46 research outputs found
The effect of inhomogenities on single molecule imaging by hard XFEL pulses
We study the local distortion of the atomic structure in small biological
samples illuminated by x-ray free electron laser (XFEL) pulses. We concentrate
on the effect of inhomogenities: heavy atoms in a light matrix and
non-homogeneous spatial distribution of atoms. In biological systems we find
both. Using molecular-dynamics type modeling it is shown that the local
distortions about heavy atoms are larger than the average distortion in the
light matrix. Further it is also shown that the large spatial density
fluctuations also significantly alter the time evolution of atomic
displacements as compared to samples with uniform density. This fact has
serious consequences on single particle imaging. This is discussed and the
possibility of a correction is envisaged.Comment: Movies: http://www.szfki.hu/~jurek/art2009_1/index.htm
Dynamics in a cluster under the influence of intense femtosecond hard x-ray pulses
In this paper we examine the behavior of small cluster of atoms in a short
(10-50 fs) very intense hard x-ray (10 keV) pulse. We use numerical modeling
based on the non-relativistic classical equation of motion. Quantum processes
are taken into account by the respective cross sections. We show that there is
a Coulomb explosion, which has a different dynamics than one finds in classical
laser driven cluster explosions. We discuss the consequences of our results to
single molecule imaging by the free electron laser pulses.Comment: 14 pages, 13 figure
Hydrodynamic model for picosecond propagation of laser-created nanoplasmas
The interaction of a free-electron-laser pulse with a moderate or large size
cluster is known to create a quasi-neutral nanoplasma, which then expands on
hydrodynamic timescale, i.e., ps. To have a better understanding of ion
and electron data from experiments derived from laser-irradiated clusters, one
needs to simulate cluster dynamics on such long timescales for which the
molecular dynamics approach becomes inefficient. We therefore propose a
two-step Molecular Dynamics-Hydrodynamic scheme. In the first step we use
molecular dynamics code to follow the dynamics of an irradiated cluster until
all the photo-excitation and corresponding relaxation processes are finished
and a nanoplasma, consisting of ground-state ions and thermalized electrons, is
formed. In the second step we perform long-timescale propagation of this
nanoplasma with a computationally efficient hydrodynamic approach.
In the present paper we examine the feasibility of a hydrodynamic two-fluid
approach to follow the expansion of spherically symmetric nanoplasma, without
accounting for the impact ionization and three-body recombination processes at
this stage. We compare our results with the corresponding molecular dynamics
simulations. We show that all relevant information about the nanoplasma
propagation can be extracted from hydrodynamic simulations at a significantly
lower computational cost when compared to a molecular dynamics approach.
Finally, we comment on the accuracy and limitations of our present model and
discuss possible future developments of the two-step strategy.Comment: 14 pages, 6 figure
The effect of tamper layer on the explosion dynamics of atom clusters
The behavior of small samples in very short and intense hard x-ray pulses is
studied by molecular dynamics type calculations. The main emphasis is put on
the effect of various tamper layers about the sample. This is discussed from
the point of view of structural imaging of single particles, including not only
the distortion of the structure but also the background conditions. A detailed
picture is given about the Coulomb explosion, with explanation of the tampering
mechanism. It is shown that a thin water layer is efficient in slowing down the
distortion of the atomic structure, but it gives a significant contribution to
the background
A molecular-dynamics approach for studying the non-equilibrium behavior of x-ray-heated solid-density matter
When matter is exposed to a high-intensity x-ray free-electron-laser pulse,
the x rays excite inner-shell electrons leading to the ionization of the
electrons through various atomic processes and creating high-energy-density
plasma, i.e., warm or hot dense matter. The resulting system consists of atoms
in various electronic configurations, thermalizing on sub-picosecond to
picosecond timescales after photoexcitation. We present a simulation study of
x-ray-heated solid-density matter. For this we use XMDYN, a Monte-Carlo
molecular-dynamics-based code with periodic boundary conditions, which allows
one to investigate non-equilibrium dynamics. XMDYN is capable of treating
systems containing light and heavy atomic species with full electronic
configuration space and 3D spatial inhomogeneity. For the validation of our
approach we compare for a model system the electron temperatures and the ion
charge-state distribution from XMDYN to results for the thermalized system
based on the average-atom model implemented in XATOM, an ab-initio x-ray atomic
physics toolkit extended to include a plasma environment. Further, we also
compare the average charge evolution of diamond with the predictions of a
Boltzmann continuum approach. We demonstrate that XMDYN results are in good
quantitative agreement with the above mentioned approaches, suggesting that the
current implementation of XMDYN is a viable approach to simulate the dynamics
of x-ray-driven non-equilibrium dynamics in solids. In order to illustrate the
potential of XMDYN for treating complex systems we present calculations on the
triiodo benzene derivative 5-amino-2,4,6-triiodoisophthalic acid (I3C), a
compound of relevance of biomolecular imaging, consisting of heavy and light
atomic species
Quantum-mechanical calculation of ionization potential lowering in dense plasmas
The charged environment within a dense plasma leads to the phenomenon of
ionization potential depression (IPD) for ions embedded in the plasma. Accurate
predictions of the IPD effect are of crucial importance for modeling atomic
processes occurring within dense plasmas. Several theoretical models have been
developed to describe the IPD effect, with frequently discrepant predictions.
Only recently, first experiments on IPD in Al plasma have been performed with
an x-ray free-electron laser (XFEL), where their results were found to be in
disagreement with the widely-used IPD model by Stewart and Pyatt. Another
experiment on Al, at the Orion laser, showed disagreement with the model by
Ecker and Kr\"oll. This controversy shows a strong need for a rigorous and
consistent theoretical approach to calculate the IPD effect. Here we propose
such an approach: a two-step Hartree-Fock-Slater model. With this
parameter-free model we can accurately and efficiently describe the
experimental Al data and validate the accuracy of standard IPD models. Our
model can be a useful tool for calculating atomic properties within dense
plasmas with wide-ranging applications to studies on warm dense matter, shock
experiments, planetary science, inertial confinement fusion and studies of
non-equilibrium plasmas created with XFELs.Comment: 13 pages, 9 figures, to be published in Phys. Rev. X; added
references [46,47
Incoherent x-ray scattering in single molecule imaging
Imaging of the structure of single proteins or other biomolecules with atomic
resolution would be enormously beneficial to structural biology. X-ray
free-electron lasers generate highly intense and ultrashort x-ray pulses,
providing a route towards imaging of single molecules with atomic resolution.
The information on molecular structure is encoded in the coherent x-ray
scattering signal. In contrast to crystallography there are no Bragg
reflections in single molecule imaging, which means the coherent scattering is
not enhanced. Consequently, a background signal from incoherent scattering
deteriorates the quality of the coherent scattering signal. This background
signal cannot be easily eliminated because the spectrum of incoherently
scattered photons cannot be resolved by usual scattering detectors. We present
an ab initio study of incoherent x-ray scattering from individual carbon atoms,
including the electronic radiation damage caused by a highly intense x-ray
pulse. We find that the coherent scattering pattern suffers from a significant
incoherent background signal at high resolution. For high x-ray fluence the
background signal becomes even dominating. Finally, based on the atomic
scattering patterns, we present an estimation for the average photon count in
single molecule imaging at high resolution. By varying the photon energy from
3.5 keV to 15 keV, we find that imaging at higher photon energies may improve
the coherent scattering signal quality
Effect of two-particle correlations on x-ray coherent diffractive imaging studies performed with continuum models
Coherent diffraction imaging (CDI) of single molecules at atomic resolution
is a major goal for the x-ray free electron lasers (XFELs). However, during an
imaging pulse, the fast laser-induced ionization may strongly affect the
recorded diffraction pattern of the irradiated sample. The radiation tolerance
of the imaged molecule should then be investigated 'a priori' with a dedicated
simulation tool. The continuum approach is a powerful tool for modeling the
evolution of irradiated large systems consisting of more than a few hundred
thousand atoms. However, this method follows the evolution of average
single-particle densities, and the experimentally recorded intensities reflect
the spatial two-particle correlations. The information on these correlations is
then inherently not accessible within the continuum approach. In this paper we
analyze this limitation of continuum models and discuss the applicability of
continuum models for imaging studies. We propose a formula to calculate
scattered intensities (including both elastic and inelastic scattering) from
the estimates obtained with a single-particle continuum model. We derive this
formula for systems under conditions typical for CDI studies with XFELs
XCALIB: a focal spot calibrator for intense X-ray free-electron laser pulses based on the charge state distributions of light atoms
We develop the XCALIB toolkit to calibrate the beam profile of an X-ray
free-electron laser (XFEL) at the focal spot based on the experimental charge
state distributions (CSDs) of light atoms. Accurate characterization of the
fluence distribution at the focal spot is essential to perform the volume
integrations of physical quantities for a quantitative comparison between
theoretical and experimental results, especially for fluence dependent
quantities. The use of the CSDs of light atoms is advantageous because CSDs
directly reflect experimental conditions at the focal spot, and the properties
of light atoms have been well established in both theory and experiment. To
obtain theoretical CSDs, we use XATOM, a toolkit to calculate atomic electronic
structure and to simulate ionization dynamics of atoms exposed to intense XFEL
pulses, which involves highly excited multiple core hole states. Employing a
simple function with a few parameters, the spatial profile of an XFEL beam is
determined by minimizing the difference between theoretical and experimental
results. We have implemented an optimization procedure employing the
reinforcement learning technique. The technique can automatize and organize
calibration procedures which, before, had been performed manually. XCALIB has
high flexibility, simultaneously combining different optimization methods, sets
of charge states, and a wide range of parameter space. Hence, in combination
with XATOM, XCALIB serves as a comprehensive tool to calibrate the fluence
profile of a tightly focused XFEL beam in the interaction region.Comment: 28 pages, 7 figure