Molecular Collisions at Near Absolute Zero Reveal Surprising Chemical Slowdown

Scientists have investigated charge-exchange collisions between calcium monohydride (CaH) and ultracold potassium atoms, revealing a significantly suppressed rate coefficient compared to classical Langevin predictions. Swapnil Patel, Dibyendu Sardar, and Jyothi Saraladevi from Duke University and the University of Warsaw, along with Michał Tomza and Kenneth R. Brown, performed both experimental measurements and quantum-chemical calculations to explore the underlying dynamics of this hybrid atomic-molecular system. This research is significant because it demonstrates access to previously inaccessible chemical complexity and collisional phenomena using cold, hybrid platforms, and highlights the limitations of current theoretical models, necessitating a full-dimensional quantum treatment to fully understand the observed charge-exchange mechanisms.

The measured charge-exchange rate coefficient is notably suppressed when compared to predictions based on the classical Langevin rate constant for this system.

This discrepancy suggests that the standard theoretical models fail to fully capture the complexities of the interaction, prompting a detailed investigation into the underlying mechanisms. Researchers employed quantum-chemical calculations to model the system in both ground and excited electronic states, aiming to identify the pathways responsible for charge transfer.
The work demonstrates a significant advancement in controlling and studying collisions between molecular ions and neutral atoms, opening avenues for exploring chemical dynamics previously inaccessible in purely atomic systems. Specifically, the study focuses on 40CaH+ molecular ions colliding with ultracold 39K atoms, meticulously characterising the charge-exchange process both experimentally and theoretically.

Analysis of the reaction rate coefficient, coupled with sophisticated quantum-chemical modelling, reveals the crucial role of molecular vibrations and the potential formation of intermediate complexes during the charge-exchange reaction. This hybrid trap system, integrating a linear Paul trap with a magneto-optical trap, allows for precise control over the overlap between the ions and atoms.
The experimental setup, detailed in accompanying documentation, utilizes a time-of-flight mass spectrometer to monitor the ion composition following the collision. By varying the interaction time and MOT parameters, researchers were able to track the evolution of the ion populations, observing the conversion of 40CaH+ and 39K into reaction products.

The observed TOF-MS signals clearly indicate the presence of potassium, calcium, and calcium hydride ions, providing direct evidence of charge transfer. Quantum-chemical calculations were performed to map the potential energy surfaces governing the collision dynamics. These calculations explored both ground and excited electronic states, identifying potential charge-exchange mechanisms and providing insights into the factors influencing the observed rate coefficient.

Despite the sophistication of these calculations, they do not fully account for the experimentally measured rate, indicating the need for more comprehensive theoretical treatment that incorporates vibrational motion and the possibility of transient intermediate complex formation during the collision process. This research establishes a platform for investigating ultracold chemical reactions with molecular ions, promising new insights into fundamental chemical processes and potentially impacting fields such as quantum information processing and precision spectroscopy.

Hybrid trap loading and calcium monohydride formation

A time-of-flight mass spectrometer serves as the primary diagnostic tool in this study of charge-exchange collisions between calcium monohydride (CaH) molecular ions and ultracold potassium atoms. The experimental setup utilizes a hybrid ion-atom trap, initially loading approximately 120 calcium ions (40Ca+) into a linear four-rod Paul trap with a radius of 9.6mm driven at 2π × 1.7MHz.

Calcium monohydride (40CaH+) ions are then generated by introducing hydrogen gas at a pressure of 4 × 10−8 torr into the chamber for four minutes, converting roughly 25% of the calcium ions to the molecular species. Sympathetic cooling from the laser-cooled calcium ions ensures the co-trapping and cooling of the molecular ions, with assumptions made regarding thermalization of vibrational and rotational degrees of freedom with room temperature black-body radiation.

The initial rotational population of 40CaH+ is distributed across the first 15 rotational states, peaking at J = 4 with approximately 13% population. A three-dimensional magneto-optical trap, employing 767nm trapping beams and an anti-Helmholtz coil configuration, is used to capture and confine ultracold 39K atoms.

The potassium MOT is initially loaded away from the ion trap centre, then overlapped with the ion crystal by deactivating an additional Helmholtz field, allowing for variable interaction times ranging up to seven seconds. Following the interaction period, the MOT is extinguished, and the resulting ions are extracted to the time-of-flight mass spectrometer, which boasts a resolution better than 1 amu and directly quantifies the number of ions present.

Example mass spectra, presented in Fig0.1(b), demonstrate the detection of K+, Ca+, and CaH+ ions. Integrated mass spectrometry signals, plotted as a function of interaction time in Fig0.1(c), are fitted with pseudo-first order rate constants to determine charge-exchange dynamics. Complementing the experimental work, ab initio quantum-chemical calculations were performed to model potential energy surfaces for both ground and excited electronic states of the CaH-K system.

The internally contracted multireference configuration interaction method restricted to single and double excitations, MRCISD, was employed alongside the coupled cluster method CCSD(T) using the augmented correlation-consistent polarized weighted core-valence quintuple-ζ basis set to investigate electronic structure and reaction pathways. These calculations included one-, two-, and three-dimensional potential energy surface explorations to characterise the system’s energetic landscape and inform the interpretation of the observed charge-exchange rates.

Suppressed charge exchange between calcium monohydride and ultracold potassium atoms

Charge-exchange rate coefficients of 0.72 × 10−9 cm3s-1 were measured for collisions between calcium monohydride (CaH) molecules and ultracold potassium atoms. These rates are significantly suppressed when compared to the Langevin rate constant for the system, indicating a more complex interaction than simple classical models predict.

The study employed a hybrid trap to facilitate these collisions and utilised quantum-chemical calculations to investigate potential charge-exchange mechanisms. Experimental determination of the rate coefficients involved observing the decay of 40CaH+ ions and the concurrent growth of 39K+ ions over varying ion-atom hold times.

Ion numbers were fitted to a pseudo-first-order rate equation, ln[Nx(t)/Nx] = −kxt, where Nx(t) represents the number of ions of species x at time t and kx is the corresponding fit rate. By manipulating the trapping laser intensity and anti-Helmholtz field strength, researchers controlled the average excited state population of the potassium atoms to probe state-dependent charge exchange.

Analysis of the data using both constant and linear models revealed a ground state rate of 0.29 × 10−9 cm3s-1 and an excited state rate of 1.99 × 10−9 cm3s-1. While the linear fit showed a modest preference, both models exhibited large reduced chi-squared values of 5.61 and 5.20 respectively, suggesting considerable scatter in the data.

The ground state rate is an order of magnitude smaller than the classical Langevin rate coefficient of 3.44 × 10−9 cm3s−1 for the S1/2 state, and the excited state rate approaches the Langevin limit within a factor of 2.5 for 100% P-state population, which is 4.99 × 10−9 cm3s−1. Even when scaling the Langevin rate by the excited state population, experimental values remained notably suppressed.

Calculations explored potential energy surfaces for the (CaH-K)+ system, revealing several energetically accessible channels including those with CaH in excited electronic states and a reactive channel leading to Ca+ and KH. These calculations, however, did not fully explain the observed suppression of the charge-exchange rate, suggesting the need for more comprehensive quantum treatments that account for vibrational motion and intermediate complex formation. This work demonstrates the potential of cold hybrid atom-molecule platforms for accessing complex chemical dynamics beyond the reach of purely atomic systems.

Charge transfer dynamics in ultracold calcium monohydride-potassium collisions

Scientists have observed charge-exchange collisions between calcium monohydride molecules and ultracold potassium atoms within a hybrid atomic system. The experimentally determined rate coefficient for these collisions is notably lower than predicted by the Langevin rate constant, which describes the classical limit of ion-atom interactions.

Quantum-chemical calculations were performed to model the interactions between calcium monohydride and potassium in both ground and excited electronic states, aiming to identify the mechanisms driving charge exchange. These calculations explored potential energy surfaces to assess the possibility of direct charge transfer or non-radiative pathways, but did not fully account for the suppressed experimental rate.

The research demonstrates the potential of utilising cold hybrid atomic systems, incorporating molecules, to investigate complex chemical dynamics that are inaccessible in purely atomic systems. The authors acknowledge limitations in their modelling, specifically the need for more comprehensive quantum treatments that incorporate vibrational motion and the formation of intermediate complexes during collisions. Future research could focus on developing such full-dimensional quantum models to better understand the observed rate suppression and to explore the role of these more complex interactions in the collision process.