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arXiv:1108.3739 · Physical Review Letters

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

Felix H. J. Hall, M Aymar, Nadia Bouloufa-Maafa, Olivier Dulieu, Stefan Willitsch

Published Dec 7, 2011• 170 citations
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Abstract

We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of nonadiabatic 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 nonradiative processes for the cold chemistry occurring in ion-atom hybrid traps.

DOI
10.1103/physrevlett.107.243202

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We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of nonadiabatic 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 nonradiative processes for the cold chemistry occurring in ion-atom hybrid traps.
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 cm3 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.
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 /k_B = 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 quantum-scattering 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 short-range 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.
Cold ion-neutral reactive processes were studied in an hybrid trap down to average collision energies <E_coll>/k_B > 20 mK. The atomic ion-neutral systems Ca^+ + Rb and Ba^+ + Rb were studied, and the results interpreted with high-level quantum chemical and quantum scattering calculations. Three reactive processes were found to be in competition, namely non-radiative charge transfer induced by non-adiabatic couplings between potential energy surfaces, radiative charge transfer, and radiative association of molecular ions, which were observed using mass spectrometry. The role of light in these processes was investigated. Enhancement of reaction rate constants from electronically excited entrance channels was observed and rationalised using computed potential energy curves. The collision-energy dependence of the reaction rate constants was investigated, and was found to be in line with predictions of both classical and quantum models uncovering general effects in cold ion-neutral systems. Cold reactions in the molecular ion-neutral system N_2^+ + Rb were also investigated with average collision energies two orders of magnitude lower than previously realised. The reaction rate constant in this system was found to depend strongly on the Rb ^2P_3/2 population, with the rate constant from the corresponding excited entrance channel being significantly faster than the collision rate predicted by Langevin theory. A classical capture model was developed taking into account the charge permanent-quadrupole intermolecular force and was found to reproduce the observed rate. The implied near unit reaction probability was rationalised by a near electronic resonance of entrance and product channels. A detailed description of the implementation of the ion-neutral hybrid trap is given. A thorough comparison of the results from state-of-the-art experiments and theory reveals general features of light assisted cold ion-neutral reactions. \n
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⟩/k B=20 mK-20 K. The lowest energies were achieved in experiments using single localised 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 analysed using high-level quantum-chemical calculations of the potential energy curves of CaRb+ and quantum-scattering 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 short-range 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.
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 2 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 within the range studied, but strongly dependent on the internal state of Rb. Highly efficient charge exchange four times faster than the Langevin rate was observed with Rb in the excited (5p) (2)P(3/2) state. This observation is rationalized by a capture process dominated by the charge-quadrupole interaction and a near resonance between the entrance and exit channels of the system. Our results provide a test of classical models for reactions of molecular ions at the lowest energies reached thus far.
We theoretically investigate the collisional dynamics of laser-cooled 87Rb{}^{87}\text{Rb}87Rb ground-state atoms and 40Ca+{}^{40}{\mathrm{Ca}}^{+}40Ca+ ground-state ions in the context of the hybrid trap experiment of Hall et al. [Phys. Rev. Lett. 107, 243202 (2011)], leading to ion losses. Cold 87Rb40Ca+{}^{87}{\mathrm{Rb}}^{40}{\mathrm{Ca}}^{+}87Rb40Ca+ ground-state molecular ions are created by radiative association, and we demonstrate that they are protected against photodissociation by blackbody radiation and by the 40Ca+{}^{40}{\mathrm{Ca}}^{+}40Ca+ cooling laser at 397 nm. This paper yields an interpretation of the direct observation of 87Rb40Ca+{}^{87}{\mathrm{Rb}}^{40}{\mathrm{Ca}}^{+}87Rb40Ca+ ions in the experiment, in contrast to other hybrid trap experiments using other species. Based on an updated model for molecular spin-orbit interaction, we also confirm that the nonradiative charge-exchange is the dominant loss process for Ca+{\mathrm{Ca}}^{+}Ca+ and obtain rates in agreement with experimental observations and a previous calculation.