66 research outputs found
Rotationally induced Penning ionization of ultracold photoassociated helium dimers
We have studied photoassociation of metastable \tripS helium atoms near the
\tripS-\tripP asymptote by both ion detection in a magneto-optical trap and
trap-loss measurements in a magnetic trap. A detailed comparison between the
results of the two experiments gives insight into the mechanism of the Penning
ionization process. We have identified four series of resonances corresponding
to vibrational molecular levels belonging to different rotational states in two
potentials. The corresponding spin states become quasi-purely quintet at small
interatomic distance, and Penning ionization is inhibited by spin conservation
rules. Only a weak rotational coupling is responsible for the contamination by
singlet spin states leading to a detectable ion signal. However, for one of
these series Bose statistics does not enable the rotational coupling and the
series detected through trap-loss does not give rise to sufficient ionization
for detection.Comment: 7 pages, 4 figures, submitted to EuroPhysics Letter
Limit on suppression of ionization in metastable neon traps due to long-range anisotropy
This paper investigates the possibility of suppressing the ionization rate in
a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms.
Suppression of the ionization is critical for the possibility of reaching
Bose-Einstein condensation with such atoms. We estimate the relevant long-range
interactions for the system, consisting of electric quadrupole-quadrupole and
dipole-induced dipole terms, and develop short-range potentials based on the
Na_2 singlet and triplet potentials. The auto-ionization widths of the system
are also calculated. With these ingredients we calculate the ionization rate
for spin-polarized and for spin-isotropic samples, caused by anisotropy of the
long-range interactions. We find that spin-polarization may allow for four
orders of magnitude suppression of the ionization rate for Ne. The results
depend sensitively on a precise knowledge of the interaction potentials,
however, pointing out the need for experimental input. The same model gives a
suppression ratio close to unity for metastable xenon in accordance with
experimental results, due to a much increased anisotropy in this case.Comment: 15 pages including figures, LaTex/RevTex, uses epsfig.st
Coherence properties of an atom laser
We study the coherence properties of an atom laser, which operates by
extracting atoms from a gaseous Bose-Einstein condensate via a two-photon Raman
process, by analyzing a recent experiment. We obtain good agreement with the
experimental data by solving the time-dependent Gross-Pitaevskii equation in
three dimensions both numerically and with a Thomas-Fermi model. The coherence
length is strongly affected by the space-dependent phase developed by the
condensate when the trapping potential is turned off.Comment: 11 pages, 2 Postscript figure
Band Gaps for Atoms in Light based Waveguides
The energy spectrum for a system of atoms in a periodic potential can exhibit
a gap in the band structure. We describe a system in which a laser is used to
produce a mechanical potential for the atoms, and a standing wave light field
is used to shift the atomic levels using the Autler-Townes effect, which
produces a periodic potential. The band structure for atoms guided by a hollow
optical fiber waveguide is calculated in three dimensions with quantised
external motion. The size of the band gap is controlled by the light guided by
the fiber. This variable band structure may allow the construction of devices
which can cool atoms. The major limitation on this device would be the
spontaneous emission losses.Comment: 7 pages, four postscript figures, uses revtex.sty, available through
http://online.anu.edu.au/Physics/papers/atom.htm
On the feasibility of cooling and trapping metastable alkaline-earth atoms
Metastability and long-range interactions of Mg, Ca, and Sr in the
lowest-energy metastable state are investigated. The calculated
lifetimes are 38 minutes for Mg*, 118 minutes for Ca*, and 17 minutes for Sr*,
supporting feasibility of cooling and trapping experiments. The
quadrupole-quadrupole long-range interactions of two metastable atoms are
evaluated for various molecular symmetries. Hund's case (c) 4_g potential
possesses a large 100-1000 K potential barrier. Therefore magnetic trap losses
can possibly be reduced using cold metastable atoms in a stretched M=2 state.
Calculations were performed in the framework of ab initio relativistic
configuration interaction method coupled with the random-phase approximation.Comment: 8 pages, 2 figures; to appear in PR
Scaling laws in velocity-selective coherent-population-trapping laser cooling
One-dimensional laser cooling based on velocity-selective coherent population trapping (VSCPT) has been investigated numerically through the solution of the optical Bloch equations and through a Monte Carlo analysis. The 1→1 and 2→2 transitions have been examined as a function of the atomic recoil frequency, the spontaneous-emission decay rate, and the Rabi frequency of the cooling laser. It has been found that for a large set of those parameters, the VSCPT cooling process may be described through scaling-law relations. The scaling laws are not valid at long atom-laser interaction times or large Rabi frequencies, where the atomic Doppler shift plays a significant role in the atomic motion evolution. Similar results for two atomic transitions suggest the validity of the scaling law for any one-dimensional VSCPT process
Coherence Properties of an Atom Laser
We study the coherence properties of an atom laser, which operates by extracting atoms from a gaseous Bose-Einstein condensate via a two-photon Raman process, by analysing a recent experiment (Hagley et al1999 Phys. Rev. Lett.833112). We obtain good agreement with the experimental data by solving the time-dependent Gross-Pitaevskii equation in three dimensions both numerically and with a Thomas-Fermi model. The coherence is strongly affected by the space-dependent phase developed by the condensate when the trapping potential is turned off
Metastable neon collisions: anisotropy and scattering length
In this paper we investigate the effective scattering length of
spin-polarized Ne*. Due to its anisotropic electrostatic interaction, its
scattering length is determined by five interaction potentials instead of one,
even in the spin-polarized case, a unique property among the Bose condensed
species and candidates. Because the interaction potentials of Ne* are not known
accurately enough to predict the value of the scattering length, we investigate
the behavior of as a function of the five phase integrals corresponding to
the five interaction potentials. We find that the scattering length has five
resonances instead of only one and cannot be described by a simple gas-kinetic
approach or the DIS approximation. However, the probability for finding a
positive or large value of the scattering length is not enhanced compared to
the single potential case. The complex behavior of is studied by comparing
a quantum mechanical five-channel numerical calculation to simpler two-channel
models. We find that the induced dipole-dipole interaction is responsible for
coupling between the different |\Omega> states, resulting in an inhomogeneous
shift of the resonance positions and widths in the quantum mechanical
calculation as compared to the DIS approach. The dependence of the resonance
positions and widths on the input potentials turns out to be rather
straightforward. The existence of two bosonic isotopes of Ne* enables us to
choose the isotope with the most favorable scattering length for efficient
evaporative cooling towards the Bose-Einstein Condensation transition, greatly
enhancing the feasibility to reach this transition.Comment: 13pages, 8 eps figures, analytical model in section V has been
remove
Long-range diatomic s + p potentials of heavy rare gases
We examine the long-range part of the rare-gas diatomic potentials that connect to the R͕(nϪ1)p 5 ns͖ ϩR͕(nϪ1)p 5 np͖ atomic states in the separated atom limit ͑nϭ3, 4, 5, and 6 for Ne, Ar, Kr, and Xe, respectively͒. We obtain our potentials by diagonalization of a Hamiltonian matrix containing the atomic energies and the electric dipole-dipole interaction, with experimentally determined parameters ͑atomic energies, lifetimes, transition wavelengths, and branching ratios͒ as input. Our numerical studies focus on Ne and Kr in this paper, but apply in principle to all other rare gases lacking hyperfine structure. These diatomic potentials are essential for applications in which homonuclear rare-gas pairs interact at large internuclear separations, greater than about 20 Bohr radii. Among such applications are the study of cold atomic collisions and photoassociative spectroscopy
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