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Conference on Precision Electromagnetic Measurements

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

T. Hasegawa, T. Shimizu

Published Nov 7, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cpem.2000.850943

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

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Conference on Precision Electromagnetic Measurements•Dec 17, 2002•0 citations
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.
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.
A large sample $(N\ensuremath{\approx}{10}^{4})$ of fullerene ions (m=720a.u)(m=720\phantom{\rule{0.3em}{0ex}}\mathrm{a}.\mathrm{u})(m=720a.u) is cooled by means of a much smaller number$(\ensuremath{\approx}100)$ of laser-cooled magnesium ions (m=24a.u)(m=24\phantom{\rule{0.3em}{0ex}}\mathrm{a}.\mathrm{u})(m=24a.u). The results demonstrate that sympathetic cooling of large molecules to low temperatures is feasible. The temperature of the 24Mg+^{24}\mathrm{Mg}^{+}24Mg+ ions in the mixed ${\mathrm{Mg}}^{+}\text{\ensuremath{-}}{{\mathrm{C}}_{60}}^{+}$ ion cloud stored in an rf trap is found to be TMg=5K{T}_{\mathrm{Mg}}=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}TMg​=5K from their fluorescence signal. Molecular dynamics simulations are used to obtain the corresponding equilibrium temperature of the fullerene ions, and show that TC60=14K{T}_{{\mathrm{C}}_{60}}=14\phantom{\rule{0.3em}{0ex}}\mathrm{K}TC60​​=14K.