38,648 research outputs found
Electronic structure and dynamics of optically excited single-wall carbon nanotubes
We have studied the electronic structure and charge-carrier dynamics of
individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using
optical and electron-spectroscopic techniques. The electronic structure of
semiconducting SWNTs in the band-gap region is analyzed using near-infrared
absorption spectroscopy. A semi-empirical expression for
transition energies, based on tight-binding calculations is found to give
striking agreement with experimental data. Time-resolved PL from dispersed
SWNT-micelles shows a decay with a time constant of about 15 ps. Using
time-resolved photoemission we also find that the electron-phonon ({\it e-ph})
coupling in metallic tubes is characterized by a very small {\it e-ph}
mass-enhancement of 0.0004. Ultrafast electron-electron scattering of
photo-excited carriers in nanotube ropes is finally found to lead to internal
thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics
A simplified picture for Pi electrons in conjugated polymers : from PPP Hamiltonian to an effective molecular crystal approach
An excitonic method proper to study conjugated oligomers and polymers is
described and its applicability tested on the ground state and first excited
states of trans-polyacetylene, taken as a model. From the Pariser-Parr-Pople
Hamiltonian, we derive an effective Hamiltonian based on a local description of
the polymer in term of monomers; the relevant electronic configurations are
build on a small number of pertinent local excitations. The intuitive and
simple microscopic physical picture given by our model supplement recent
results, such as the Rice and Garstein ones. Depending of the parameters, the
linear absorption appears dominated by an intense excitonic peak.Comment: 41 Pages, 6 postscript figure
Stellar (n,gamma) cross sections of p-process isotopes PartI: 102Pd, 120Te, 130,132Ba,and 156Dy
We have investigated the (n,gamma) cross sections of p-process isotopes with
the activation technique. The measurements were carried out at the Karlsruhe
Van de Graaff accelerator using the 7Li(p,n)7Be source for simulating a
Maxwellian neutron distribution of kT = 25 keV. Stellar cross section
measurements are reported for the light p-process isotopes 102Pd, 120Te,
130,132Ba, and 156Dy. In a following paper the cross sections of 168Yb, 180W,
184Os, 190Pt, and 196Hg will be discussed. The data are extrapolated to
p-process energies by including information from evaluated nuclear data
libraries. The results are compared to standard Hauser-Feshbach models
frequently used in astrophysics.Comment: 13 pages, 4 figure
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Atmospheric observations of the water vapour continuum in the near-infrared windows between 2500 and 6600 cm-1
Water vapour continuum absorption is potentially important for both closure of the Earth's energy budget and remote sensing applications. Currently, there are significant uncertainties in its characteristics in the near-infrared atmospheric windows at 2.1 and 1.6â”m. There have been several attempts to measure the continuum in the laboratory; not only are there significant differences amongst these measurements, but there are also difficulties in extrapolating the laboratory data taken at room temperature and above to temperatures more widely relevant to the atmosphere. Validation is therefore required using field observations of the real atmosphere. There are currently no published observations in atmospheric conditions with enough water vapour to detect a continuum signal within these windows or where the self-continuum component is significant. We present observations of the near-infrared water vapour continuum from Camborne, UK, at sea level using a Sun-pointing, radiometrically calibrated Fourier transform spectrometer in the window regions between 2000 and 10â000âcmâ1. Analysis of these data is challenging, particularly because of the need to remove aerosol extinction and the large uncertainties associated with such field measurements. Nevertheless, we present data that are consistent with recent laboratory datasets in the 4 and 2.1â”m windows (when extrapolated to atmospheric temperatures). These results indicate that the most recent revision (3.2) of the MT_CKD foreign continuum, versions of which are widely used in atmospheric radiation models, requires strengthening by a factor of âŒ5 in the centre of the 2.1â”m window. In the higher-wavenumber window at 1.6â”m, our estimated self- and foreign-continua are significantly stronger than MT_CKD. The possible contribution of the self- and foreign-continua to our derived total continuum optical depth is estimated by using laboratory or MT_CKD values of one, to estimate the other. The obtained self-continuum shows some consistency with temperature-extrapolated laboratory data in the centres of the 4 and 2.1â”m windows. The 1.6â”m region is more sensitive to atmospheric aerosol and continuum retrievals and therefore more uncertain than the more robust results at 2.1 and 4â”m. We highlight the difficulties in observing the atmospheric continuum and make the case for additional measurements in both the laboratory and field and discuss the requirements for any future field campaign
Physical properties of galaxies and their evolution in the VIMOS VLT Deep Survey. I. The evolution of the mass-metallicity relation up to z~0.9
We derive the mass-metallicity relation of star-forming galaxies up to
, using data from the VIMOS VLT Deep Survey. Automatic measurement of
emission-line fluxes and equivalent widths have been performed on the full
spectroscopic sample. This sample is divided into two sub-samples depending on
the apparent magnitude selection: wide () and deep
). These two samples span two different ranges of stellar
masses. Emission-line galaxies have been separated into star-forming galaxies
and active galactic nuclei using emission line ratios. For the star-forming
galaxies the emission line ratios have also been used to estimate gas-phase
oxygen abundance, using empirical calibrations renormalized in order to give
consistent results at low and high redshifts. The stellar masses have been
estimated by fitting the whole spectral energy distributions with a set of
stellar population synthesis models. We assume at first order that the shape of
the mass-metallicity relation remains constant with redshift. Then we find a
stronger metallicity evolution in the wide sample as compared to the deep
sample. We thus conclude that the mass-metallicity relation is flatter at
higher redshift. The observed flattening of the mass-metallicity relation at
high redshift is analyzed as an evidence in favor of the open-closed model.Comment: 21 pages, revised version submitted to A&
The nature and strength of inter-layer binding in graphite
We computed the inter-layer bonding properties of graphite using an ab-initio
many body theory. We carried out variational and diffusion quantum Monte Carlo
calculations and found an equilibrium inter-layer binding energy in good
agreement with most recent experiments. We also analyzed the behavior of the
total energy as a function of interlayer separation at large distances
comparing the results with the predictions of the random phase approximation.Comment: 5 pages; to appear in Phys. Rev. Let
Ultraviolet Spectroscopy of Circumnuclear Star Clusters in M83
We analyze archival HST/STIS/FUV-MAMA imaging and spectroscopy of 13 compact
star clusters within the circumnuclear starburst region of M83, the closest
such example. We compare the observed spectra with semi-empirical models, which
are based on an empirical library of Galactic O and B stars observed with IUE,
and with theoretical models, which are based on a new theoretical UV library of
hot massive stars computed with WM-Basic. The models were generated with
Starburst99 for metallicities of Z=0.020 and Z=0.040, and for stellar IMFs with
upper mass limits of 10, 30, 50, and 100 M_sol. We estimate the ages and masses
of the clusters from the best fit model spectra, and find that the ages derived
from the semi-empirical and theoretical models agree within a factor of 1.2 on
average. A comparison of the spectroscopic age estimates with values derived
from HST/WFC3/UVIS multi-band photometry shows a similar level of agreement for
all but one cluster. The clusters have a range of ages from about 3 to 20 Myr,
and do not appear to have an age gradient along M83's starburst. Clusters with
strong P-Cygni profiles have masses of a few times 10^4 M_sol, seem to have
formed stars more massive than 30 M_sol, and are consistent with a Kroupa IMF
from 0.1-100 M_sol. Field regions in the starburst lack P-Cygni profiles and
are dominated by B stars.Comment: 38 pages, 13 figures, accepted for publication in the Ap
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