951 research outputs found
Numerical simulations of X-rays Free Electron Lasers (XFEL)
We study a nonlinear Schr\"odinger equation which arises as an effective
single particle model in X-ray Free Electron Lasers (XFEL). This equation
appears as a first-principles model for the beam-matter interactions that would
take place in an XFEL molecular imaging experiment in \cite{frat1}. Since XFEL
is more powerful by several orders of magnitude than more conventional lasers,
the systematic investigation of many of the standard assumptions and
approximations has attracted increased attention.
In this model the electrons move under a rapidly oscillating electromagnetic
field, and the convergence of the problem to an effective time-averaged one is
examined. We use an operator splitting pseudo-spectral method to investigate
numerically the behaviour of the model versus its time-averaged version in
complex situations, namely the energy subcritical/mass supercritical case, and
in the presence of a periodic lattice.
We find the time averaged model to be an effective approximation, even close
to blowup, for fast enough oscillations of the external field. This work
extends previous analytical results for simpler cases \cite{xfel1}.Comment: 14 page
Stochastic stimulated electronic x-ray Raman spectroscopy
Resonant inelastic x-ray scattering (RIXS) is a well-established tool for
studying electronic, nuclear and collective dynamics of excited atoms,
molecules and solids. An extension of this powerful method to a time-resolved
probe technique at x-ray free electron lasers (XFELs) to ultimately unravel
ultrafast chemical and structural changes on a femtosecond time scale is often
challenging, due to the small signal rate in conventional implementations at
XFELs that rely on the usage of a monochromator set up to select a small
frequency band of the broadband, spectrally incoherent XFEL radiation. Here, we
suggest an alternative approach, based on stochastic spectroscopy, that uses
the full bandwidth of the incoming XFEL pulses. Our proposed method is relying
on stimulated resonant inelastic x-ray scattering, where in addition to a pump
pulse that resonantly excites the system a probe pulse on a specific electronic
inelastic transition is provided, that serves as seed in the stimulated
scattering process. The limited spectral coherence of the XFEL radiation
defines the energy resolution in this process and stimulated RIXS spectra of
high resolution can be obtained by covariance analysis of the transmitted
spectra. We present a detailed feasibility study and predict signal strengths
for realistic XFEL parameters for the CO molecule resonantly pumped at the
O1s-{\pi}* transition. Our theoretical model describes the evolution of the
spectral and temporal characteristics of the transmitted x-ray radiation, by
solving the equation of motion for the electronic and vibrational degrees of
freedom of the system self consistently with the propagation by Maxwell's
equations
Corrugated structure insertion for extending the SASE bandwidth up to 3% at the European XFEL
The usage of x-ray free electron laser (XFEL) in femtosecond
nanocrystallography involves sequential illumination of many small crystals of
arbitrary orientation. Hence a wide radiation bandwidth will be useful in order
to obtain and to index a larger number of Bragg peaks used for determination of
the crystal orientation. Considering the baseline configuration of the European
XFEL in Hamburg, and based on beam dynamics simulations, we demonstrate here
that the usage of corrugated structures allows for a considerable increase in
radiation bandwidth. Data collection with a 3% bandwidth, a few microjoule
radiation pulse energy, a few femtosecond pulse duration, and a photon energy
of 5.4 keV is possible. For this study we have developed an analytical modal
representation of the short-range wake function of the flat corrugated
structures for arbitrary offsets of the source and the witness particles.Comment: 29 pages, 17 figure
Superradiant Undulator Radiation for Selective THz Control Experiments at XFELs
The generation of frequency-tunable, narrow-bandwidth and
carrier-envelope-phase stable THz pulses with fields in the MV/cm regime that
can be appropriately timed to the femtosecond X-ray pulses from
free-electron-lasers is of highest scientific interest. It will enable to
follow the electronic and structural dynamics stimulated by (non)linear
selective excitations of matter on few femtosecond time and {\AA}ngstrom length
scales. In this article, a scheme based on superradiant undulator radiation
generated just after the XFEL is proposed. The concept utilizes cutting edge
superconducting undulator technology and provides THz pulses in a frequency
range between 3 and 30 THz with exceptional THz pulse energies. Relevant
aspects for realization and operation are discussed point by point on the
example of the European XFEL
Efficient electronic structure calculation for molecular ionization dynamics at high x-ray intensity
We present the implementation of an electronic-structure approach dedicated
to ionization dynamics of molecules interacting with x-ray free-electron laser
(XFEL) pulses. In our scheme, molecular orbitals for molecular core-hole states
are represented by linear combination of numerical atomic orbitals that are
solutions of corresponding atomic core-hole states. We demonstrate that our
scheme efficiently calculates all possible multiple-hole configurations of
molecules formed during XFEL pulses. The present method is suitable to
investigate x-ray multiphoton multiple ionization dynamics and accompanying
nuclear dynamics, providing essential information on the chemical dynamics
relevant for high-intensity x-ray imaging.Comment: 28 pages, 6 figure
A split-beam probe-pump-probe scheme for femtosecond time resolved protein X-ray crystallography
In order to exploit the femtosecond pulse duration of X-ray Free-Electron Lasers (XFEL) operating in the hard X-ray regime for ultrafast time-resolved protein crystallography experiments, critical parameters that determine the crystallographic signal-to-noise (I/ĻI) must be addressed. For single-crystal studies under low absorbed dose conditions, it has been shown that the intrinsic pulse intensity stability as well as mode structure and jitter of this structure, significantly affect the crystallographic signal-to-noise. Here, geometrical parameters are theoretically explored for a three-beam scheme: X-ray probe, optical pump, X-ray probe (or āprobe-pump-probeā) which will allow experimental determination of the photo-induced structure factor amplitude differences, ĪF, in a ratiometric manner, thereby internally referencing the intensity noise of the XFEL source. In addition to a non-collinear split-beam geometry which separates un-pumped and pumped diffraction patterns on an area detector, applying an additional convergence angle to both beams by focusing leads to integration over mosaic blocks in the case of well-ordered stationary protein crystals. Ray-tracing X-ray diffraction simulations are performed for an example using photoactive yellow protein crystals in order to explore the geometrical design parameters which would be needed. The specifications for an X-ray split and delay instrument that implements both an offset angle and focused beams are discussed, for implementation of a probe-pump-probe scheme at the European XFEL. We discuss possible extension of single crystal studies to serial femtosecond crystallography, particularly in view of the expected X-ray damage and ablation due to the first probe pulse
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