811 research outputs found
Ab initio investigation of intermolecular interactions in solid benzene
A computational strategy for the evaluation of the crystal lattice constants
and cohesive energy of the weakly bound molecular solids is proposed. The
strategy is based on the high level ab initio coupled-cluster determination of
the pairwise additive contribution to the interaction energy. The
zero-point-energy correction and non-additive contributions to the interaction
energy are treated using density functional methods. The experimental crystal
lattice constants of the solid benzene are reproduced, and the value of 480
meV/molecule is calculated for its cohesive energy
Low-Energy Scale Excitations in the Spectral Function of Organic Monolayer Systems
Using high-resolution photoemission spectroscopy we demonstrate that the
electronic structure of several organic monolayer systems, in particular
1,4,5,8-naphthalene tetracarboxylic dianhydride and Copper-phtalocyanine on
Ag(111), is characterized by a peculiar excitation feature right at the Fermi
level. This feature displays a strong temperature dependence and is immediatly
connected to the binding energy of the molecular states, determined by the
coupling between the molecule and the substrate. At low temperatures, the
line-width of this feature, appearing on top of the partly occupied lowest
unoccupied molecular orbital of the free molecule, amounts to only
meV, representing an unusually small energy scale for electronic excitations in
these systems. We discuss possible origins, related e.g. to many-body
excitations in the organic-metal adsorbate system, in particular a generalized
Kondo scenario based on the single impurity Anderson model.Comment: 6 pages, 3 figures, accepted as PRB Rapid Communication
Kinetics of the triplet state of 2,3-dichloroquinoxaline from microwave-induced phosphorescence transients
The reflectivity of relativistic ultra-thin electron layers
The coherent reflectivity of a dense, relativistic, ultra-thin electron layer
is derived analytically for an obliquely incident probe beam. Results are
obtained by two-fold Lorentz transformation. For the analytical treatment, a
plane uniform electron layer is considered. All electrons move with uniform
velocity under an angle to the normal direction of the plane; such electron
motion corresponds to laser acceleration by direct action of the laser fields,
as it is described in a companion paper. Electron density is chosen high enough
to ensure that many electrons reside in a volume \lambda_R^3, where \lambda_R
is the wavelength of the reflected light in the rest frame of the layer. Under
these conditions, the probe light is back-scattered coherently and is directed
close to the layer normal rather than the direction of electron velocity. An
important consequence is that the Doppler shift is governed by
\gamma_x=(1-(V_x/c)^2)^{-1/2} derived from the electron velocity component V_x
in normal direction rather than the full \gamma-factor of the layer electrons.Comment: 7 pages, 4 figures, submitted to the special issue "Fundamental
Physics with Ultra-High Fields" in The European Physical Journal
All-optical measurement of the hot electron sheath driving laser ion acceleration from thin foils
We present experimental results from an all-optical diagnostic method to directly measure the evolution of the hot-electron distribution driving the acceleration of ions from thin foils using high-intensity lasers. Central parameters of laser ion acceleration such as the hot-electron density, the temperature distribution and the conversion efficiency from laser pulse energy into hot electrons become comprehensively accessible with this technique.Deutsche Forschungsgemeinschaft (DFG) (contract number TR18)Germany. Bundesministerium für Bildung und Forschung (contract number 03ZIK445)Germany. Bundesministerium für Bildung und Forschung (contract number 03ZIK052
Intense high-quality medical proton beams via laser fields
During the past decade, the interaction of high-intensity lasers with solid
targets has attracted much interest, regarding its potential in accelerating
charged particles. In spite of tremendous progress in laser-plasma based
acceleration, it is still not clear which particle beam quality will be
accessible within the upcoming multi petawatt (1 PW = 10 W) laser
generation. Here, we show with simulations based on the coupled relativistic
equations of motion that protons stemming from laser-plasma processes can be
efficiently post-accelerated using crossed laser beams focused to spot radii of
a few laser wavelengths. We demonstrate that the crossed beams produce
monoenergetic accelerated protons with kinetic energies MeV, small
energy spreads ( 1) and high densities as required for hadron
cancer therapy. To our knowledge, this is the first scheme allowing for this
important application based on an all-optical set-up.Comment: 14 pages, 3 figures, 1 tabl
Nuclear dynamics of singlet exciton fission: a direct observation in pentacene single crystals
Singlet exciton fission (SEF) is a key process in the development of efficient opto-electronic devices. An aspect that is rarely probed directly, and yet has a tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density functional theory and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the disintegration of the electronically correlated triplet pairs, and shed light on why SEF occurs on ultrafast timescales
Lattice dynamics and correlated atomic motion from the atomic pair distribution function
The mean-square relative displacements (MSRD) of atomic pair motions in
crystals are studied as a function of pair distance and temperature using the
atomic pair distribution function (PDF). The effects of the lattice vibrations
on the PDF peak widths are modelled using both a multi-parameter Born
von-Karman (BvK) force model and a single-parameter Debye model. These results
are compared to experimentally determined PDFs. We find that the near-neighbor
atomic motions are strongly correlated, and that the extent of this correlation
depends both on the interatomic interactions and crystal structure. These
results suggest that proper account of the lattice vibrational effects on the
PDF peak width is important in extracting information on static disorder in a
disordered system such as an alloy. Good agreement is obtained between the BvK
model calculations of PDF peak widths and the experimentally determined peak
widths. The Debye model successfully explains the average, though not detailed,
natures of the MSRD of atomic pair motion with just one parameter. Also the
temperature dependence of the Debye model largely agrees with the BvK model
predictions. Therefore, the Debye model provides a simple description of the
effects of lattice vibrations on the PDF peak widths.Comment: 9 pages, 11 figure
Spectral shaping of laser generated proton beams
The rapid progress in the field of laser particle acceleration has stimulated a debate about the promising perspectives of laser based ion beam sources. For a long time, the beams produced exhibited quasi-thermal spectra. Recent proof-of-principle experiments demonstrated that ion beams with narrow energy distribution can be generated from special target geometries. However, the achieved spectra were strongly limited in terms of monochromacity and reproducibility. We show that microstructured targets can be used to reliably produce protons with monoenergetic spectra above 2 MeV with less than 10% energy spread. Detailed investigations of the effects of laser ablation on the target resulted in a significant improvement of the reproducibility. Based on statistical analysis, we derive a scaling law between proton peak position and laser energy, underlining the suitability of this method for future applications. Both the quality of the spectra and the scaling law are well reproduced by numerical simulations
Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa
The partial density of vibrational states has been measured for Fe in
compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering.
Substantial changes have been observed in the overall shape of the density of
states close to the magnetic transiton around 20 GPa from the paramagnetic (low
pressure) to the antiferromagnetic (high pressure) state. Our data indicate a
substantial softening of the aggregate sound velocities far below the
transition, starting between 5 and 10 GPa. This is consistent with recent
radial x-ray diffraction measurements of the elastic constants in FeO. The
results indicate that strong magnetoelastic coupling in FeO is the driving
force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure
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