Photoassociation spectroscopy of laser-cooled ytterbium atoms

Ab initio studies of the ground and first excited states of the Sr-H2 and Yb-H2 complexes

Accurate intermolecular potential-energy surfaces (IPESs) for the ground and first excited states of the Sr-H₂ and Yb-H₂ complexes were calculated. After an extensive methodological study, the coupled cluster with single, double, and non-iterative triple excitation method with the Douglas-Kroll-Hess Hamiltonian and correlation-consistent basis sets of triple-ζ quality extended with 2 sets of diffuse functions and a set of midbond functions were chosen. The obtained ground-state IPESs are similar in both complexes, being relatively isotropic with two minima and two transition states (equivalent by symmetry). The global minima correspond to the collinear geometries with R = 5.45 and 5.10 Å and energies of -27.7 and -31.7 cm^-1 for the Sr-H₂ and Yb-H₂ systems, respectively. The calculated surfaces for the Sr(³P)-H₂ and Yb(³P)-H₂ states are deeper and more anisotropic, and they exhibit similar patterns within both complexes. The deepest surfaces, where the singly occupied p-orbital of the metal atom is perpendicular to the intermolecular axis, are characterised by the global minima of ca. -2053 and -2260 cm^-1 in the T-shape geometries at R = 2.41 and 2.29 Å for Sr-H₂ and Yb-H₂, respectively. Additional calculations for the complexes of Sr and Yb with the He atom revealed a similar, strong dependence of the interaction energy on the orientation of the p-orbital in the Sr(³P)-He and Yb(³P)-He states.

High efficiency demagnetization cooling by suppression of light-assisted collisions

Demagnetization cooling utilizes dipolar relaxations that couple the internal degree of freedom (spin) to the external (angular momentum) in order to cool an atomic cloud efficiently. Optical pumping into a dark state constantly recycles the atoms that were thermally excited to higher spin states. The net energy taken away by a single photon is very favorable since the lost energy per atom is the Zeeman energy rather than the recoil energy. As the density of the atomic sample rises the presence of the photons leads to limiting processes. In our previous publication [Volchkov et al. (2014)] we have shown that light-assisted collisions are such an important limiting process. In this paper we suppress light-assisted collisions by detuning the optical pumping light such that the Condon point coincides with the first node of the ground state wave function of two colliding atoms. This leads to an increased cooling efficiency χ ≥ 17 as well as to increased maximum densities of n ≈ 1 · 10(20) m(-3). However, due to the high number of involved molecular states the net suppression is not strong enough to reach quantum degeneracy.

Creation of quantum-degenerate gases of ytterbium in a compact 2D-/3D-magneto-optical trap setup

We report on the first experimental setup based on a 2D-/3D-magneto-optical trap (MOT) scheme to create both Bose-Einstein condensates and degenerate Fermi gases of several ytterbium isotopes. Our setup does not require a Zeeman slower and offers the flexibility to simultaneously produce ultracold samples of other atomic species. Furthermore, the extraordinary optical access favors future experiments in optical lattices. A 2D-MOT on the strong (1)S0 → (1)P1 transition captures ytterbium directly from a dispenser of atoms and loads a 3D-MOT on the narrow (1)S0 → (3)P1 intercombination transition. Subsequently, atoms are transferred to a crossed optical dipole trap and cooled evaporatively to quantum degeneracy.

Determination of the s-wave scattering length and the C6 van der Waals coefficient of 174Yb via photoassociation spectroscopy

We report photoassociation spectroscopy of 174Yb for the 1S(0)-1P1 transition at 1 microK, where only the s-wave scattering state contributes to the spectra. The wave function of the s-wave scattering state is obtained from the photoassociation efficiency, and we determine that the C6 potential coefficient is 2300+/-250 a.u. and the s-wave scattering length is 5.53+/-0.11 nm. Based on these parameters, we discuss the scattering properties of s- and d-wave states.

Narrow line photoassociation in an optical lattice

With ultracold 88Sr in a 1D magic wavelength optical lattice, we performed narrow-line photoassociation spectroscopy near the 1S0 - 3P1 intercombination transition. Nine least-bound vibrational molecular levels associated with the long-range 0u and 1u potential energy surfaces were measured and identified. A simple theoretical model accurately describes the level positions and treats the effects of the lattice confinement on the line shapes. The measured resonance strengths show that optical tuning of the ground state scattering length should be possible without significant atom loss.

High-resolution photoassociation spectroscopy of ultracold ytterbium atoms by using the intercombination transition

We observed high-resolution photoassociation spectra of laser-cooled ytterbium (Yb) atoms in the spin-forbidden 1S0 - 3P1 intercombination line. The rovibrational levels in the 0u+ state were measured for red detunings of the photoassociation laser ranging from 2.9 MHz to 1.97 GHz with respect to the atomic resonance. The rotational splitting of the vibrational levels near the dissociation limit were fully resolved due to the sub-MHz linewidth of the spectra in contrast to previous measurements using the spin-allowed singlet transition. In addition, from a comparison between the spectra of 174Yb and those of 176Yb, a d-wave shape resonance for 174Yb is strongly suggested.

Spectroscopic determination of the s-wave scattering lengths of 86Sr and 88Sr

We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, 86Sr and 88Sr. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the 1S0-1P1 atomic resonance at 461 nm. A minimum in the transition amplitude for 86Sr at -494 +/- 5 GHz allows us to determine the scattering lengths 610a0 < a86 < 2300a0 for 86Sr and a much smaller value of -1a0 < a88 < 13a0 for 88Sr.

Photoassociative spectroscopy at long range in ultracold strontium

We report photoassociative spectroscopy of 88Sr(2) in a magneto-optical trap operating on the 1S0-->3P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the 1S0-->1P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1P1 atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.