Dark resonances for ground state transfer of molecular quantum gases

One possible way to produce ultracold, high-phase-space-density quantum gases of molecules in the rovibronic ground state is given by molecule association from quantum-degenerate atomic gases on a Feshbach resonance and subsequent coherent optical multi-photon transfer into the rovibronic ground state. In ultracold samples of Cs_2 molecules, we observe two-photon dark resonances that connect the intermediate rovibrational level |v=73,J=2> with the rovibrational ground state |v=0,J=0> of the singlet $X^1\Sigma_g^+$ ground state potential. For precise dark resonance spectroscopy we exploit the fact that it is possible to efficiently populate the level |v=73,J=2> by two-photon transfer from the dissociation threshold with the stimulated Raman adiabatic passage (STIRAP) technique. We find that at least one of the two-photon resonances is sufficiently strong to allow future implementation of coherent STIRAP transfer of a molecular quantum gas to the rovibrational ground state |v=0,J=0>.Comment: 7 pages, 4 figure

On the role of the magnetic dipolar interaction in cold and ultracold collisions: Numerical and analytical results for NH(3Σ−^3\Sigma^-) + NH(3Σ−^3\Sigma^-)

We present a detailed analysis of the role of the magnetic dipole-dipole interaction in cold and ultracold collisions. We focus on collisions between magnetically trapped NH molecules, but the theory is general for any two paramagnetic species for which the electronic spin and its space-fixed projection are (approximately) good quantum numbers. It is shown that dipolar spin relaxation is directly associated with magnetic-dipole induced avoided crossings that occur between different adiabatic potential curves. For a given collision energy and magnetic field strength, the cross-section contributions from different scattering channels depend strongly on whether or not the corresponding avoided crossings are energetically accessible. We find that the crossings become lower in energy as the magnetic field decreases, so that higher partial-wave scattering becomes increasingly important \textit{below} a certain magnetic field strength. In addition, we derive analytical cross-section expressions for dipolar spin relaxation based on the Born approximation and distorted-wave Born approximation. The validity regions of these analytical expressions are determined by comparison with the NH + NH cross sections obtained from full coupled-channel calculations. We find that the Born approximation is accurate over a wide range of energies and field strengths, but breaks down at high energies and high magnetic fields. The analytical distorted-wave Born approximation gives more accurate results in the case of s-wave scattering, but shows some significant discrepancies for the higher partial-wave channels. We thus conclude that the Born approximation gives generally more meaningful results than the distorted-wave Born approximation at the collision energies and fields considered in this work.Comment: Accepted by Eur. Phys. J. D for publication in Special Issue on Cold Quantum Matter - Achievements and Prospects (2011

Optimal trapping wavelengths of Cs2_2 molecules in an optical lattice

The present paper aims at finding optimal parameters for trapping of Cs$_2$ molecules in optical lattices, with the perspective of creating a quantum degenerate gas of ground-state molecules. We have calculated dynamic polarizabilities of Cs$_2$ molecules subject to an oscillating electric field, using accurate potential curves and electronic transition dipole moments. We show that for some particular wavelengths of the optical lattice, called "magic wavelengths", the polarizability of the ground-state molecules is equal to the one of a Feshbach molecule. As the creation of the sample of ground-state molecules relies on an adiabatic population transfer from weakly-bound molecules created on a Feshbach resonance, such a coincidence ensures that both the initial and final states are favorably trapped by the lattice light, allowing optimized transfer in agreement with the experimental observation

Spin-orbit relaxation of Cl(2P½) and F(2P½) in a gas of H2

Contains fulltext : 36485.pdf (publisher's version ) (Open Access)The authors present quantum scattering calculations of rate coefficients for the spin-orbit relaxation of F(P-2(1/2)) atoms in a gas of H-2 molecules and Cl(P-2(1/2)) atoms in a gas of H-2 and D-2 molecules. Their calculation of the thermally averaged rate coefficient for the electronic relaxation of chlorine in H-2 agrees very well with an experimental measurement at room temperature. It is found that the spin-orbit relaxation of chlorine atoms in collisions with hydrogen molecules in the rotationally excited state j = 2 is dominated by the near-resonant electronic-to-rotational energy transfer accompanied by rotational excitation of the molecules. The rate of the spin-orbit relaxation in collisions with D-2 molecules increases to a great extent with the rotational excitation of the molecules. They have found that the H-2/D-2 isotope effect in the relaxation of Cl(P-2(1/2)) is very sensitive to temperature due to the significant role of molecular rotations in the nonadiabatic transitions. Their calculation yields a rate ratio of 10 for the electronic relaxation in H-2 and D-2 at room temperature, in qualitative agreement with the experimental measurement of the isotope ratio of about 5. The isotope effect becomes less significant at higher temperatures. (c) 2007 American Institute of Physics

Spin-flipping transitions in (2)Sigma molecules induced by collisions with structureless atoms

Contains fulltext : 103948.pdf (publisher's version ) (Open Access

Quantum and semiclassical study of the intramultiplet transitions in collisions of Cl(2P) and O(3P) with He, Ar and Xe

Quantum and semiclassical (SC) close coupling and coupled states approaches, as well as the analytical two-level model of Nikitin, are implemented for studying Cl(2pj) and O(3pj) intramultiplet mixing dynamics in collisions with the rare gas atoms He, Ar and Xe. The validity of different approximations and the effects of electrostatic (ES) and Coriolis couplings are investigated in the calculations of room-temperature rate constants. SC methods are used to analyze the mechanism for the non-adiabatic dynamics. In particular, it is shown that the j = 0→1 transition in oxygen atoms, forbidden to first order, proceeds through post-collision relaxation induced by a complicated interplay between ES and Coriolis interactions.Financial support came from NATO (under grant CRGLG 974215) and Russian Fund of Fundamental Research (under grant 00-15-97346). AAB also thanks the support from the Spanish Ministry of Education and Culture for sabbatical leave at CSIC.Peer Reviewe

Electronic interaction anisotropy between atoms in arbitrary angular momentum states

Contains fulltext : 60382.pdf (publisher's version ) (Closed access)A general tensorial expansion for the interaction potential between two atoms in arbitrary angular momentum states is derived and the relations between the expansion coefficients and the Born-Oppenheimer potentials of the diatomic molecule are obtained. It is demonstrated that a complete expansion of the interaction potential must employ tensors that are invariant under the inversion of the coordinate system, and the expansion in terms of conventional spherical harmonics is not adequate for the case of two atoms in states with nonzero electronic orbital angular momenta. The concept of the interaction anisotropy between two open-shell atoms is introduced. The correctness of the formalism is demonstrated by the example of two atoms in P states

Six-dimensional potential energy surface for NaK-NaK collisions: Gaussian process representation with correct asymptotic form

Contains fulltext : 201526.pdf (publisher's version ) (Open Access)12 p

The He-CaH((2)Sigma(+)) interaction. II. Collisions at cold and ultracold temperatures

Contains fulltext : 103950.pdf (publisher's version ) (Closed access

Molecular regimes in ultracold Fermi gases

The use of Feshbach resonances for tuning the interparticle interaction in ultracold Fermi gases has led to remarkable developments, in particular to the creation and Bose-Einstein condensation of weakly bound diatomic molecules of fermionic atoms. These are the largest diatomic molecules obtained so far, with a size of the order of thousands of angstroms. They represent novel composite bosons, which exhibit features of Fermi statistics at short intermolecular distances. Being highly excited, these molecules are remarkably stable with respect to collisional relaxation, which is a consequence of the Pauli exclusion principle for identical fermionic atoms. The purpose of this review is to introduce theoretical approaches and describe the physics of molecular regimes in two-component Fermi gases and Fermi-Fermi mixtures, focusing attention on quantum statistical effects