105 research outputs found
Ultracold mixtures of metastable He and Rb: scattering lengths from ab initio calculations and thermalization measurements
We have investigated the ultracold interspecies scattering properties of
metastable triplet He and Rb. We performed state-of-the-art ab initio
calculations of the relevant interaction potential, and measured the
interspecies elastic cross section for an ultracold mixture of metastable
triplet He and Rb in a quadrupole magnetic trap at a temperature of
0.5 mK. Our combined theoretical and experimental study gives an interspecies
scattering length , which prior to this work was
unknown. More general, our work shows the possibility of obtaining accurate
scattering lengths using ab initio calculations for a system containing a
heavy, many-electron atom, such as Rb.Comment: 11 pages, 11 figures, accepted for publication in Phys. Rev.
Interatomic potentials of van der Waals dimers and : probing discrepancies between theory and experiment
Results of new all-electron ab initio calculations and revisit of experimental studies of the interatomic potentials of lower-lying ungerade excited and ground electronic energy states of the Hg_{2} and Cd_{2} van der Waals complexes are used as probes of discrepancies between theory and experiment. From simulations of the previously and presently measured LIF excitation and dispersed emission spectra new analytical representations of the excited- and the ground-state interatomic potentials are proposed. An inverted perturbation approach was also used to improve the studied interatomic potentials. The comparison of the new ab-initio calculated potentials with the results of the analyses illustrates an improve theory-to-experiment agreement for such a demanding system like Hg_{2} or Cd_{2}
Ab initio potential energy surfaces for NH-NH with analytical long range
We present four-dimensional ab initio potential energy surfaces for the three
spin states of the NH-NH complex. The potentials are partially based on the
work of Dhont et al. [J. Chem. Phys. 123, 184302 (2005)]. The surface for the
quintet state is obtained at the RCCSD(T)/aug-cc-pVTZ level of theory and the
energy diferences with the singlet and triplet states are calculated at the
CASPTn/aug-cc-pVTZ (n = 2; 3) level of theory. The ab initio potentials are
fitted to coupled spherical harmonics in the angular coordinates, and the long
range is further expanded as a power series in 1/R. The RCCSD(T) potential is
corrected for a size-consistency error prior to fitting. The long-range
coeficients obtained from the fit are found to be in good agreement with
perturbation theory calculations.Comment: submitted to JCP, supporting information available from authors on
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Sisyphus Cooling of Electrically Trapped Polyatomic Molecules
The rich internal structure and long-range dipole-dipole interactions
establish polar molecules as unique instruments for quantum-controlled
applications and fundamental investigations. Their potential fully unfolds at
ultracold temperatures, where a plethora of effects is predicted in many-body
physics, quantum information science, ultracold chemistry, and physics beyond
the standard model. These objectives have inspired the development of a wide
range of methods to produce cold molecular ensembles. However, cooling
polyatomic molecules to ultracold temperatures has until now seemed
intractable. Here we report on the experimental realization of opto-electrical
cooling, a paradigm-changing cooling and accumulation method for polar
molecules. Its key attribute is the removal of a large fraction of a molecule's
kinetic energy in each step of the cooling cycle via a Sisyphus effect,
allowing cooling with only few dissipative decay processes. We demonstrate its
potential by reducing the temperature of about 10^6 trapped CH_3F molecules by
a factor of 13.5, with the phase-space density increased by a factor of 29 or a
factor of 70 discounting trap losses. In contrast to other cooling mechanisms,
our scheme proceeds in a trap, cools in all three dimensions, and works for a
large variety of polar molecules. With no fundamental temperature limit
anticipated down to the photon-recoil temperature in the nanokelvin range, our
method eliminates the primary hurdle in producing ultracold polyatomic
molecules. The low temperatures, large molecule numbers and long trapping times
up to 27 s will allow an interaction-dominated regime to be attained, enabling
collision studies and investigation of evaporative cooling toward a BEC of
polyatomic molecules
On the role of the magnetic dipolar interaction in cold and ultracold collisions: Numerical and analytical results for NH() + NH()
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
Vocal imitations and the identification of sound events
International audienceIt is commonly observed that a speaker vocally imitates a sound that she or he intends to communicate to an interlocutor. We report on an experiment that examined the assumption that vocal imitations can e ffectively communicate a referent sound, and that they do so by conveying the features necessary for the identifi cation of the referent sound event. Subjects were required to sort a set of vocal imitations of everyday sounds. The resulting clusters corresponded in most of the cases to the categories of the referent sound events, indicating that the imitations enabled the listeners to recover what was imitated. Furthermore, a binary decision tree analysis showed that a few characteristic acoustic features predicted the clusters. These features also predicted the classi fication of the referent sounds, but did not generalize to the categorization of other sounds. This showed that, for the speaker, vocally imitating a sound consists of conveying the acoustic features important for recognition, within the constraints of human vocal production. As such vocal imitations prove to be a phenomenon potentially useful to study sound identifi cation
Global potential energy surface for the O2 + N2 interaction. Applications to the collisional, spectroscopic, and thermodynamic properties of the complex
A detailed characterization of the interaction between the most abundant
molecules in air is important for the understanding of a variety of phenomena
in atmospherical science. A completely {\em ab initio} global potential energy
surface (PES) for the O + N interaction is
reported for the first time. It has been obtained with the symmetry-adapted
perturbation theory utilizing a density functional description of monomers
[SAPT(DFT)] extended to treat the interaction involving high-spin open-shell
complexes. The computed interaction energies of the complex are in a good
agreement with those obtained by using the spin-restricted coupled cluster
methodology with singles, doubles and noniterative triple excitations
[RCCSD(T)]. A spherical harmonics expansion containing a large number of terms
due to the anisotropy of the interaction has been built from the {\em ab
initio} data. The radial coefficients of the expansion are matched in the long
range with the analytical functions based on the recent {\em ab initio}
calculations of the electric properties of the monomers [M. Bartolomei et al.,
J. Comp. Chem., {\bf 32}, 279 (2011)]. The PES is tested against the second
virial coefficient data and the integral cross sections measured with
rotationally hot effusive beams, leading in both cases to a very good
agreement. The first bound states of the complex have been computed and
relevant spectroscopic features of the interacting complex are reported. A
comparison with a previous experimentally derived PES is also provided
Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms
We report the simultaneous production of Bose-Einstein condensates (BECs) of
Rb and Cs atoms in separate optical traps. The two samples are
mixed during laser cooling and loading but are separated by m for the
final stage of evaporative cooling. This is done to avoid considerable
interspecies three-body recombination, which causes heating and evaporative
loss. We characterize the BEC production process, discuss limitations, and
outline the use of the dual-species BEC in future experiments to produce
rovibronic ground state molecules, including a scheme facilitated by the
superfluid-to-Mott-insulator (SF-MI) phase transition
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