10,892 research outputs found
Near threshold rotational excitation of molecular ions by electron-impact
New cross sections for the rotational excitation of H by electrons are
calculated {\it ab initio} at low impact energies. The validity of the
adiabatic-nuclei-rotation (ANR) approximation, combined with -matrix
wavefunctions, is assessed by comparison with rovibrational quantum defect
theory calculations based on the treatment of Kokoouline and Greene ({\it Phys.
Rev. A} {\bf 68} 012703 2003). Pure ANR excitation cross sections are shown to
be accurate down to threshold, except in the presence of large oscillating
Rydberg resonances. These resonances occur for transitions with
and are caused by closed channel effects. A simple analytic formula is derived
for averaging the rotational probabilities over such resonances in a 3-channel
problem. In accord with the Wigner law for an attractive Coulomb field,
rotational excitation cross sections are shown to be large and finite at
threshold, with a significant but moderate contribution from closed channels.Comment: 3 figures, a5 page
A search for clusters and groups of galaxies on the line of sight towards 8 lensed quasars
In this paper we present new ESO/VLT FORS1 and ISAAC images of the fields
around eight gravitationally lensed quasars: CTQ414, HE0230-2130,
LBQS1009-0252, B1030+074, HE1104-1805, B1359+154, H1413+117 and HE2149-2745.
When available and deep enough, HST/WFPC2 data were also used to infer the
photometric redshifts of the galaxies around the quasars. The search of galaxy
overdensities in space and redshift, as well as a weak-shear analysis and a
mass reconstruction are presented in this paper. We find that there are most
probably galaxy groups towards CTQ414, HE0230-2130, B1359+154, H1413+117 and
HE2149-2745, with a mass ~ 4x10^14 M_sol h^-1. Considering its photometric
redshift, the galaxy group discovered in the field around HE1104-1805 is
associated with the quasar rather than with the lensing potential.Comment: 14 pages, 11 figures(.jpg
Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
The interpretation of water line emission from existing observations and
future HIFI/Herschel data requires a detailed knowledge of collisional rate
coefficients. Among all relevant collisional mechanisms, the rotational
(de)excitation of H2O by H2 molecules is the process of most interest in
interstellar space. To determine rate coefficients for rotational de-excitation
among the lowest 45 para and 45 ortho rotational levels of H2O colliding with
both para and ortho-H2 in the temperature range 20-2000 K. Rate coefficients
are calculated on a recent high-accuracy H2O-H2 potential energy surface using
quasi-classical trajectory calculations. Trajectories are sampled by a
canonical Monte-Carlo procedure. H2 molecules are assumed to be rotationally
thermalized at the kinetic temperature. By comparison with quantum calculations
available for low lying levels, classical rates are found to be accurate within
a factor of 1-3 for the dominant transitions, that is those with rates larger
than a few 10^{-12}cm^{3}s^{-1}. Large velocity gradient modelling shows that
the new rates have a significant impact on emission line fluxes and that they
should be adopted in any detailed population model of water in warm and hot
environments.Comment: 8 pages, 2 figures, 1 table (the online material (4 tables) can be
obtained upon request to [email protected]
Plasma wake inhibition at the collision of two laser pulses in an underdense plasma
An electron injector concept for laser-plasma accelerator was developed in
ref [1] and [2] ; it relies on the use of counter-propagating ultrashort laser
pulses. In [2], the scheme is as follows: the pump laser pulse generates a
large amplitude laser wakefield (plasma wave). The counter-propagating
injection pulse interferes with the pump laser pulse to generate a beatwave
pattern. The ponderomotive force of the beatwave is able to inject plasma
electrons into the wakefield. We have studied this injection scheme using 1D
Particle in Cell (PIC) simulations. The simulations reveal phenomena and
important physical processes that were not taken into account in previous
models. In particular, at the collision of the laser pulses, most plasma
electrons are trapped in the beatwave pattern and cannot contribute to the
collective oscillation supporting the plasma wave. At this point, the fluid
approximation fails and the plasma wake is strongly inhibited. Consequently,
the injected charge is reduced by one order of magnitude compared to the
predictions from previous models.Comment: 4 pages, 4 figure
Many parameter Hoelder perturbation of unbounded operators
If is a -mapping, for , having
as values unbounded self-adjoint operators with compact resolvents and common
domain of definition, parametrized by in an (even infinite dimensional)
space, then any continuous (in ) arrangement of the eigenvalues of is
indeed in .Comment: LaTeX, 4 pages; The result is generalized from Lipschitz to Hoelder.
Title change
Quasimonoenergetic electron beams produced by colliding cross-polarized laser pulses in underdense plasmas
The interaction of two laser pulses in an underdense plasma has proven to be
able to inject electrons in plasma waves, thus providing a stable and tunable
source of electrons. Whereas previous works focused on the "beatwave" injection
scheme in which two lasers with the same polarization collide in a plasma, this
present letter studies the effect of polarization and more specifically the
interaction of two colliding cross-polarized laser pulses. It is shown both
theoretically and experimentally that electrons can also be pre-accelerated and
injected by the stochastic heating occurring at the collision of two
cross-polarized lasers and thus, a new regime of optical injection is
demonstrated. It is found that injection with cross-polarized lasers occurs at
higher laser intensities.Comment: 4 pages, 4 figure
Collisional excitation of singly deuterated ammonia NHD by H
The availability of collisional rate coefficients with H is a
pre-requisite for interpretation of observations of molecules whose energy
levels are populated under non local thermodynamical equilibrium conditions. In
the current study, we present collisional rate coefficients for the NHD /
para--H() collisional system, for energy levels up to (735 K) and for gas temperatures in the range K. The
cross sections are obtained using the essentially exact close--coupling (CC)
formalism at low energy and at the highest energies, we used the
coupled--states (CS) approximation. For the energy levels up to
(215 K), the cross sections obtained through the CS formalism are
scaled according to a few CC reference points. These reference points are
subsequently used to estimate the accuracy of the rate coefficients for higher
levels, which is mainly limited by the use of the CS formalism. Considering the
current potential energy surface, the rate coefficients are thus expected to be
accurate to within 5\% for the levels below , while we estimate
an accuracy of 30\% for higher levels
A seven square degrees survey for galaxy-scale gravitational lenses with the HST imaging archive
We present the results of a visual search for galaxy-scale gravitational
lenses in nearly 7 square degrees of Hubble Space Telescope (HST) images. The
dataset comprises the whole imaging data ever taken with the Advanced Camera
for Surveys (ACS) in the filter F814W (I-band) up to August 31st, 2011, i.e.
6.03 square degrees excluding the field of the Cosmic Evolution Survey (COSMOS)
which has been the subject of a separate visual search. In addition, we have
searched for lenses in the whole Wide Field Camera 3 (WFC3) near-IR imaging
dataset in all filters (1.01 square degrees) up to the same date. Our primary
goal is to provide a sample of lenses with a broad range of different
morphologies and lens-source brightness contrast in order estimate a lower
limit to the number of galaxy-scale strong lenses in the future Euclid survey
in its VIS band. Our criteria to select lenses are purely morphological as we
do not use any colour or redshift information.The final candidate selection is
very conservative hence leading to a nearly pure but incomplete sample. We find
49 new lens candidates: 40 in the ACS images and 9 in the WFC3 images. Out of
these, 16 candidates are secure lenses owing to their striking morphology, 21
more are very good candidates, and 12 more have morphologies compatible with
gravitational lensing but also compatible with other astrophysical objects. It
is therefore insensitive to cosmic variance and allows to estimate the number
of galaxy-scale strong lenses on the sky for a putative survey depth, which is
the main result of the present work. Because of the incompleteness of the
sample, the estimated lensing rates should be taken as lower limits. Using
these, we anticipate that a 15 000 square degrees space survey such as Euclid
will find at least 60 000 galaxy-scale strong lenses down to a limiting AB
magnitude of I = 24.5 (10-sigma) or I = 25.8 (3-sigma).Comment: 13 pages, 12 figures, Accepted for publication in MNRA
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