XUV pulses at 26.2 nm wavelength were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort XUV pulses and cross correlated by ultrashort optical laser pulses. This scheme enabled for the first time the measurement of a phase transition on a timescale of ~150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that ultrashort XUV pulses can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.
Modern free-electron lasers produce femtosecond X-ray pulses sufficiently intense to induce modifications of material properties. Energetic photoelectrons created upon the X-ray absorption and Auger electrons emitted after relaxation of core-hole states trigger secondary electron cascades, which increase the transient electron density. In this talk, we present and apply the XCASCADE(3D) code , based on classical event-by-event individual-particle Monte-Carlo approach, which is valid for relatively low absorbed fluences. The code takes into account photoabsorption, electron impact ionization, electron elastic scattering, and Auger decays of core holes. It uses atomistic cross sections and atomistic ionization potentials. Such approximation is justified for high-energy X-ray absorption and for high-energy electron-atom collisions, since they both excite electrons from the deeply lying core levels. At the same time, it makes the approach very efficient and flexible, since the corresponding cross sections can be found in the literature for many elements. In this approximation, one can treat any practical material assuming that the total cross section is the sum of cross sections for each individual atom. Elastic and inelastic anisotropic scatterings of photo- and secondary electrons on atoms are taken into account. The validity of the model is confirmed by comparing the calculated electron ranges with published data for silicon and gold.
We apply the code to study the consequences of the electron cascading in ruthenium (Ru). Ru, which is a promising material for coating of X-ray mirrors , is simulated at different incoming photon energies, from extreme ultraviolet (XUV) to hard x-ray, in a grazing incidence geometry. The generated data then serve as an input to Two-Temperature Model, enabling analysis of the temperature evolution and damage thresholds .
According to the simulations, much larger electron ranges for hard x-rays case result in spread of absorbed laser energy into a larger volume compared to the XUV case and, consequently, into smaller temperature gradients. The latter is the source of thermo-induced stresses that eventually lead to material damage. As a consequence of the electron transport, one should expect higher damage thresholds with increasing energy of incident photons, even for the same photon penetration depth, which can be achieved by adjusting the incident angle.
 V. Lipp, N. Medvedev, B. Ziaja, Proc. SPIE 10236, 102360H (2017).
 Igor Milov, Igor A. Makhotkin, Ryszard Sobierajski, Nikita Medvedev, Vladimir Lipp et al., Opt. Express 26, 19665-19685 (2018).
We employ start–to-end simulations to model coherent diffractive imaging of single biomolecules using x-ray free electron lasers. This technique is expected to yield new structural information about biologically relevant macromolecules thanks to the ability to study the isolated sample in its natural environment as opposed to crystallized or cryogenic samples. The effect of the solvent on the diffraction pattern and interpretability of the data is an open question. We present first results of calculations where the solvent is taken into account explicitly. They were performed with a molecular dynamics scheme for a sample consisting of a protein and a hydration layer of varying thickness. Through R–factor analysis of the simulated diffraction patterns from hydrated samples, we show that the scattering background from realistic hydration layers of up to 3 Å thickness presents no obstacle for the resolution of molecular structures at the sub–nm level.
Simulations of experiments at modern light sources, such as optical laser laboratories, synchrotrons, and
free electron lasers, become increasingly important for the successful preparation, execution, and analysis of these
experiments investigating ever more complex physical systems, e.g. biomolecules, complex materials, and ultra–short
lived states of matter at extreme conditions. We have implemented a platform for complete start–to–end simulations
of various types of photon science experiments, tracking the radiation from the source through the beam transport
optics to the sample or target under investigation, its interaction with and scattering from the sample, and registration
in a photon detector. This tool allows researchers and facility operators to simulate their experiments and instruments
under real life conditions, identify promising and unattainable regions of the parameter space and ultimately make
better use of valuable beamtime. In this paper, we present an overview about status and future development of the
simulation platform and discuss three applications: 1.) Single–particle imaging of biomolecules using x–ray free
electron lasers and optimization of x–ray pulse properties, 2.) x–ray scattering diagnostics of hot dense plasmas in
high power laser–matter interaction and identification of plasma instabilities, and 3.) x–ray absorption spectroscopy
in warm dense matter created by high energy laser–matter interaction and pulse shape optimization for low–isentrope
In this paper, we present an extension to our code, XCASCADE [Medvedev, Appl. Phys. B 118, 417], that enables to model time evolution of electron cascades following low-intensity X-ray excitation in various materials consisting of elements with atomic numbers Z = 1 - 92. The code is based on a classical Monte-Carlo scheme and uses atomistic cross sections to describe electron impact ionization. The new extended version, XCascade-3D, also tracks the electron trajectories with 3D spatial resolution. This model takes into account anisotropic scattering of electrons on atoms. We show that the calculated electron ranges in various materials are in a good agreement with the available data, confirming the potential for high accuracy applications at FEL pulse diagnostics.
Silicon irradiated with an ultrashort laser pulse can experience two competing damage processes: the ultrafast ’nonthermal melting’ or the picoseconds ‘thermal melting’. The first one occurs if the density of excited electrons within the conduction band overcomes a certain threshold value, which leads to modification of the atomic potential energy surface and triggers a phase transition. The second one heats a material due to the electron-ion (electron-phonon) coupling, which in case of atomic temperature exceeding melting temperature also induces a phase transition. Our recently developed code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, Phys. Rev. B 91 (2015) 054113), can model both effects simultaneously. Nonadiabatic electron-ion coupling is treated within tight binding molecular dynamics model beyond the Born-Oppenheimer approximation. Two different channels of phase transition emerge at different irradiation dose: thermal melting of silicon into low-density-liquid phase occurs for deposited energies above ~0.65 eV/atom; nonthermal melting into high-density liquid takes place for doses higher than ~0.9 eV/atom. Here we discuss in detail electronic processes during such phase transitions. Evolution of the electronic structure is presented.
When a semiconductor or a dielectric is irradiated with ultrashort intense X-ray pulse, several processes occur: first the photoabsorption brings the electron subsystem out of equilibrium, bringing valence or deeper shells electrons into high energy states of the conduction band. Then, secondary electron cascading promotes further electrons of the valence to conduction band increasing their number there. These electrons also influence the atomic motion, modifying the interatomic forces. This process is known as a nonthermal melting. It can turn a material into a new phase state on ultrashort timescales. Recently developed hybrid model for treating all of these processes with different computational tools was reported in [N. Medvedev et al, New J. Phys. 15, 015016 (2013)]. Based on this model, we present here further investigations of nonthermal processes occurring in diamond under irradiation with a FLASH pulse of 10 fs FWHM and 92 eV photon energy. It is shown that the diamond turns into graphite under such irradiation, independently whether constant pressure or constant volume modeling is performed. However, for the latter case, the time of the nonthermal phase transition is longer (few tens of fs for P=const vs few hundreds of fs for V=const) and the damage threshold is slightly higher (0.69 eV/atom vs 0.74 eV/atom, correspondingly).
Kinetic Boltzmann equations are used to model the ionization and expansion dynamics of xenon clusters irradiated with short, intense VUV pulses from free-electron-laser (FEL). This unified model
includes all of the predominant interactions that contribute to the cluster dynamics induced by this radiation. The dependence of the evolution dynamics on cluster size and pulse fluence is investigated.
It is found that the highly charged ions observed in the experiments are mainly due to Coulomb explosion of the outer shell of the cluster while ions formed in the interior of the cluster predominantly
recombine with plasma electrons. As a result, a large fraction of neutral atoms is formed within the core, the proportion depending on the cluster size. The predictions of ion charge distribution,
average ion charge and average energy absorbed per ion made with our model are found to be in good agreement with the experimental data. To our knowledge, our model is the first and only one
that gives a full and quantitatively accurate description of all of the experimental data collected from irradiated atomic clusters at 100 nm photon wavelength.
We have analyzed the evolution and the interaction dynamics of secondary cascade electrons generated by a single Auger electron in diamond and in amorphous carbon. The elastic mean free path was calculated as a function of impact energy, using the muffin-tin potential approximation, while the differential mean free path and the inelastic mean free path were estimated from two different optical models as a function of the impact energy. A Monte-Carlo model for describing the time evolution of the cascade was constructed, and numerical simulations were performed. The results show that the maximal average ionization rate caused by a single Auger electron corresponds to about $20$ to $40$ ionization events in a macroscopic sample. These electrons are liberated within 100 fs, following the Auger emission.