6,107 research outputs found
Inelastic collisions of relativistic electrons with atomic targets assisted by a laser field
We consider inelastic collisions between relativistic electrons and atomic
targets assisted by a low-frequency laser field in the case when this field is
still much weaker than the typical internal fields in the target. Concentrating
on target transitions we show that they can be substantially affected by the
presence of the laser field. This may occur either via strong modifications in
the motion of the relativistic electrons caused by the electron-laser
interaction or via the Compton effect when the incident electrons convert laser
photon(s) into photons with frequencies equal to target transition frequencies.Comment: 4 pages, 2 figure
Time-resolved X-ray microscopy of nanoparticle aggregates under oscillatory shear
Of all current detection techniques with nanometer resolution, only X-ray
microscopy allows imaging nanoparticles in suspension. Can it also be used to
investigate structural dynamics? When studying response to mechanical stimuli,
the challenge lies in applying them with precision comparable to spatial
resolution. In the first shear experiments performed in an X-ray microscope, we
accomplished this by inserting a piezo actuator driven shear cell into the
focal plane of a scanning transmission X-ray microscope (STXM). Thus
shear-induced reorganization of magnetite nanoparticle aggregates could be
demonstrated in suspension. As X-ray microscopy proves suitable for studying
structural change, new prospects open up in physics at small length scales.Comment: submitted to J. Synchrot. Radia
A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,
rigorously based on many-body Green's function theory but incorporating
information from density functional theory, has already been demonstrated for
the optical gaps and spectra of solid-state systems. Interest in photoactive
hybrid organic/inorganic systems has recently increased, and so has the use of
the BSE for computing neutral excitations of organic molecules. However, no
systematic benchmarks of the BSE for neutral electronic excitations of organic
molecules exist. Here, we study the performance of the BSE for the 28 small
molecules in Thiel's widely-used time-dependent density functional theory
benchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. We
observe that the BSE produces results that depend critically on the mean-field
starting point employed in the perturbative approach. We find that this
starting point dependence is mainly introduced through the quasiparticle
energies obtained at the intermediate GW step, and that with a judicious choice
of starting mean-field, singlet excitation energies obtained from BSE are in
excellent quantitative agreement with higher-level wavefunction methods. The
quality of the triplet excitations is slightly less satisfactory
A new mechanism for electron transfer in fast ion-atomic collisions
We discuss a new mechanism for the electron capture in fast ion-atom
collisions. Similarly like in the radiative capture, where the electron
transfer occurs due to photon emission, within the mechanism under
consideration the electron capture takes place due to the emission of an
additional electron. This first order capture process leads to the so called
transfer-ionization and has a number of interesting features, in particular, in
the target frame it results in the electron emission mainly into the backward
semi-sphere.Comment: 4 pages, two figure
Imaging Molecules from Within: Ultra-fast, {\AA}ngstr\"om Scale Structure Determination of Molecules via Photoelectron Holography using Free Electron Lasers
A new scheme based on (i) upcoming brilliant X-ray Free Electron Laser (FEL)
sources, (ii) novel energy and angular dispersive, large-area electron imagers
and (iii) the well-known photoelectron holography is elaborated that provides
time-dependent three-dimensional structure determination of small to medium
sized molecules with {\AA}ngstr\"om spatial and femtosecond time resolution.
Inducing molecular dynamics, wave-packet motion, dissociation, passage through
conical intersections or isomerization by a pump pulse this motion is
visualized by the X-ray FEL probe pulse launching keV photoelectrons within few
femtoseconds from specific and well-defined sites, deep core levels of
individual atoms, inside the molecule. On their way out the photoelectrons are
diffracted generating a hologram on the detector that encodes the molecular
structure at the instant of photoionization, thus providing 'femtosecond
snapshot images of the molecule from within'. Detailed calculations in various
approximations of increasing sophistication are presented and three-dimensional
retrieval of the spatial structure of the molecule with {\AA}ngstr\"om spatial
resolution is demonstrated. Due to the large photo-absorption cross sections
the method extends X-ray diffraction based, time-dependent structure
investigations envisioned at FELs to new classes of samples that are not
accessible by any other method. Among them are dilute samples in the gas phase
such as aligned, oriented or conformer selected molecules, ultra-cold ensembles
and/or molecular or cluster objects containing mainly light atoms that do not
scatter X-rays efficiently.Comment: 18 pages, 11 figure
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