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 a new experimental method to study reactive ion-molecule collisions at very low temperatures. A source of laser-cooled ions in a linear Paul trap has been combined with a quadrupole-guide velocity selector to investigate the reaction of Ca+ with CH3F at collision energies E[over](coll)/k(B)> or =1 K with single-particle sensitivity. The technique represents a general approach to study reactive collisions between ions and polar molecules over a wide temperature range down to the cold regime.
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.
We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with lasercooled calcium ions. Using N þ 2 ions as a test system, we achieve >90% selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with stateselected molecular ions and molecular-ion qubits. [5], and novel schemes for quantum-information processing [6]. Such experiments not only require precise control over the translational motion of the molecules, but also over their internal, in particular, rotational-vibrational, quantum state. The preparation of fully quantum-state-selected ultracold molecules and molecular ions constituted one of the major challenges in the field over the past decade and has only very recently been achieved for neutral diatomics synthesized from ultracold atoms (see, e.g., Refs. [1,3,4] and references therein).Translationally cold molecular ions, on the other hand, are conventionally produced from ''hot'' samples by sympathetic cooling using the Coulomb interaction with lasercooled atomic ions [7]. Because low-energy collisions between ions are dominated by the Coulomb interaction which does not couple to the internal degrees of freedom, sympathetically cooled ions exhibit broad distributions of rotational-state populations [8,9]. In such translationally cold, but internally warm samples population can be accumulated in the rotational ground state using optical pumping schemes as demonstrated in two recent studies by Staanum et al. [10] and Schneider et al. [11]. In their experiments, continuous excitation of selected rovibrational transitions in combination with population redistribution aided by blackbody radiation (BBR) enabled them to increase the ground-state population to 37% in MgH þ [10] and 78% in HD þ [11]. Although these values are about an order of magnitude higher than the corresponding thermal populations at room temperature, the state preparation is not complete which reflects the challenges associated with optical pumping in systems with a large number of simultaneously populated levels. Moreover, because the schemes used thus far rely on population transfer via dipole-allowed transitions, they cannot be applied to fundamental apolar ions such as H þ 2 , N þ 2 , and O þ 2 . In the present Letter, we demonstrate a complementary method for the production of rovibrationally state-selected, translationally cold molecular ions which circumvents the problems associated with optical pumping applied to a broad distribu...
Ensembles of cold atomic and molecular ions in ion traps prepared at millikelvin temperatures by laser and sympathetic cooling have recently found considerable interest in both physics and chemistry. At very low temperatures the ions form ordered structures in the trap also known as "Coulomb crystals". Ion Coulomb crystals exhibit a range of intriguing properties which render them attractive systems for novel experiments in chemical dynamics, ultrahigh-resolution spectroscopy and quantum-information processing. In this article we review the methods used to prepare atomic and molecular ion Coulomb crystals and discuss some recent studies in mass spectrometry, low-temperature chemistry and precision spectroscopy to illustrate their scientific potential for chemical applications. Finally, we conclude with an outlook on outstanding challenges and prospective further developments in the field.
Many molecules exhibit multiple rotational isomers (conformers) that interconvert thermally and are difficult to isolate. Consequently, a precise characterization of their role in chemical reactions has proven challenging. We have probed the reactivity of specific conformers by using an experimental technique based on their spatial separation in a molecular beam by electrostatic deflection. The separated conformers react with a target of Coulomb-crystallized ions in a trap. In the reaction of Ca(+) with 3-aminophenol, we find a twofold larger rate constant for the cis compared with the trans conformer (differentiated by the O-H bond orientation). This result is explained by conformer-specific differences in the long-range ion-molecule interaction potentials. Our approach demonstrates the possibility of controlling reactivity through selection of conformational states.
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.
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