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arXiv:1306.0035 · Physical Review A

Production of translationally cold barium monohalide ions

Michael V. DePalatis, Michael Chapman

Published Aug 5, 2013• 8 citations
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Abstract

We have produced sympathetically cooled barium monohalide ions Ba${X}^{+}$ ($X=\text{F}$, Cl, Br) by reacting trapped, laser-cooled Ba${}^{+}$ ions with room-temperature gas-phase neutral halogen-containing molecules. Reaction rates for two of these (SF${}_{6}$ and CH${}_{3}$Cl) have been measured and were found to be in agreement with classical models. Ba${X}^{+}$ ions are promising candidates for cooling to the rovibrational ground state, and our method presents a straightforward way to produce these polar molecular ions.

DOI
10.1103/physreva.88.023403

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Physical Review A•Oct 24, 2008•28 citations

Production of cold barium monohalide ions

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We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N$_2^+$ ions as a test system, we achieve > 90 % selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.

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We have produced sympathetically cooled barium monohalide ions BaX+{X}^{+}X+ (X=FX=\text{F}X=F, Cl, Br) by reacting trapped, laser-cooled Ba+{}^{+}+ ions with room-temperature gas-phase neutral halogen-containing molecules. Reaction rates for two of these (SF6{}_{6}6​ and CH3{}_{3}3​Cl) have been measured and were found to be in agreement with classical models. BaX+{X}^{+}X+ ions are promising candidates for cooling to the rovibrational ground state, and our method presents a straightforward way to produce these polar molecular ions.
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.
Ion traps are an incredibly versatile tool which have many applications throughout the physical sciences, including such diverse topics as mass spectrometry, precision frequency metrology, tests of fundamental physics, and quantum computing. In this thesis, experiments are presented which involve trapping and measuring properties of Th³⁺. Th³⁺ ions are of unique interest in part because they are a promising platform for studying an unusually low-lying nuclear transition in the 229Th nucleus which could eventually be used as an exceptional optical clock. Here, experiments to measure electronic lifetimes of Th³⁺ are described. A second experimental topic explores the production of sympathetically cooled molecular ions. The study of cold molecular ions has a number of applications, some of which include spectroscopy to aid the study of astrophysical objects, precision tests of quantum electrodynamics predictions, and the study of chemical reactions in the quantum regime. The experiments presented here involve the production of barium monohalide ions, BaX⁺ (X = F, Cl, Br). This type of molecular ion proves to be particularly promising for cooling to the rovibrational ground state. The method used for producing BaX⁺ ions involves reactions between cold, trapped Ba⁺ ions and neutral gas phase reactants at room temperature. The Ba⁺ ion reaction experiments presented in this thesis characterize these reactions for producing Coulomb crystals composed of laser cooled Ba⁺ ions and sympathetically cooled BaX⁺ ions.
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.
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.
Stationary molecules in well-defined internal states are of broad interest in both physics and chemistry. Through high-resolution spectroscopy, fundamental physics can be tested and lead to, e.g., improved values of the electron-to-proton mass ratio [1], new boundaries on the electron electric dipole moment [2] and observation of the potential evolution of the fine structure constant over time [3]. Translationally and internally cold molecules have as well been considered promising candidates for qubits in new quantum computing senarios [4], and molecular ions could in the future likely become an excellent alternative to atomic qubits in the realization of a practical ion trap based quantum computer due to favourable internal state decoherence rates. In chemistry, state prepared molecular targets are an ideal starting point for uni-molecular reactions, including coherent control of photofragmentation through the application of various laser sources [5,6]. In cold bi-molecular reactions, where the effect of even tiny potential barriers becomes significant, experiments with state prepared molecules can yield important information on the details of the potential curves of the molecular complexes [7,8,9]. Furthermore, in order to learn more about the chemistry in interstellar clouds, astrochemists can benefit greatly from direct measurements on cold reactions in laboratories [9]. Working with MgH+ molecular ions in a linear Paul trap, we routinely cool their translational degree of freedom by sympathetic cooling with Doppler laser cooled Mg+ ions. Giving the time for the molecules to equilibrate internally to the room temperature blackbody radiation, the vibrational degree of freedom will freeze out, leaving only the rotational degree of freedom to be cooled. We report here on the implementation of a new technique for laser-induced rotational ground-state cooling of vibrationally and translationally cold MgH+ ions [10]. The scheme is based on excitation of a single rovibrational transition [11], and it should be generalizable to any diatomic polar molecular ion, given appropriate mid-infrared laser sources such as a quantum cascade laser are available. In recent experiments, a nearly 15-fold increase in the rotational ground-state population was obtained, with the resulting ground-state population of 36,7±1,2 %, equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that, through this technique, cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature set-ups. References [1] Koelemeij, J. C. J., Roth, B., Wicht, A., Ernsting, I. and Schiller, S., Phys. Rev. Lett. 98, 173002 (2007). [2] Hudson, J. J., Sauer, B. E., Tarbutt, M. R. and Hinds, E. A., Phys. Rev. Lett. 89, 023003 (2002). [3] Hudson, E. R., Lewandowski, H. J., Sawyer, B. C. and Ye, J, Phys. Rev. Lett. 96, 143004 (2006). [4] DeMille, D, Phys. Rev. Lett. 88, 067901 (2002). [5] Rice, S. A. and Zaho, M. Optical control of molecular dynamics (Wiley, New York, 2000). [6] Shapiro, M. and Brumer, P. W. Principles of the Quantum Control of Molecular Processes (Wiley, New York, 2003). [7] Willitsch, S., Bell, M. T., Gingell, A. D. and Softley, T. P., Phys. Chem. Chem. Phys. 10, 7200 ( 2008). [8] Krems, R. V., Phys. Chem. Chem. Phys. 10, 4079 (2008). [9] Smith, I. W. M., Low temperatures and cold molecules (Imperial College Press, London, 2008). [10] Staanum, P. F., Højbjerre, K., Skyt, P.S., Hansen, A. K. and Drewsen, M., Nat. Phys. 6, 271 (2010). [11] Vogelius, I. S., Madsen, L. B. and Drewsen, M., Phys. Rev. Lett. 89, 173003 (2002).
We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N2+_2^+2+​ ions as a test system, we achieve > 90 % selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.