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Physical Review A

Sympathetic crystallization of CaH + produced by a laser-induced reaction

Naoki Kimura, K. Okada, Toshinobu Takayanagi, M. Wada, Shunsuke Ohtani, Hans A. Schuessler

Published Mar 30, 2011• 39 citations
Publisher

Abstract

We investigated sympathetic Coulomb crystallization of CaH${}^{+}$ ions produced by a laser-induced reaction through an excited state according to $^{40}\mathrm{Ca}$${}^{+}$($4p {}^{2}{P}_{1/2}$) $+$ ${\mathrm{H}}_{2}$ $\ensuremath{\rightarrow}$ $^{40}\mathrm{Ca}$H${}^{+}$ $+$ H. $^{40}\mathrm{Ca}$H${}^{+}$ ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca${}^{+}$ ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH${}^{+}$ ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm${}^{3}$/s was obtained.

DOI
10.1103/physreva.83.033422

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Coulomb crystals, containing up to a few hundred ions of which more than 50% were cooled sympathetically by the Coulomb interaction with laser cooled Mg+ ions, have been produced in a linear Paul trap. By controlling the balance of the radiation pressure from the two cooling lasers, the Coulomb crystals could be segregated according to ion species. Previous studies of ion crystals and molecular dynamics simulations suggest that the temperature may be around 10 mK or lower. The obtained results indicate that a wide range of atomic and molecular ions, which due to their internal structures are not amenable to direct laser cooling, can be effectively cooled and localized (crystallized) in linear Paul traps. For high resolution spectroscopy of such ions this may turn out to be very useful.
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 report on the sympathetic cooling and Coulomb crystallization of xenon highly charged ions (HCIs) with laser-cooled Ca+^++ ions. The HCIs are produced in a compact electron beam ion trap, then charge selected, decelerated, and finally injected into a cryogenic linear Paul trap. There, they are captured into 40^{40}40Ca+^++ Coulomb crystals, and co-crystallized within them, causing dark voids in their fluorescence images. Fine control over the number of trapped ions and HCIs allows us to realize mixed-species crystals with arbitrary ordering patterns. By investigating Xeq+^{q+}q+--Ca+^++ strings, we confirm the HCI charge states, measure their lifetime and characterize the mixed-species motional modes. Our system effectively combines the established quantum control toolbox for Ca+^++ with the rich set of atomic properties of Xe highly charged ions, providing a resourceful platform for optical frequency metrology, searches for signatures of new physics, and quantum information science.
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.
We have created multispecies Coulomb crystals in a linear Paul trap containing up to a few hundred ions of which more than 5050%50 were cooled only sympathetically through the Coulomb interaction with laser-cooled Mg+{\mathrm{Mg}}^{+}Mg+ ions. In an extreme case, one laser-cooled ion maintained order in a 15 ion string. Ion species segregation was obtained by radiation pressure. Previous experiments and molecular dynamics simulations suggest the temperature is 10 mK or lower. These results indicate that a wide range of atomic and molecular ions can be cooled and localized in linear Paul traps which is important for improvements in spectroscopic studies of such ions.
We have produced ensembles of cold 16O+2, 40Ar+, 12C16O+2, and various isotopes of barium ions (135Ba+, 136Ba+ and 137Ba+) via sympathetic cooling with laser-cooled 138Ba+ in a linear radiofrequency trap. The sympathetically cooled species were embedded in the centre of large 138Ba+ Coulomb crystals containing up to 2000 ions and were identified by motional resonance excitation. Crystals with molecular fractions exceeding 70% were obtained. The observed multi-species crystal structures agree well with results from molecular dynamics simulations. The simulations were also used to deduce an upper limit for the translational temperature of the molecular ions, as low as 20 mK.