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Lasermanipulation von rotationsgekühlten Molekülionen in einer neuen kryogenen Paul-Falle

Schwarz, Maria

Published Jan 1, 2012• 0 citations
Publisher

Abstract

A new cryogenic linear Paul trap for atomic, molecular and highly charged ions was designed and constructed at MPIK. Experiments with laser-cooled Mg+ ions in the mK-regime and with sympathetically cooled MgH+ molecular ions provided evidence for an excellent vacuum as well as a strongly reduced inuence of the black body radiation at the trap center. Measurements proved that more than 90% of the population of the MgH+ ions can be found in the lowest three rotational states. By laser excitation it was possible to transfer the main part of the ions into the absolute ground state. The population transfer rates for the lowest four rotational states were investigated, additionally to measuring for the first time the decay constant of a deterministically prepared rovibronic state of the MgH+ ion. A decay constant Γ = (7,02 +/- 1,26) Hz was determined for the radiative decay of the |ν = 1, J = 1> state to the ground state.

DOI
10.11588/heidok.00013535

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AIP conference proceedings•Jan 1, 1999•0 citations
A new cryogenic linear Paul trap for atomic, molecular and highly charged ions was designed and constructed at MPIK. Experiments with laser-cooled Mg+ ions in the mK-regime and with sympathetically cooled MgH+ molecular ions provided evidence for an excellent vacuum as well as a strongly reduced inuence of the black body radiation at the trap center. Measurements proved that more than 90% of the population of the MgH+ ions can be found in the lowest three rotational states. By laser excitation it was possible to transfer the main part of the ions into the absolute ground state. The population transfer rates for the lowest four rotational states were investigated, additionally to measuring for the first time the decay constant of a deterministically prepared rovibronic state of the MgH+ ion. A decay constant Γ = (7,02 +/- 1,26) Hz was determined for the radiative decay of the |ν = 1, J = 1> state to the ground state.
Working with MgH+molecular ions in a linear Paul trap, we routinely cool their translational degrees of freedom by sympathetic cooling with Doppler laser cooled Mg+ions. Giving the time for the molecules to equilibrate internally to room temperature blackbody radiation, the vibrational degree of freedom is frozen out, leaving only the rotational degree of freedom to be cooled. We have implemented a new technique for laser-induced rotational ground-state cooling of vibrationally and translationally cold MgH+ions, using a scheme based on excitation of a single rovibrational transition. This technique is generalizable to any diatomic polar molecular ion, given an appropriate mid-infrared laser source such as a quantum cascade laser.
Electron beam ion traps used for spectroscopy of highly charged ions (HCI) produce a deep trapping potential leading to high temperatures of the stored ions, and thus limiting the achievable spectral resolution. A novel device at the Max-Planck-Institut für Kernphysik, the Cryogenic linear Paul Trap Experiment (CryPTEx), attached to an electron beam ion trap, provides a new experimental platform to overcome these limitations. The trap assembly operates at a temperature of 4 K and offers optical access for quantum manipulation and imaging of the trapped ions. Since forbidden optical transitions in HCI do not support direct laser cooling, sympathetic cooling with Coulomb crystals of singly charged ions such as Be+ or Mg+ will be applied in order to reach the natural linewidth of optical forbidden transitions in HCI of interest. With the added advantage of long ion trapping times resulting from residual gas pressures of H2 at 4 K below 10−15 mbar, CryPTEx has been commissioned in collaboration with the Ion Trap Group in Århus using rovibrationally cooled MgH+ ions. Strong suppression of the black body radiation at the trap center, ion storage times of about 28 hours, and largely enhanced population of the rovibrational ground state were achieved.
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).
Storage and cooling of highly charged ions require ultra-high vacuum levels obtainable by means of cryogenic methods. We have developed a linear Paul trap operating at 4 K capable of very long ion storage times of about 30 h. A conservative upper bound of the H(2) partial pressure of about 10(-15) mbar (at 4 K) is obtained from this. External ion injection is possible and optimized optical access for lasers is provided, while exposure to black body radiation is minimized. First results of its operation with atomic and molecular ions are presented. An all-solid state laser system at 313 nm has been set up to provide cold Be(+) ions for sympathetic cooling of highly charged ions.
We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the $|\ensuremath{\nu}=1,J=1{⟩}_{X}$ towards the $|\ensuremath{\nu}=0,J=0{⟩}_{X}$ level in MgH+{\mathrm{MgH}}^{+}MgH+ by saturated laser excitation of the $|\ensuremath{\nu}=0,J=2{⟩}_{X}\mathrm{\text{\ensuremath{-}}}|\ensuremath{\nu}=1,J=1{⟩}_{X}$ transition followed by state selective resonance enhanced two-photon dissociation out of the $|\ensuremath{\nu}=0,J=2{⟩}_{X}$ level. The experimentally observed rate of $6.32(0.69)\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ is in excellent agreement with the theory value of $6.13(0.03)\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.
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.