Associative detachment in anion-atom reactions involving a dipole-bound electron

Abstract

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.

Fig. 1. Description of the Rb–OH⁻ system.

a Potential energy surfaces as a function of the distance R_Rb between the Rb atom and the center of mass of O–H (left panel). The angle θ between the OH axis and the Rb atom is chosen to be 80^∘. The diabatic crossing between the excited state RbOH⁻ complex (red curves) and its neutral counterpart RbOH (gray curve), is indicated by the two red dots. The crossing for the ground state RbOH⁻ complex occurs at the inner part of the potentials (blue dot). R_opt stands for the optimized R_Rb distance (minimum of the interaction well) and R_c corresponds to the distance at which the detachment occurs. The crossing in the repulsive region between the anionic and neutral PES for θ = 0^∘ is shown in the right panel. b Reaction path for the AED reaction between ground state Rb and OH⁻ shown for two different collisional angles: θ = 0^∘ (dashed blue line) and θ = 80^∘ (solid blue line). The zero corresponds to the energy of the Rb+OH⁻ entrance channel. The orbital corresponding to the excess electron (highest occupied molecular orbital, HOMO) is shown.

Fig. 2. Hybrid atom-ion system.

a The experimental hybrid atom ion trap system. The OH⁻ ions (purple cloud) are created and loaded from the source chamber and trapped in an octupole radio-frequency (rf) wire trap. A far-threshold laser beam (green) is used to determine the ion density via photodetachment tomography. A laser-cooled cloud of ultracold Rb atoms (orange cloud) is overlapped with the ion cloud. As shown here, the spatial extent of the ion ensemble is significantly larger than that of the atomic cloud. After interaction with the laser or atoms, the time of flight of the ions are extracted onto the detector. b The detected normalized OH⁻ ion count after reaction with the laser-cooled rubidium atoms for excited state fractions of 0.10(2) and 0.28(5) (blue and red data points, respectively). The gray data points depict the ion losses without the presence of rubidium atoms. The ion losses are fitted by Eq. (1) (solid lines). The error bars represent the statistical errors. c Reaction rate coefficient as a function of the excited state fraction. The solid red line is a linear fit through the data points. The slope and intercept of the fit yields the reaction rate coefficient for the excited state and ground state rubidium interacting with OH⁻, respectively. The gray shaded area represents the 1-σ range of statistical uncertainty.