Light-Assisted Ion-Neutral Reactive Processes in the Cold Regime: Radiative Molecule Formation versus Charge Exchange

Abstract

We present a combined experimental and theoretical study of cold reactive collisions between lasercooled Ca + ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of non-adiabatic and radiative charge exchange as well as radiative molecule formation using high-level electronic structure calculations. We study the role of light-assisted processes and show that the efficiency of the dominant chemical pathways is considerably enhanced in excited reaction channels. Our results illustrate the importance of radiative and non-radiative processes for the cold chemistry occurring in ion-atom hybrid traps.Over the past few years, impressive progress has been achieved in the study of reactive collisions at ultralow energies. Recent landmark studies using neutral molecules highlighted the distinct quantum character of reactive processes in this regime and demonstrated new approaches for an unprecedented control of molecular collisions [3,4]. Ion-neutral reactions are another class of processes which exhibit different long-range interactions and therefore a different chemical behavior in comparison to neutrals [5][6][7][8][9][10][11]. With the development of hybrid traps in which laser-cooled atomic ions stored in a radiofrequency ion trap are combined with ultracold neutral atoms in a magneto-optical trap [12][13][14] or a BoseEinstein-condensate [15,16], the study of ion-neutral reactions in the energy range between 1 and 10 −3 Kelvin (usually termed the "cold" regime) has recently become possible. Under these conditions, only a few partial waves contribute to the collision so that resonance as well as radiative effects can become important [5,6,10,17].One key question pertains to the types of chemical processes which can occur in hybrid traps. So far, either fast near-resonant homonuclear charge exchange (in Yb-Yb . For Rb-Yb + , the latter observation was rationalized in terms of radiative and non-radiative charge exchange [18]. The feasibility of molecular-ion formation has also been considered, and evidence for a radiative mechanism has recently been found in the Ca-Yb + system [14]. However, a general understanding of the interplay between these reactive processes and in particular the role of light remains to be established.In the current study, we present a combined experimental and theoretical study of ion-neutral reactive collisions in a Rb-Ca + hybrid trap. Our experimental results are interpreted using high-level electronic structure calculations of the CaRb + potential energy curves (PECs) up to the twenty-second dissociation limit. We observe rich chemical dynamics which we rationalize in terms of nonadiabatic and radiative effects. We show that the efficiency of the dominant chemical processes (radiative molecule formation, radiative and non-radiative charge exchange) is considerably enhanced in excited reaction channels populated in the presence of radiation. Using Rb-Ca + as a model system, our results illustrate the reactive processes which can occu...

Charge-exchange collisions between ultracold fermionic lithium atoms and calcium ions

“…Similar results have been reported for experimental trials using the Rb-Yb + system, in which the reaction rates of ions in the metastable D 3/2 and F 7/2 states were investigated in detail [7]. Energy level crossing may thus occur in the Li + +Ca (3d4s 3 D) level and Li + Ca + (D 3/2 or D 5/2 ) levels at sufficiently short distances at which the interaction potentials may be modified when working at high energy levels [16,21]. A quantitative understanding of the charge exchange collision coefficients determined for our system will require further studies of the related energy levels.…”

“…The inelastic collisions can result in either charge exchange or molecular ion formation and, in order to determine a detailed mechanism, mass spectrometry [5,15,16] of the reaction products is performed following the inelastic collisions. In these trials, a pure Ca + ion cloud consisting of ∼400 ions is mixed with Li atoms for 4 s. Subsequently, an oscillating electric field is applied to one of the ion trap electrodes to excite the radial motions of the ions.…”

Quantum-defect theory for−1/r4-type interactions

“…Renewed interest in QDT for −C 4 /r 4 polarization potential has arisen with the emergence of cold ion-atom [11][12][13][14][15][16][17][18][19][20] and ion-molecule interactions and reactions [21][22][23][24][25][26]. For such heavier systems, the QDT takes on a different magnitude of importance for three primary reasons.…”

“…(42), and the F ǫsl (ν), defined by Eq. (19), which is the result of careful analytic continuation through entire functions [47]. The M ǫsl function, which is function of the scaled energy ǫ s , is one of the most important mathematical entities in QDT for the −1/r 4 potential.…”

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

“…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].…”