103,698 research outputs found
Lifetime determination of the 5d~F state in barium using trapped atoms
Magneto-optically trapped atoms enable the determination of lifetimes of
metastable states and higher lying excited states like the
state in barium. The state is efficiently populated by
driving strong transitions from metastable states within the cooling cycle of
the barium MOT. The lifetime is inferred from the increase of MOT fluorescence
after the transfer of up to of the trapped atoms to this state. The
radiative decay of the state cascades to the cooling
cycle of the MOT with a probability of corresponding to a trap
loss of and its lifetime is determined to .
This is in good agreement with the theoretically calculated lifetime of
[J. Phys. B, {\bf 40}, 227 (2007)]. The determined loss of
from the cooling cycle is compared with the theoretically
calculated branching ratios. This measurement extends the efficacy of trapped
atoms to measure lifetimes of higher, long-lived states and validate the atomic
structure calculations of heavy multi-electron systems.Comment: 5 pages, accepted for publication in Physical Review
Isotope shifts of 6s5d D-states in neutral Barium
Laser spectroscopy of the low lying P and D states in atomic barium
has been performed. This work contributes substantially to the development of
an effective laser cooling and trapping for heavy alkaline earth elements and
aims in particular for a better understanding of the atomic wave function of
these systems. Isotope shifts and hyperfine structures are ideal probes for the
wave functions at the position of the nucleus. This is essential input for a
theoretical evaluation of the sensitivity to fundamental symmetry breaking
properties like permanent electric dipole moments. We report the first isotope
shift measurements of the D-P transitions. A deviation of
the King plot from its expected behavior has been observed. Further we have
optically resolved the hyperfine structure of the D states.Comment: 7 pages, 7 figure
Magneto optical trapping of Barium
First laser cooling and trapping of the heavy alkaline earth element barium
has been achieved based on the strong 6s S - 6s6p P
transition for the main cooling. Due to the large branching into metastable
D-states several additional laser driven transitions are required to provide a
closed cooling cycle. A total efficiency of for slowing
a thermal atomic beam and capturing atoms into a magneto optical trap was
obtained. Trapping lifetimes of more than 1.5 s were observed. This lifetime is
shortened at high laser intensities by photo ionization losses. The developed
techniques will allow to extend significantly the number of elements that can
be optically cooled and trapped.Comment: 4 pages, 5 figure
Radiative feedback and cosmic molecular gas: the role of different radiative sources
We present results from multifrequency radiative hydrodynamical chemistry
simulations addressing primordial star formation and related stellar feedback
from various populations of stars, stellar energy distributions (SEDs) and
initial mass functions. Spectra for massive stars, intermediate-mass stars and
regular solar-like stars are adopted over a grid of 150 frequency bins and
consistently coupled with hydrodynamics, heavy-element pollution and
non-equilibrium species calculations. Powerful massive population III stars are
found to be able to largely ionize H and, subsequently, He and He, causing
an inversion of the equation of state and a boost of the Jeans masses in the
early intergalactic medium. Radiative effects on star formation rates are
between a factor of a few and 1 dex, depending on the SED. Radiative processes
are responsible for gas heating and photoevaporation, although emission from
soft SEDs has minor impacts. These findings have implications for cosmic gas
preheating, primordial direct-collapse black holes, the build-up of "cosmic
fossils" such as low-mass dwarf galaxies, the role of AGNi during reionization,
the early formation of extended disks and angular-momentum catastrophe.Comment: 19 pages on MNRA
Instanton Solutions for the Universal Hypermultiplet
We expand our previous analysis on fivebrane and membrane instanton solutions
in the universal hypermultiplet, including near-extremal multi-centered
solutions and mixed fivebrane-membrane charged instantons. The results are most
conveniently described in terms of a double-tensor multiplet.Comment: Minor changes, clarification added. Contribution to the proceedings
of the 36th International Symposium Ahrenshoop, August 200
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
Effect of Poisson ratio on cellular structure formation
Mechanically active cells in soft media act as force dipoles. The resulting
elastic interactions are long-ranged and favor the formation of strings. We
show analytically that due to screening, the effective interaction between
strings decays exponentially, with a decay length determined only by geometry.
Both for disordered and ordered arrangements of cells, we predict novel phase
transitions from paraelastic to ferroelastic and anti-ferroelastic phases as a
function of Poisson ratio.Comment: 4 pages, Revtex, 4 Postscript figures include
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