We report on the design of a segmented linear Paul trap for optical clock applications using trapped ion Coulomb crystals. For an optical clock with an improved short-term stability and a fractional frequency uncertainty of 10 −18 , we propose 115 In + ions sympathetically cooled by 172 Yb + . We discuss the systematic frequency shifts of such a frequency standard. In particular, we elaborate on high precision calculations of the electric radiofrequency field of the ion trap using the finite element method. These calculations are used to find a scalable design with minimized excess micromotion of the ions at a level at which the corresponding secondorder Doppler shift contributes less than 10 −18 to the relative uncertainty of the frequency standard.

We present an experiment to characterize our new linear ion trap designed for the operation of a many-ion optical clock using 115 In + as clock ions. For the characterization of the trap as well as the sympathetic cooling of the clock ions we use 172 Yb + . The trap design has been derived from finite element method (FEM) calculations and a first prototype based on glass-reinforced thermoset laminates was built. This paper details on the trap manufacturing process and micromotion measurement. Excess micromotion is measured using photon-correlation spectroscopy with a resolution of 1.1 nm in motional amplitude, and residual axial rf fields in this trap are compared to FEM calculations. With this method, we demonstrate a sensitivity to systematic clock shifts due to excess micromotion of |(∆ν/ν) mm | = 8.5 × 10 −20 . Based on the measurement of axial rf fields of our trap, we estimate a number of twelve ions that can be stored per trapping segment and used as an optical frequency standard with a fractional inaccuracy of ≤ 1 × 10 −18 due to micromotion.Submitted to: New J. Phys.

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We report loading of 1.5ϫ10 9 metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams. We fully characterized this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: 5.3(9)ϫ10 Ϫ9 cm 3 /s at Ϫ35 MHz and 3.7(6)ϫ10 Ϫ9 cm 3 /s at Ϫ44 MHz. Also, we find that the loss rate is maximum at Ϫ5 MHz detuning, where the rate is 1.3(3)ϫ10 Ϫ8 cm 3 /s, much larger than recent theoretical and experimental values. In the absence of light the S-S ionization rate constant is measured to be 1.3(2)ϫ10 Ϫ10 cm 3 /s.

We observe vibrational states by photoassociation spectroscopy of cold He͑2 3 S͒ atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3 S 1 -2 3 P 2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed. PACS numbers: 33.80.Eh, 32.80.Pj, 33.20.Tp Photoassociation spectroscopy of cold atoms is a new and powerful technique to investigate long-range interactions between atoms [1,2]. Experiments in alkali-atom traps have revealed precise information on the long-range part of molecular potentials. This has resulted in the precise determination of s-wave scattering lengths for these systems, of high interest for studies of Bose-Einstein condensation in dilute trapped gases [3][4][5]. In these experiments a probe laser beam is sent through a cloud of cold atoms in a trap and the trap loss as a function of the laser frequency is detected. When the laser frequency is tuned to a photoassociative resonance, trap losses increase through the process of radiative escape or the formation of molecules, which are not trapped [2]. In some cases the excited molecules can also be photoionized by absorption of a second photon and the produced ions can be detected with high efficiency [1].In view of this success it is obvious that application of the same method to cold rare gas metastables is of great value. Especially in the case of metastable He͑2 3 S͒ atoms ͑He ‫ء‬ ͒, the detailed information that can be obtained on the long-range interactions of collision systems in ground and photoexcited states is of fundamental interest. Metastable rare gas systems differ from alkali systems mainly by the possibility of Penning ionization occurring at small internuclear distances. A molecule formed by photoassociation would contain an internal energy of 40 eV and its lifetime due to Penning ionization is expected to be short compared with a vibrational period, thereby excluding photoassociation spectroscopy for this system. For He͑2 3 S͒ atoms, close collisions in the molecular 1 S 1 g or 3 S 1 g potentials have ionization probability close to unity [6], while for the 5 S 1 g potential the ionization probability is reduced by orders of magnitude [7] due to the total spin conservation selection rule. An ionization rate constant of 1.3 3 10 210 cm 3 ͞s in laser-cooled unpolarized He ‫ء‬ atomic clouds [8] is attributed to these close collisions. It is to be expected that, for the molecular potentials relevant for the photoassociation process, i.e., potentials that asymptotically belong to He͑2 3 S͒-He͑2 3 P͒, the situation at small internuclear distances is equivalent: the singlet and triplet states decay with high probability, while the quintet states are stable. However, the fine-structure interaction implies the possibility of spin mixing at large distances. It is thus an open question how this mixing affects the for...

We measure the rates of elastic and inelastic two-body collisions of cold spin-polarized neon atoms in the metastable 3P2 state for 20Ne and 22Ne in a magnetic trap. From particle loss, we determine the loss parameter of inelastic collisions beta=6.5(18) x 10(-12) cm(3) s(-1) for 20Ne and beta=1.2(3) x 10(-11) cm(3) s(-1) for 22Ne. These losses are caused by ionizing (i.e., Penning) collisions and occur less frequently than for unpolarized atoms. This proves the suppression of Penning ionization due to spin polarization. From cross-dimensional relaxation measurements, we obtain elastic scattering lengths of a=-180(40)a(0) for 20Ne and a = +150(+80)(-50)a(0) for 22Ne, where a(0)=0.0529 nm.

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Using transverse resonant two-photon, single-mode laser excitation of metastable 40 Ar(4s 3 P 2 ) atoms in a collimated beam in combination with a calibrated travelling Michelson interferometer involving digital fringe interpolation, we have measured the energies for the Ar(4p, J = 3 → nd, J = 4) transitions with principal quantum numbers n = 12-100 with a relative uncertainty of 8 × 10 −8 . The Rydberg atoms are formed in a region of low electric and magnetic fields and detected by electron transfer to SF 6 molecules in a skimmed supersonic beam. A single-channel quantum defect analysis of the experimental data is performed, revealing weak perturbations of the n = 12 and 20 levels by interaction with the Ar( 2 P 1/2 7g , J = 4) and Ar( 2 P 1/2 8g , J = 4) levels, respectively. The value for the ionization energy of the 40 Ar + ( 2 P 3/2 ) limit has been determined as 21 647.076(2) cm −1 relative to the 40 Ar(4p, J = 3) level and as 127 109.836(3)(±0.05) cm −1 relative to the ground state of 40 Ar (see also note added in proof). Low-lying autoionizing ng , J = 4 resonances (n = 9-13) have been investigated and their quantum defects and reduced width determined for the first time. A multichannel quantum defect theory analysis yields an estimate (0.76 cm −1 ) for the energy separation between the perturbed 20d level and the perturbing 8g level, predicts the transition intensity to the 8g level to be much weaker than that to the 20d level and shows that the g -g coupling in the autoionization region is much stronger than the g -d coupling determined from the 8g -induced perturbation in the discrete nd, J = 4 spectrum.