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...

We report on a study of cold reactive collisions between sympathetically-cooled molecular ions and laser-cooled atoms in an ion-atom hybrid trap. Chemical reactions were studied at average collision energies E coll /k B 20 mK, about two orders of magnitude lower than has been achieved in previous experiments with molecular ions. Choosing N + 2 +Rb as a prototypical system, we find that the reaction rate is independent of the collision energy, but strongly dependent on the internal state of Rb. Highly efficient charge exchange about four times faster than the Langevin rate was observed with Rb in the excited (5p) 2 P 3/2 state. This observation is rationalized in terms of a capture process dominated by the chargequadrupole interaction and a near resonance between the entrance and exit channels of the reaction. Our results provide a test of classical models for reactions of molecular ions at the lowest energies reached thus far.

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.

Light-assisted reactive collisions between laser-cooled Ba + ions and Rb atoms were studied in an ion-atom hybrid trap. The reaction rate was found to strongly depend on the electronic state of the reaction partners with the largest rate constant (7(2) × 10 −11 cm 3 s −1 ) obtained for the excited Ba + (6s)+Rb(5p) reaction channel. Similar to the previously studied Ca + +Rb system, charge transfer and radiative association were found to be the dominant reactive processes. The generation of molecular ions by radiative association could directly be observed by their sympathetic cooling into a Coulomb crystal. Potential energy curves up to the Ba + (6s)+Rb(5p) asymptote and reactive-scattering cross sections for the radiative processes were calculated. The theoretical rate constant obtained for the lowest reaction channel Ba + (6s)+Rb(5s) is compatible with the experimental estimates obtained thus far. * Electronic address: stefan.willitsch@unibas.ch arXiv:1301.0724v1 [physics.atom-ph]

Double-resonance spectroscopy has been employed to characterise the autoionising and predissociating Rydberg states of NO converging to the υ + = 0 and υ + = 1 levels of the X 1 + ground state of NO + . Below the lowest ionisation limit, we monitor the formation of the N( 2 D) predissociation product and observe a spectrum dominated by the p(N + = 0) series, with smaller contributions from the p(2) and f(2) series. Many of the lineshapes can be fit to a simple Fano line profile. Upon vibrational excitation, a competing autoionisation channel is opened and we monitor the products of both the dissociation and ionisation channels. The υ + = 1 predissociation spectrum appears much more complex than the υ + = 0 predissociation spectrum, with significant contributions from the p(0), f(2), p(2), s (1) and d(1) series. In contrast, the υ + = 1 ionisation spectrum is dominated by the p(0) and f(2) Rydberg series, with much weaker contributions from s(1), d(1) and p(2) series. The lineshapes in the υ + = 1 predissociation and autoionisation spectra are perturbed and cannot be fit to simple Fano line profiles. In some extreme cases, these perturbations result in the complete disappearance of peaks from either the autoionisation or predissociation spectrum.