Alexander Windberger

Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas

The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations, high-resolution spectroscopy of complex molecules, cold chemistry and astrochemistry. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion or a cryogenic trap environment. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH+ molecular ions, which have been trapped and sympathetically cooled in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second—that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings—we have cooled a single molecular ion to a rotational temperature of  kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest.

Coulomb crystallization of highly charged ions

Highly charged ions in cold confines High-energy irradiation can strip many electrons away from individual atoms, producing ions with charges of +10 or more. However, many of the interesting properties of such highly charged ions are hard to study or exploit under the extreme conditions needed to prepare them. Schmöger et al. cooled down argon ions with +13 charges from the megakelvin temperatures needed for their generation to millikelvin temperatures appropriate for high-precision spectroscopy. The method relies on sympathetic cooling by a cold sample of singly charged beryllium ions and is likely to be applicable to a broad range of other elements. Science, this issue p. 1233 Cold singly charged ions can be used to cool down and confine ions with charges of +13 for precise study of their properties. Control over the motional degrees of freedom of atoms, ions, and molecules in a field-free environment enables unrivalled measurement accuracies but has yet to be applied to highly charged ions (HCIs), which are of particular interest to future atomic clock designs and searches for physics beyond the Standard Model. Here, we report on the Coulomb crystallization of HCIs (specifically 40Ar13+) produced in an electron beam ion trap and retrapped in a cryogenic linear radiofrequency trap by means of sympathetic motional cooling through Coulomb interaction with a directly laser-cooled ensemble of Be+ ions. We also demonstrate cooling of a single Ar13+ ion by a single Be+ ion—the prerequisite for quantum logic spectroscopy with a potential 10−19 accuracy level. Achieving a seven-orders-of-magnitude decrease in HCI temperature starting at megakelvin down to the millikelvin range removes the major obstacle for HCI investigation with high-precision laser spectroscopy.

Cryogenic linear Paul trap for cold highly charged ion experiments

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.

Decay Rate Measurement of the First Vibrationally Excited State of MgH+ in a Cryogenic Paul Trap

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 |ν = 1, J = 1)(X) towards the |ν = 0, J = 0)(X) level in MgH+ by saturated laser excitation of the |ν = 0, J = 2)(X)-|ν = 1, J = 1)(X) transition followed by state selective resonance enhanced two-photon dissociation out of the |ν = 0, J=2)(X) level. The experimentally observed rate of 6.32(0.69) s(-1) is in excellent agreement with the theory value of 6.13(0.03) s(-1) (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.

Analysis of the fine structure of Sn11+ - Sn14+ ions by optical spectroscopy in an electron-beam ion trap

We experimentally re-evaluate the fine structure of Sn11+-Sn14+ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole M1 transitions, in a wavelength range covering 260 to 780 nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of ab initio calculations based on Fock space coupled cluster theory. We validate our line identification using semiempirical COWAN calculations with adjustable wave-function parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited.

Optical spectroscopy of complex open-4d-shell ions Sn7+–Sn10+

We analyze the complex level structure of ions with many-valence-electron open-[Kr] 4*d*m subshells (m=7–4) with *ab initio* calculations based on configuration-interaction many-body perturbation theory (CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra of Sn7+–Sn10+ ions were obtained using an electron beam ion trap. Semiempirical spectral fits carried out with the orthogonal parameters technique and cowan code calculations lead to 90 identifications of magnetic-dipole transitions and the determination of 79 energy ground-configuration levels, questioning some earlier EUV-line assignments. Our results confirm the ab initio predictive power of CI+MBPT calculations for these complex electronic systems.

Identifications of and EUV transitions of promethium-like Pt, Ir, Os and Re

We investigated Pm-, Nd-, and Pr-like spectra in the extreme ultra-violet region around 20 nm of Pt, Ir, Os, and Re (Z = 78–75) produced in the Heidelberg electron beam ion trap. Identification of the transitions was supported by several theoretical calculations, including collisional radiative modeling of the observed spectra. Special attention is given to the identifications of the alkaline-like – resonance lines in promethium-like highly charged ions. Previous identifications of these lines have been tentative at best due to disagreements with theory and doubts about the experimental charge state identifications. Our experimental results for the – wavelengths are accurate at the 0.005%-level. Understanding the level-structure of ions near the – level crossing is of particular importance for future searches of a possible fine-structure constant variation, and new optical clocks.

Identifications of 5s1/2−5p3/2 and 5s2−5s5p EUV transitions of promethium-like Pt, Ir, Os and Re

We investigated Pm-, Nd-, and Pr-like spectra in the extreme ultra-violet region around 20 nm of Pt, Ir, Os, and Re (Z = 78–75) produced in the Heidelberg electron beam ion trap. Identification of the transitions was supported by several theoretical calculations, including collisional radiative modeling of the observed spectra. Special attention is given to the identifications of the alkaline-like – resonance lines in promethium-like highly charged ions. Previous identifications of these lines have been tentative at best due to disagreements with theory and doubts about the experimental charge state identifications. Our experimental results for the – wavelengths are accurate at the 0.005%-level. Understanding the level-structure of ions near the – level crossing is of particular importance for future searches of a possible fine-structure constant variation, and new optical clocks.

Forbidden optical transition in Ti-like Xe, Ba, and Ir

We present measurements of the (3d4)5D2−5D3 transitions in the Ti-like ions Xe32+, Ba34+, and Ir55+ produced and trapped in the Heidelberg electron beam ion trap. The obtained wavelengths have a precision at the few ppm-level and are thereby the most precise measurements of these transitions up to date. For Z=60−75 semi-empirical calculations have shown excellent agreement, however our measurements combined with data from other works shows that outside this range predictions quickly deviate. The value obtained for Ir55+ 357.434(2) nm confirms the linear mismatch to ab initio calculations for Z > 70, as hypothesized in Utter et al., Phys. Rev. A 67, 012508 (2003).

Coulomb crystals in a cryogenic Paul trap for sympathetic cooling of molecular ions and highly charged ions

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 fur 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 T...