrational motion and intermediate complexes, with implications for sympathetic cooling and internal-state control of molecular ions.

Abstract

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.

Figures (4)

• Figure 1: (a) Experimental sequence for interactions between CaH+ and K. The ion-atom interaction time and the 39K MOT parameters are varied to characterize the CE reaction. (b) Example TOF-MS signal indicating the presence of mass at 39, 40, 41 amu corresponding to K+, Ca+, CaH+ ions in the trap. The blue and the red curve show the signal after ion-atom hold time of 0 and 3 seconds respectively. The inset shows the complete signal range. (c) Integrated MS signal as a function of ion-atom hold time for K+, Ca+, and CaH+ with pseudo-first order rate fits (solid lines). Error bars correspond to one standard deviation. The MOT density and average P-state population are $4.97(42) \times 10^8$ cm-3 and $13.6(2)\%$ respectively.
• Figure 2: Measured charge exchange rate coefficients as a function of the average excited state population of 39K atoms (2P3/2). Two model fits are shown: a constant fit (dashed red line) assuming no internal state dependence, and a linear fit (solid blue line) allowing for state dependence. The fit is represented by $k_{eff} = (k_P - k_S)p + k_S$, where $p$ is the average excited state population of the K atoms, and $k_S$ and $k_P$ are the rates for the ground and the excited state. The shaded regions represent $1\sigma$ confidence bands. The effective Langevin rate (solid green line) represents a weighted average of the S- and P-state Langevin rates.
• Figure 3: Energetics of charge-exchange collisions. Asymptotic energies of possible atom-molecule entrance and product channels below 22,000$\,$cm$^{-1}$ for the (Ca-H-K)$^+$ system. Channels on the left correspond to the arrangements with a charge formally assigned to Ca and on the right -- to K. The channels in bold on the left represent the energetically accessed initial states.
• Figure 4: One-dimensional cut through the potential energy surfaces for the ground and excited electronic states of the (CaH-K)$^+$ system in the linear geometry. Pink circles highlight the crossings between the entrance channel with higher-energy states of different symmetry. The solid orange curve represents the entrance channel potential shifted by the photon energy ($h\nu \approx 13,000\,$cm$^{-1}$), illustrating the resonance with the exit channel. The dashed gray line represents the ground-state interaction for Ca$^+$ with KH (note a different definition of the $x$ axis for this curve resulting in an artificial crossing with the lowest PES for CaH+K$^+$).