Ion-neutral chemistry at ultralow energies: dynamics of reactive collisions between laser-cooled Ca⁺ ions and Rb atoms in an ion-atom hybrid trap

Abstract

Cold chemical reactions between laser-cooled Ca+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies E coll /kB = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca + ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb + molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb + and quantumscattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by shortrange non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.

Ground- and excited-state properties of the polar and paramagnetic RbSr molecule: A comparative study

“…Other studies concern CaLi [41][42][43], LiBe [44,45]. More recently the electronic structure of the related molecular ions containing one alkali atom and Ca + [46], Sr + [47] or Ba + [48][49][50][51] with various high-level approaches have been published in relation with experiments aiming at creating cold molecular ions in merged cold ion and cold atom traps.…”

A Dynamic Ion–Atom Hybrid Trap for High‐Resolution Cold‐Collision Studies

“…Instead, it is preferable to use strings of ions located on the central axis of the ion trap. In this configuration, the micromotion energies are minimised and the ions can be assumed to have negligible kinetic energies compared to the moving atoms [13,27]. However, due to the necessarily small number of ions in this case, collision events are rare and extensive averaging is required.…”

“…The first beam was sent through an AOM for shifting the frequency slightly to the red of the Rb (5s) 2 S 1/2 F = 2 → (5p) 2 P 3/2 F = 3 resonance to provide laser radiation for Doppler laser cooling and magneto-optical trapping. The firstorder diffracted output of the AOM was further passed through an electro-optic modulator (EOM) for generating sidebands repumping population on the Rb (5s) 2 S 1/2 F = 1 → (5p) 2 P 3/2 F = 2 transition [13]. The laser beam was subsequently coupled into a PM fibre, transported to the experiment table and split into two components propagating along the y and z axes of the trap, see Fig.…”

“…Over the past years, these experiments helped to unravel the rich and complex chemistry underlying ion-atom collisions at low temperatures and paved the way for the exploration of a range of interesting phenomena. Charge exchange processes have been observed in many systems [12][13][14][15][16][17]. Reaction rates were found to increase considerably with the electronic excitation of the collision partners [12,[18][19][20].…”

“…However, in a double-trap experiment there is no straightforward way of tuning the kinetic energies without increasing their spread and thereby compromising the energy resolution. For instance, in previous studies the collision energies were varied by increasing the micromotion energy of the ions, i.e., the energy associated with their motion in the oscillating electric field of the RF trap [13,17,18,26]. However, because of the oscillatory nature of the micromotion, this approach also vastly increased the collision-energy spread thus obscuring subtle details of the underlying collisional dynamics [13,21,27].…”