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IEEE Transactions on Instrumentation and Measurement

Observation and temperature measurement of sympathetically cooled ions in an RF trap

Taro Hasegawa, T. Shimizu

Published Apr 1, 2001• 3 citations
Publisher

Abstract

Barium ions are trapped simultaneously with laser-cooled magnesium ions in a radio-frequency (RF) trap and cooled sympathetically. The barium ions are produced by charge transfer with the magnesium ions and are caught in a shallow trapping potential. The temperature of the trapped barium ions is measured, and it is confirmed that sympathetic cooling is effective. It is demonstrated that charge-transfer ionization is applicable to produce other ion species.

DOI
10.1109/19.918190

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Observation and temperature measurement of the sympathetically cooled ions in an rf trap

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The barium ions are trapped with the laser-cooled magnesium ions simultaneously in an rf trap and cooled sympathetically. Sympathetically cooled ions are one of the candidates of accurate optical frequency standard. The barium ions are ionized by the charge transfer with the magnesium ions for avoiding the loss due to the shallow trapping potential. The temperature of the barium ions is measured and the effect of the sympathetic cooling is observed. It is also found that the application of the ionization by the charge transfer to other ion species is possible.

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Physical Review Letters•Jul 7, 1986•375 citations
Barium ions are trapped simultaneously with laser-cooled magnesium ions in a radio-frequency (RF) trap and cooled sympathetically. The barium ions are produced by charge transfer with the magnesium ions and are caught in a shallow trapping potential. The temperature of the trapped barium ions is measured, and it is confirmed that sympathetic cooling is effective. It is demonstrated that charge-transfer ionization is applicable to produce other ion species.
The barium ions are trapped with the laser-cooled magnesium ions simultaneously in an rf trap and cooled sympathetically. Sympathetically cooled ions are one of the candidates of accurate optical frequency standard. The barium ions are ionized by the charge transfer with the magnesium ions for avoiding the loss due to the shallow trapping potential. The temperature of the barium ions is measured and the effect of the sympathetic cooling is observed. It is also found that the application of the ionization by the charge transfer to other ion species is possible.
A detailed investigation of ion crystals produced by laser and sympathetic cooling in a linear RF trap has been conducted.\n\nThe laser cooling methods were examined and applied to the trapped ^24Mg^(positive) ions. The crystals produced by the laser cooling were studied, including the dependence on RF voltage, end cap DC voltage, laser power and laser frequency. By manipulating the different RF voltages and endcap DC voltages, the structure phase transition of the ion crystals was observed.\n\nIn addition, the sympathetic cooling of different ion species with the laser cooled 24Mg^(positive) was carried out. In this process, the mixed Mg^(positive) and He^(positive) crystals were created andidentified, and mixed Mg^(positive) and H2^(positive) crystals were produced. The effect of an unwanted chemical reaction of Mg^(positive) and H2 was observed and minimized. After sympathetic cooling of light ion species, the sympathetic cooling of heavy molecular ions such as fullerene ions was also carried out. The efficiencies and final temperature in both cases are very different. Theoretically to interpret the results, molecular dynamics simulations of the laser cooling and sympathetic cooling were implemented. And the simulations were compared with the experimental results.\n\nIn the process of carrying out this research, the optics were rebuilt to provide reliable UV sources for the photoionization and laser cooling of Mg ions. The imaging\nsystem was reconfigured to take the images of ion crystals. New elements were added tin the ion trap to improve the ability to manipulate ions.
Abstract Sympathetic cooling is an important technique for achieving low motional temperatures in trapped ion systems that cannot be directly laser cooled. In this paper, we report on the sympathetic cooling of large samples (5 × 10 4 ) of Ba + ions which have been optically pumped into a dark state, using laser cooled Ba + ions in a linear radiofrequency (RF) ion trap. The trap lifetime was evaluated with different dark ion fractions as a function of the RF amplitude, the DC trapping potentials, the power and frequency of the cooling lasers, the quality of the vacuum, and the total number of ions. We found that the RF amplitude was the dominant factor contributing to the loss of ions, and that under optimal conditions and with a dark ion fraction as high as 85%, no losses were observed over 100 s long measurement periods, and fewer than 10% of the ions were lost after 1000 s.
This thesis reports on experimental and theoretical studies of the sympathetic cooling of complex molecular ions demonstrating that this general method for cooling atomic and molecular ions is reliable and efficient. For this purpose, complex molecular ions and barium ions have been confined simultaneously in a linear Paul trap. The complex molecular ions are generated in an electrospray ionization system and transferred to the trap via a 2 m long octopole ion guide. These molecular ions are pre-cooled by room temperature helium buffer gas so that they can be captured by the trap. The atomic barium ions are loaded from a barium evaporator oven and are laser-cooled by a 493 nm cooling laser and a 650 nm repumping laser. Due to the mutual Coulomb interaction among these charged particles, the kinetic energy of the complex molecular ions can be reduced significantly. In our experiments we have demonstrated the sympathetic cooling of various molecules (CO2, Alexa Fluor 350, glycyrrhetinic acid, cytochrome c) covering a wide mass range from a few tens to 13000 amu. In every case the molecular ions could be cooled down to millikelvin temperatures. Photo-chemical reactions of the 138Ba+ ions in the (2P1/2) excited state with gases such as O2, CO2, or N2O, could be observed. If the initial 138Ba+ ion ensemble is cold, the produced 138BaO+ ions are cold as well, with a similar temperature as the laser-cooled barium ions (a few tens of millikelvin). The back-reaction of 138BaO+ ions with neutral CO to 138Ba+ is possible and was observed in our experiments as well. A powerful molecular dynamics (MD) simulation program has been developed. With this program dynamic properties of ion ensembles, such as sympathetic interactions or heating effects, have been investigated and experimental results have been analyzed to obtain, for example, ion numbers and temperatures. Additionally, the feasibility of nondestructive spectroscopy via an optical dipole excitation and the lineshape of rovibrational transitions of 1.4 and 5.1 μm have been studied.
Sympathetic cooling of trapped ions has been demonstrated in an experiment where 198Hg+^{198}\mathrm{Hg}^{+}198Hg+ ions were confined in a Penning ion trap with laser-cooled 9Be+^{9}\mathrm{Be}^{+}9Be+ ions. 198Hg+^{198}\mathrm{Hg}^{+}198Hg+ temperatures below 1 K were achieved. Ion plasma sizes, shapes, and rotation frequencies were measured. Dramatic changes in the 9Be+^{9}\mathrm{Be}^{+}9Be+ plasma were observed when the 198Hg+^{198}\mathrm{Hg}^{+}198Hg+ ions were introduced into the trap. These observations are consistent with the prediction of centrifugal separation for ions of different charge-to-mass ratios.