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 (40)Ar(13+)) 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 Ar(13+) 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.

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 7.5(+0.9)(-0.7) 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.

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 measure optical spectra of Nd-like W, Re, Os, Ir, and Pt ions of particular interest for studies of a possibly varying fine-structure constant. Exploiting characteristic energy scalings we identify the strongest lines, confirm the predicted 5s-4f level crossing, and benchmark advanced calculations. We infer two possible values for optical M2=E3 and E1 transitions in Ir 17þ that have the highest predicted sensitivity to a variation of the fine-structure constant among stable atomic systems. Furthermore, we determine the energies of proposed frequency standards in Hf 12þ and W 14þ . DOI: 10.1103/PhysRevLett.114.150801 PACS numbers: 06.20.Jr, 31.15.am, 31.15.bw, 32.30.Jc Highly charged ions (HCIs) are currently in the focus of theoretical studies analyzing their applications to frequency metrology and tests of a variation of the fine-structure constant α [1][2][3][4][5][6][7][8][9][10][11][12]. In most of the proposed HCIs with atomic number Z ¼ 55-98 and in charge states from 7 to 35, the complex electronic structures are experimentally unknown, and accurate calculations are extremely difficult. In view of novel techniques for sympathetically cooling HCIs in Paul traps [13][14][15] aiming at quantum logic spectroscopy on highly forbidden transitions [16], such data are urgently required.Observations from quasar absorption spectra have suggested a spatial variation of the value of the fine-structure constant α over cosmological dimensions [17], characterized by a dipolar distribution with a value of 10 −6 GLyr −1 . Laboratory experiments [18][19][20][21] have not yet reached the accuracy needed to test this dipolar pattern, which translates to a temporal variation of 10 −19 yr −1 [22] due to the motion of the Earth. Future optical clocks based on HCIs [3] could improve such tests. Interconfiguration transitions in HCIs have a high sensitivity to a variation of α due to large relativistic contributions to their binding energies. Advantageously, they have a strongly suppressed sensitivity to external perturbations [12]. However, interconfiguration transitions quickly shift from the optical laser range into the extreme ultraviolet or x-ray region with increasing charge state [23]. Nonetheless, at level crossings with two or more nearly degenerate electronic configurations, forbidden, and thus narrow, optical transitions with an enhanced sensitivity [1,2] arise. In particular, the Nd-like system Ir 17þ offers narrow lines between three electronic configurations 4f 14 , 4f 13 5s 1 , and 4f 12 5s 2 with the highest ever predicted sensitivity in a stable atomic system [2]. However, calculations for this system are exceptionally difficult, and the predicted energies for intraconfiguration M1 transitions ideally suited for clock applications [6] can exhibit errors on the 10% level, as can be seen below. For interconfiguration transitions, uncertainties at the eV level are expected. Thus, optical line identification becomes extremely difficult.In this Letter, we demonstrate a method to reliably identify transitions in...

We experimentally re-evaluate the fine structure of Sn 11+...14+ 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 M 1 transitions, in a wavelength range covering 260 nm 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 semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited. arXiv:1605.04236v1 [physics.atom-ph]