21,719 research outputs found
Overlapping Resonances Interference-induced Transparency: The Photoexcitation Spectrum of Pyrazine
The phenomenon of "overlapping resonances interference-induced transparency"
(ORIT) is introduced and studied in detail for the
photoexcitation of cold pyrazine (CHN). In ORIT a molecule becomes
transparent at specific wavelengths due to interferences between envelopes of
spectral lines displaying overlapping resonances. An example is the
internal conversion in pyrazine where destructive
interference between overlapping resonances causes the light
absorption to disappear at certain wavelengths. ORIT may be of practical
importance in multi-component mixtures where it would allow for the selective
excitation of some molecules in preference to others. Interference induced
cross section enhancement is also shown.Comment: 13 pages, 7 figure
Piecewise adiabatic population transfer in a molecule via a wave packet
We propose a class of schemes for robust population transfer between quantum
states that utilize trains of coherent pulses and represent a generalized
adiabatic passage via a wave packet. We study piecewise Stimulated Raman
Adiabatic Passage with pulse-to-pulse amplitude variation, and piecewise
chirped Raman passage with pulse-to-pulse phase variation, implemented with an
optical frequency comb. In the context of production of ultracold ground-state
molecules, we show that with almost no knowledge of the excited potential,
robust high-efficiency transfer is possibleComment: 4 pages, 5 figures. Submitted to Phys. Rev. Let
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Complete transfer of populations from a single state to a pre-selected superposition of states using Piecewise Adiabatic Passage
We develop a method for executing robust and selective transfer of
populations between a single level and pre-selected superpositions of energy
eigenstates. Viewed in the frequency domain, our method amounts to executing a
series of simultaneous adiabatic passages into each component of the target
superposition state. Viewed in {the} time domain, the method works by
accumulating the wavefunction of the target wave packet as it revisits the
Franck Condon region, in what amounts to an extension of the Piecewise
Adiabatic Passage technique [ Shapiro et.al., Phys. Rev. Lett. 99, 033002
(2007)] to the multi-state regime. The viability of the method is verified by
performing numerical tests for the Na_2 molecule.Comment: 8 pages, 4 figure
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Hadron-quark mixed phase in hyperon stars
We analyze the different possibilities for the hadron-quark phase transition
occurring in beta-stable matter including hyperons in neutron stars. We use a
Brueckner-Hartree-Fock approach including hyperons for the hadronic equation of
state and a generalized MIT bag model for the quark part. We then point out in
detail the differences between Maxwell and Gibbs phase transition constructions
including the effects of surface tension and electromagnetic screening. We find
only a small influence on the maximum neutron star mass, whereas the radius of
the star and in particular its internal structure are more affected.Comment: 11 pages, 9 figure
Reducing the weak lensing noise for the gravitational wave Hubble diagram using the non-Gaussianity of the magnification distribution
Gravitational wave sources are a promising cosmological standard candle
because their intrinsic luminosities are determined by fundamental physics (and
are insensitive to dust extinction). They are, however, affected by weak
lensing magnification due to the gravitational lensing from structures along
the line of sight. This lensing is a source of uncertainty in the distance
determination, even in the limit of perfect standard candle measurements. It is
commonly believed that the uncertainty in the distance to an ensemble of
gravitational wave sources is limited by the standard deviation of the lensing
magnification distribution divided by the square root of the number of sources.
Here we show that by exploiting the non-Gaussian nature of the lensing
magnification distribution, we can improve this distance determination,
typically by a factor of 2--3; we provide a fitting formula for the effective
distance accuracy as a function of redshift for sources where the lensing noise
dominates.Comment: matches PRD accepted version (expanded description of the
cosmological parameter space + minor changes
Structure of hybrid protoneutron stars within the Nambu--Jona-Lasinio model
We investigate the structure of protoneutron stars (PNS) formed by hadronic
and quark matter in -equilibrium described by appropriate equations of
state (EOS). For the hadronic matter, we use a finite temperature EOS based on
the Brueckner-Bethe-Goldstone many-body theory, with realistic two- and
three-body forces. For the quark sector, we employ the Nambu--Jona-Lasinio
model. We find that the maximum allowed masses are comprised in a narrow range
around 1.8 solar masses, with a slight dependence on the temperature.
Metastable hybrid protoneutron stars are not found.Comment: 7 pages, 6 figures, revised version accepted for publication in Phys.
Rev.
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