115 research outputs found
Density measurement in air with saturable absorbing seed gas
Approaches which have the potential to make density measurements in a compressible flow, where one or more laser beams are used as probes, were investigated. Saturation in sulfur hexafluoride iodine and a crossed beam technique where one beam acts as a saturating beam and the other is at low intensity and acts as a probe beam are considered. It is shown that a balance between an increase in fluorescence intensity with increasing pressure from line broadening and the normal decrease in intensity with increasing pressure from quenching can be used to develop a linear relation between fluorescence intensity and number density and lead to a new density measurement scheme. The method is used to obtain a density image of the cross section of an iodine seeded underexpanded supersonic jet of nitrogen, by illuminating the cross section by a sheet of laser light
Absorption and Emission in the non-Poisson case
This letter adresses the challenging problems posed to the Kubo-Anderson (KA)
theory by the discovery of intermittent resonant fluorescence with a
non-exponential distribution of waiting times. We show how to extend the KA
theory from aged to aging systems, aging for a very extended time period or
even forever, being a crucial consequence of non-Poisson statistics.Comment: 4 pages 3 figures. accepted for publication on Physical Review
Letter
Absorption spectrum of a weakly n-doped semiconductor quantum well
We calculate, as a function of temperature and conduction band electron
density, the optical absorption of a weakly n-doped, idealized semiconductor
quantum well. In particular, we focus on the absorption band due to the
formation of a charged exciton. We conceptualize the charged exciton as an
itinerant excitation intimately linked to the dynamical response of itinerant
conduction band electrons to the appearance of the photo-generated valence band
hole. Numerical results for the absorption in the vicinity of the exciton line
are presented and the spectral weights associated with, respectively, the
charged exciton band and the exciton line are analyzed in detail. We find, in
qualitative agreement with experimental data, that the spectral weight of the
charged exciton grows with increasing conduction band electron density and/or
decreasing temperature at the expense of the exciton.Comment: 5 pages, 4 figure
Non-fermi-liquid single particle lineshape of the quasi-one-dimensional non-CDW metal Li_{0.9}Mo_{6}O_{17} : comparison to the Luttinger liquid
We report the detailed non-Fermi liquid (NFL) lineshape of the dispersing
excitation which defines the Fermi surface (FS) for quasi-one-dimensional
Li_{0.9}Mo_{6}O_{17}. The properties of Li_{0.9}Mo_{6}O_{17} strongly suggest
that the NFL behavior has a purely electronic origin. Relative to the
theoretical Luttinger liquid lineshape, we identify significant similarities,
but also important differences.Comment: 5 pages, 3 eps figure
QM/MM Lineshape Simulation of the Hydrogen-bonded Uracil NH Stretching Vibration of the Adenine:Uracil Base Pair in CDCl
A hybrid Car-Parrinello QM/MM molecular dynamics simulation has been carried
out for the Watson-Crick base pair of 9-ethyl-8-phenyladenine and
1-cyclohexyluracil in deuterochloroform solution at room temperature. The
resulting trajectory is analyzed putting emphasis on the N-HN Hydrogen
bond geometry. Using an empirical correlation between the \NN-distance and
the fundamental NH-stretching frequency, the time-dependence of this energy gap
along the trajectory is obtained. From the gap-correlation function we
determine the infrared absorption spectrum using lineshape theory in
combination with a multimode oscillator model. The obtained average transition
frequency and the width of the spectrum is in reasonable agreement with recent
experimental data.Comment: revised version with small changes submitted to Chem. Phys. Let
Phonon lineshapes in the vortex state of the phonon-mediated superconductor YNiBC
We present an inelastic neutron scattering study of phonon lineshapes in the
vortex state of the type-II superconductor YNiBC. In a previous study
[Phys. Rev. Lett. \textbf{101}, 237002 (2008)] it was shown that certain
phonons exhibit a clear signature of the superconducting gap on
entering the superconducting state. Our interest was to find out whether or not
the lineshape of such phonons reflects the inhomogeneous nature of the vortex
state induced by a magnetic field smaller than the upper critical field
.We found that this is indeed the case because the observed phonon
lineshapes can be well described by a model considering the phonon as a local
probe of the spatial variation of the superconducting gap. We found that even
at , where the inter-vortex distance is less than \AA, the
phonon lineshape still shows evidence for a variation of the gap
Generalized Kasha's Scheme for Classifying Two-Dimensional Excitonic Molecular Aggregates: Temperature Dependent Absorption Peak Frequency Shift
We propose a generalized theoretical framework for classifying
two-dimensional (2D) excitonic molecular aggregates based on an analysis of
temperature dependent spectra. In addition to the monomer-aggregate absorption
peak shift, which defines the conventional J- and H-aggregates, we incorporate
the peak shift associated with increasing temperature as a measure to
characterize the exciton band structure. First we show that there is a
one-to-one correspondence between the monomer-aggregate and the T-dependent
peak shifts for Kasha's well-established model of 1D aggregates, where
J-aggregates exhibit further redshift upon increasing temperature and
H-aggregates exhibit further blueshift. On the contrary, 2D aggregate
structures are capable of supporting the two other combinations: blueshifting
J-aggregates and redshifting H-aggregates, owing to their more complex exciton
band structures. Secondly, using spectral lineshape theory, the T-dependent
shift is associated with the relative abundance of states on each side of the
bright state. We further establish that the density of states can be connected
to the microscopic packing condition leading to these four classes of
aggregates by separately considering the short and long-range contribution to
the excitonic couplings. In particular the T-dependent shift is shown to be an
unambiguous signature for the sign of net short-range couplings: Aggregates
with net negative (positive) short-range couplings redshift (blueshift) with
increasing temperature. Lastly, comparison with experiments shows that our
theory can be utilized to quantitatively account for the observed but
previously unexplained T-dependent absorption lineshapes. Thus, our work
provides a firm ground for elucidating the structure-function relationships for
molecular aggregates and is fully compatible with existing experimental and
theoretical structure characterization tools.Comment: 29 pages, 4 figure
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