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Journal of Physics B Atomic Molecular and Optical Physics

Acceleration of the chemical reaction of trapped Ca ions with H2O molecules by laser excitation

K. Okada, M. Wada, L.G.J. Boesten, T. Nakamura, Ichiro Katayama, S. Ohtani

Published Dec 20, 2002• 26 citations
Publisher

Abstract

The reactivity of laser-excited Ca +* ions in 3 2 D J ( J = 3/2, 5/2) states with neutral H 2 O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled 40 Ca + ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca +* (D J ) + H 2 O → products was found to be 1.8(1) × 10 3 s −1 Pa −1 at an ion temperature of T = 3.1(1) × 10 3 K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca + ions.

DOI
10.1088/0953-4075/36/1/304

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The reactivity of laser-excited Ca +* ions in 3 2 D J ( J = 3/2, 5/2) states with neutral H 2 O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled 40 Ca + ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca +* (D J ) + H 2 O → products was found to be 1.8(1) × 10 3 s −1 Pa −1 at an ion temperature of T = 3.1(1) × 10 3 K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca + ions.
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Using a laser-cooled ion trapping apparatus, we have investigated laser-induced chemical reactions between cold trapped Ba+{\mathrm{Ba}}^{+}Ba+ ions and several neutral molecular gases at room temperature, O2{\mathrm{O}}_{2}O2​, CO2\mathrm{C}{\mathrm{O}}_{2}CO2​, and N2O{\mathrm{N}}_{2}\mathrm{O}N2​O, leading to the production of cold trapped $(\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ BaO+\mathrm{Ba}{\mathrm{O}}^{+}BaO+ ions. The BaO+\mathrm{Ba}{\mathrm{O}}^{+}BaO+ ions were converted back to Ba+{\mathrm{Ba}}^{+}Ba+ ions via reaction with room-temperature CO. Reaction rates were determined by employing molecular dynamics simulations. The cold mixed-species ion ensembles produced were used for studying the efficiency of sympathetic cooling, by variation of the ratio of laser-cooled to sympathetically cooled ion numbers. In one extreme case, 20 laser-cooled 138Ba+^{138}\mathrm{Ba}^{+}138Ba+ ions were capable of maintaining the translational temperature of 120 sympathetically cooled barium isotopes (135−−137Ba+)(^{135--137}\mathrm{Ba}^{+})(135−−137Ba+) and 430 138Ba16O+^{138}\mathrm{Ba}^{16}{\mathrm{O}}^{+}138Ba16O+ molecules at approximately 25mK25\phantom{\rule{0.3em}{0ex}}\mathrm{mK}25mK.
We have studied laser-induced reactions between a Ca+ Coulomb crystal and the polar molecules of NH3 and ND3 at room temperature. The reaction rates of Ca+*(3d2D3/2, 4p2P1/2) + NH3 (ND3) → products have been determined from time variation of the size of fluorescence images of Ca+ Coulomb crystals. The results show that the reaction rates are very slow, and therefore the reaction-rate measurements between sympathetically cooled molecular ions and slow NH3 (ND3) molecules can be performed using a Ca+ Coulomb crystal as coolant.
We have studied laser-induced reactions between a Ca + Coulomb crystal and the polar molecules of NH 3 and ND 3 at room temperature. The reaction rates of Ca +* (3 d 2 D 3/2 , 4 p 2 P 1/2 ) + NH 3 (ND 3 ) → products have been determined from time variation of the size of fluorescence images of Ca + Coulomb crystals. The results show that the reaction rates are very slow, and therefore the reaction-rate measurements between sympathetically cooled molecular ions and slow NH 3 (ND 3 ) molecules can be performed using a Ca + Coulomb crystal as coolant.
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.