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Laser and particle beams 37(2), 235 - 241 (2019). doi:10.1017/S0263034619000326, Modern free-electron lasers (FEL) operating in XUV (extreme ultraviolet) or X-ray range allow an access to novel research areas. An example is the ultrafast ionization of a solid by an intense femtosecond FEL pulse in XUV which consequently leads to a change of the complex index of refraction on an ultrashort timescale. The photoionization and subsequent impact ionization resulting in electronic and atomic dynamics are modeled with our hybrid code XTANT(X-ray thermal and non-thermal transitions) and a Monte Carlo code XCASCADE(X-ray-induced electron cascades). The simulations predict the temporal kinetics of FEL-induced electron cascades and thus yield temporally and spatially resolved information on the induced changes of the optical properties. In a series of experiments at FERMI and LCLS, single shot measurements with spatio-temporal encoding of the ionization process have been performed by a correlation of the FEL pump pulse with an optical femtosecond probe pulse. An excellent agreement between the experiment and the simulation has been found. We also show that such kind of experiments forms the basis for pulse duration and arrival time jitter monitoring as currently under development for XUV-FELs, Published by Cambridge Univ. Press, Cambridge

A swift heavy ion leaves a damaged trail (SHI track) while penetrating through a solid. The radius of the track depends on the ion energy loss and velocity, and thereby on its penetration depth. Applying developed Monte Carlo code TREKIS and molecular dynamics approach we describe the damage distribution along the trajectory of an SHI (Xe and U) in olivine. This enables us to obtain the dependencies of the track radius and structure on the ion energy and its energy losses, as well as to identify the track formation thresholds in forsterite (Mg2SiO4) for fast and slow ions. It is found that the position of the maximal damage produced by an ion in a solid does not coincide with the Bragg peak of its electronic stopping. Loop-like dependence of the track radius on the electronic energy loss is identified.

We experimentally discovered that Al2O3 and MgO exhibit well-pronounced nanometric modifications on the surfaces when irradiated under grazing incidence with 23 MeV I beam, in contrast to normal incidence irradiation with the same ion beam when no damage was found. Moreover, ions in these two materials produce notably different structures: grooves surrounded with nanohillocks on MgO surfaces vs. smoother, roll-like discontinuous structures on the surfaces of Al2O3. To explain these results, detailed numerical simulations were performed. We identified that a presence of the surface inhibits recrystallization process, thereby preventing transient tracks from recovery, and thus forming observable nanopatterns. Furthermore, a difference in the viscosities in molten states in Al2O3 vs. MgO explains the differences in the created nanostructures. Our results thus provide a deeper understanding of the fundamental processes of surface nanostructuring, potentially allowing for controlled production of periodic surface nanopatterns.

We recently developed an approach for calculation of the electron-phonon (electron-ion in a more general case) coupling in materials based on tight-binding molecular dynamics simulations. In the present work we utilize this approach to study partial contributions of inter- and intraband electron scattering events into total electron-phonon coupling in Al, Au, Cu elemental metals and in AlCu alloy. We demonstrate that the interband scattering plays an important role in electron-ion energy exchange process in Al and AlCu, whereas intraband $d-d$ transitions are dominant in Au and Cu. Moreover, inter- and intraband transitions exhibit qualitatively different dependencies on the electron temperature. Our findings should be taken into account for interpretation of experimental results on electron-phonon coupling parameter., Comment: To be submitted

The European physical journal / D 75, 224 (2021). doi:10.1140/epjd/s10053-021-00235-z, In this paper, we report on the Boltzmann kinetic equation approach adapted for simulations of warm dense matter created by irradiation of bulk gold with intense ultrashort X-ray pulses. X-rays can excite inner-shell electrons, which triggers creation of deep-lying core holes. Their relaxation, especially in heavier elements such as gold (atomic number Z=79) takes complicated pathways, involving collisional processes, and leading through a large number of active configurations. This number can be so high that solving a set of evolution equations for each configuration becomes computationally inefficient, and another modeling approach should be used instead. Here, we use the earlier introduced ���predominant excitation and relaxation path��� approach. It still uses true atomic configurations but limits their number by restricting material relaxation to a selected set of predominant pathways for material excitation and relaxation. With that, we obtain time-resolved predictions for excitation and relaxation in X-ray irradiated bulk of gold, including the respective change of gold optical properties. We compare the predictions with the available data from high-energy-density experiments. Their good agreement indicates ability of the Boltzmann kinetic equation approach to describe warm dense matter created from high-Z materials after their irradiation with X rays, which can be validated in future experiments., Published by Springer, Heidelberg

This paper investigates an effect of a target temperature on lattice excitation by the relaxing electron ensemble in a swift heavy ion track. A new version of the Monte-Carlo code TREKIS is used for this purpose. Cross sections implemented in TREKIS are constructed within the dynamic structure factor/complex dielectric function (DSF-CDF) formalism. This enables us to take into account unusual pathways of collective responses of the electronic and ionic subsystems to their femtosecond excitations in SHI tracks. The paper demonstrates that the temperature dependent fundamental asymmetry of the lattice DSF manifests itself in different kinetics of energy exchange between the electronic and atomic systems of alumina in SHI tracks at different temperatures (80 K vs. 300 K).

Soft x-rays 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 soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. 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 soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale., Comment: 27 pages, 17 figures (includes supplementary materials)

Parameters of point defects halos (F-color centers) created due to decays of self-trapped valence holes generated in nanometric vicinities of trajectories of gold ions of 275 MeV and 2187 MeV in LiF are estimated in absorption spectroscopy experiments. Such ions have approximately the same electronic stopping: 24.6 keV/nm and 22.9 keV/nm, respectively. In contrast to the usual concept of the velocity effect that a slower ion produces larger structure changes due to a higher density of the deposited energy, the opposite effect occurs for the defect halo revealing a larger radius and a larger defect concentration for an ion of the higher velocity realizing the same energy loss. Spatial spreading of generated valence holes before their self-trapping (500 fs) forms the size of the defect halos around the trajectories of the applied ions. Simulations with Monte-Carlo code TREKIS show no significant difference in the initial spatial distributions of these valence holes by the times of finishing of ionization cascades (∼10 fs after the projectile passage) within the radii of the defect halos deduced from the experiments. Using these distributions as initial conditions for spatial spreading of generated valence holes and taking into account the difference between the defect halo radii, the diffusion coefficients of these holes near the trajectories of 275 and 2187 MeV Au ions in LiF are estimated showing about six times larger value in tracks of the faster ion for irradiations at room temperatures. Presence of H-color centers changes considerably the kinetics of the created defect ensemble in the defect halo resulting in differences between the defect halo parameters in LiF crystals irradiated at 8 K vs. 300 K.

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 [1], 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 [2], 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 [2]. 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. [1] V. Lipp, N. Medvedev, B. Ziaja, Proc. SPIE 10236, 102360H (2017). [2] Igor Milov, Igor A. Makhotkin, Ryszard Sobierajski, Nikita Medvedev, Vladimir Lipp et al., Opt. Express 26, 19665-19685 (2018).

This contribution discusses challenges in the modeling of formation of the warm dense matter (WDM) state in solids exposed to femtosecond X-ray free-electron laser pulses. It is based upon our previously reported code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, 4open 1, 3, 2018), which combines tight-binding (TB) molecular dynamics for atoms with Monte Carlo modeling of high-energy electrons and core-holes, and Boltzmann collision integrals for nonadiabatic electron-ion coupling. The current version of the code, XTANT-3, includes LCAO basis sets sp3, sp3s*, and sp3d5, and can operate with both orthogonal and nonorthogonal Hamiltonians. It includes the TB parameterizations by Goodwin et al., a transferrable version of Vogl's et al. TB, NRL, and DFTB. Considering that other modules of the code are applicable to any chemical element, this makes XTANT-3 capable of treating a large variety of materials. In order to extend it to the WDM regime, a few limitations that must be overcome are discussed here: short-range repulsion potential must be sufficiently strong; basis sets must span large enough energy space within the conduction band; dependence of the electronic scattering cross sections on the electronic and atomic temperatures and structure needs to be considered. Directions at solving these issues are outlined in this proceeding., To be submitted as conference proceeding to Proc. SPIE

The event-by-event Monte Carlo code, TREKIS, was recently developed to describe excitation of the electron subsystems of solids in the nanometric vicinity of a trajectory of a nonrelativistic swift heavy ion (SHI) decelerated in the electronic stopping regime. The complex dielectric function (CDF) formalism was applied in the used cross sections to account for collective response of a matter to excitation. Using this model we investigate effects of the basic assumptions on the modeled kinetics of the electronic subsystem which ultimately determine parameters of an excited material in an SHI track. In particular, (a) effects of different momentum dependencies of the CDF on scattering of projectiles on the electron subsystem are investigated. The ‘effective one-band’ approximation for target electrons produces good coincidence of the calculated electron mean free paths with those obtained in experiments in metals. (b) Effects of collective response of a lattice appeared to dominate in randomization of electron motion. We study how sensitive these effects are to the target temperature. We also compare results of applications of different model forms of (quasi-) elastic cross sections in simulations of the ion track kinetics, e.g. those calculated taking into account optical phonons in the CDF form vs. Mott’s atomic cross sections. (c) It is demonstrated that the kinetics of valence holes significantly affects redistribution of the excess electronic energy in the vicinity of an SHI trajectory as well as its conversion into lattice excitation in dielectrics and semiconductors. (d) It is also shown that induced transport of photons originated from radiative decay of core holes brings the excess energy faster and farther away from the track core, however, the amount of this energy is relatively small.

Applied physics letters 113(11), 114102 (2018). doi:10.1063/1.5046070, Recent studies confirmed that the materials used in the extreme UV and soft X-ray regime for precise characterization of intense free-electron laser pulses (e.g., Si$_3$N$_4$) do not work efficiently in the hard X-ray regime, which is due to the fact that the impact of a hard X-ray photon is followed by a series of electron cascading processes. Following theoretical indication, we show that this limitation can be circumvented and the cascading time can be significantly reduced if the X-ray photon energy is double the ionization energy. We investigate an alternative material for pulse diagnostics, SnO$_2$, using the Linac Coherent Light Source at photon energies of 5 keV and 9 keV. We prove the validity of the concept and show that it has a large potential for practical applications. By applying the proposed criteria, the temporal accuracy of the non-invasive pulse diagnostic tools can be improved in current and emerging hard X-ray facilities., Published by American Inst. of Physics, Melville, NY

A considerable part of the excess energy of the electronic subsystem of a solid penetrated by a swift heavy ion (SHI) is accumulated in valence holes. Spatial redistribution of these holes can affect subsequent relaxation, resulting in ionizations of new electrons by hole impacts as well as energy transfer to the target lattice. A new version of the Monte Carlo code TREKIS is applied to study this effect in Al2O3 for SHI tracks. The complex dielectric function (CDF) formalism is used to calculate the cross sections of interaction of involved charged particles (an ion, electrons, holes) with the target giving us ability to take into account collective response of a target to excitations. We compare the radial distributions of the densities and energies of excited electrons and valence holes at different times to those obtained under the assumption of immobile holes used in earlier works. The comparison shows a significant difference between these distributions within the track core, where the majority of slow electrons and valence holes are located at femtosecond timescales after the ion impact. The study demonstrates that the energy deposited by valence holes into the lattice in nanometric tracks is comparable to the energy transferred by excited electrons. Radii of structure transformations in tracks produced by these energy exchange channels are in a good agreement with experiments.

Biological samples are highly radiation sensitive. The rapid progress of their radiation damage prevents accurate structure determination of single macromolecular assemblies in standard diffraction experiments. However, computer simulations of the damage formation have shown that the radiation tolerance might be extended at very high intensities with ultrafast imaging such as is possible with the presently developed and operating x-ray free-electron lasers. Recent experiments with free-electron lasers on nanocrystals have demonstrated proof of the imaging principle at resolutions down to 1:6 Angstroms. However, there are still many physical and technical problems to be clarified on the way to imaging of single biomolecules at atomic resolution. In particular, theoretical simulations try to address an important question: How does the radiation damage progressing within an imaged single object limit the structural information about this object recorded in its diffraction image during a 3D imaging experiment? This information is crucial for adjusting pulse parameters during imaging so that high-resolution diffraction patterns can be obtained. Further, dynamics simulations should be used to verify the accuracy of the structure reconstruction performed from the experimental data. This is an important issue as the experimentally recorded diffraction signal is recorded from radiation-damaged samples. It also contains various kinds of background. In contrast, the currently used reconstruction algorithms assume perfectly coherent scattering patterns with shot noise only. In this review paper, we discuss the most important processes and effects relevant for imaging-related simulations that are not yet fully understood, or omitted in the irradiation description. We give estimates for their contribution to the overall radiation damage. In this way we can identify unsolved issues and challenges for simulations of x-ray irradiated single molecules relevant for imaging studies. They should be addressed during further development of these simulation tools.

SPIE Optics + Optoelectronics, Prague, Czech Republic; Proceedings of SPIE 10239, 102360H (2017). doi:10.1117/12.2267939, 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., Published by SPIE, Bellingham, Wash.

High energy density physics 24, 15 - 21 (2017). doi:10.1016/j.hedp.2017.05.012, In this paper we present the current status of our theoretical studies on ultrafast relaxation of X-ray/XUV excited gallium arsenide. First, we discuss our previous approach, the unified model based on rate equations, two-temperature model and the extended Drude approach. By fitting the model to the available experimental data, we obtained realistic estimates on transient electronic temperature and electron-lattice thermalization timescale. Next, we make a step towards a rigorous description of the relaxation process with our hybrid code, XTANT. We extend the XTANT to include the band-specific effect of the suppression of collisional processes in GaAs, and perform dedicated simulations. We find that the extended model correctly describes the predicted transient non-isothermality of conduction and valence bands, however, currently, it cannot reproduce the experimentally observed reflectivity overshooting at 5 - 10 ps. The reason for this discrepancy is that the electron-phonon coupling rate implemented in XTANT, although successfully applied for diamond and silicon, clearly underestimates the strength of this coupling in GaAs. The outline for a respective model improvement is discussed., Published by Elsevier, Amsterdam [u.a.]

Physical review / B covering condensed matter and materials physics 95(1), 014309(2017). doi:10.1103/PhysRevB.95.014309, In the present work, a theoretical study of electron-phonon (electron-ion) coupling rates in semiconductors driven out of equilibrium is performed. Transient change of optical coefficients reflects the band gap shrinkage in covalently bonded materials, and thus, the heating of atomic lattice. Utilizing this dependence, we test various models of electron-ion coupling. The simulation technique is based on tight-binding molecular dynamics. Our simulations with the dedicated hybrid approach (XTANT) indicate that the widely used Fermi's golden rule can break down describing material excitation on femtosecond time scales. In contrast, dynamical coupling proposed in this work yields a reasonably good agreement of simulation results with available experimental data., Published by Inst., Woodbury, NY

SPIE Optics + Optoelectronics, Prague, Czech Republic; Proceedings of SPIE 10236, 102360I (2017). doi:10.1117/12.2266953, In this contribution an analysis of influence of model parameters on the results of simulations of material properties under free-electron laser irradiation is presented. It is based on the in-house hybrid code XTANT (X-ray-induced Thermal And Nonthermal Transition; N. Medvedev et. al, Phys. Rev. B 91 (2015) 054113), which combines tight binding molecular dynamics for atoms with Monte Carlo treatment of high-energy electrons and core-holes, and Boltzmann collision integrals for nonadiabatic (electron-phonon) coupling. Different parameterizations of transferable tight binding method for silicon are analyzed, namely basis sets sp$^3$ and sp$^3$s*. The sp$^3$ parameterization seems to provide a better agreement of the silicon damage threshold with experimental data. Further, the influence of different schemes of molecular dynamics periodic boundaries simulation is compared: constant volume vs Parrinello-Rahman constant pressure. Constant-volume scheme gives a better agreement with experimental transient properties, as could be expected. Parameters entering the calculations of optical properties are analyzed, showing virtually no effect on the outcome beyond trivial broadening of the peaks of the optical coefficients., Published by SPIE, Bellingham, Wash.

The presented Monte Carlo model simulates excitation of the electron subsystem of semiconductors by a penetrating swift heavy ion (SHI). The cross sections of interaction of an ion with the electron subsystem of a target are calculated via the complex dielectric function formalism, which accounts for all the collective modes of the electron ensemble of the target. The predicted electron inelastic mean free paths are in a very good agreement with those from the NIST database. The calculated SHI energy losses coincide well with SRIM and CasP codes. The model is used to calculate the spectra of electrons emitted from germanium and silicon targets during SHI irradiation. These spectra agree well with the experimental data.

The Monte-Carlo (MC) model simulating the femto-second kinetics of the electron subsystem in a track of a swift heavy ion decelerated in the electronic stopping regime is developed. The complex dielectric function (CDF) formalism is used to calculate the cross sections of interactions of an ion and fast electrons with the electron subsystem of a target. It accounts for all collective modes in the electron ensemble. The applied method of CDF reconstruction from the experimental optical data provided a very good agreement of the calculated electron inelastic mean free paths with the NIST database as well as of the calculated SHI energy loss with those from SRIM and CasP codes. The MC model was applied to determine the radial distributions of delocalized electrons and their energy density in tracks of Au (2187 MeV) ions in insulators (Al 2 O 3 and polyethylene) at different times. The femtosecond electron kinetics reveals two fronts of the spatial propagation: the primary fast delta-electrons form the front of excitations while electrons appearing due to decay of plasmons generated in a track form the second slow front following behind.

A multiscale model was developed to describe excitation and initial relaxation of an insulator after an impact of a swift heavy ion (SHI) decelerated in the electronic stopping regime. This model consists of a combination of three methods: (a) Monte Carlo simulations of the nonequilibrium kinetics of the electron subsystem of a solid at the femtosecond scale after the projectile passage. The complex dielectric function (CDF) is used to construct the cross sections for the MC model taking into account a collective response of the electron ensemble to excitation. (b) A molecular-kinetic approach describing further spatial spreading of electrons after finishing of ionization cascades up to hundred femtoseconds. And (c) molecular dynamics simulations of a reaction of the lattice on the excess energy transferred from the relaxing electron subsystem at the picosecond time scale. The dynamic structure factor (DSF) formalism is used to calculate the electron-lattice energy exchange in a general way which is valid for sub-picosecond timescales, beyond the phononic approximation of the lattice dynamics. The calculations were performed for 2 GeV Au ion in olivine, demonstrating a heating of the lattice up to 700 K in the nanometric scale picoseconds after the projectile passage.

Nuclear instruments & methods in physics research / B 315, 173 - 178 (2013). doi:10.1016/j.nimb.2013.04.082, Published by Elsevier, Amsterdam [u.a.]

Complex dielectric function formalism is applied to obtain the cross-sections, mean free path of electrons, and energy losses of swift heavy ions (SHI) in solid LiF and Y 2 O 3 out of the experimentally known loss function. The calculated electron inelastic mean free paths in these materials agree very well with the NIST database; the inelastic energy losses of swift Pb and Au ions agree well with those calculated with the widely used SRIM and CasP codes. The obtained cross-sections are used in Monte Carlo simulations of the electronic kinetics after SHI impacts. The radial distributions of the electron and valence hole densities as well as their energy densities were calculated.

In this review we discuss four examples of environment-dependent processes triggered by free-electron laser radiation within systems of various complexity. Those are: (i) non-thermal structural changes within irradiated solids, (ii) inverse bremsstrahlung during VUV irradiation of atomic clusters, (iii) shifts of ionic potentials due to a charged ion environment within a plasma, and (iv) fast thermalization of the electrons created after FEL irradiation. We show that FEL excitation enables access to a unique class of environment-influenced processes, depending on the radiation wavelength, pulse fluence and the structure of sample.

We report on the kinetic Boltzmann approach adapted for simulations of highly ionized matter created from a solid by its x-ray irradiation. X rays can excite inner-shell electrons, which leads to the creation of deeply lying core holes. Their relaxation, especially in heavier elements, can take complicated paths, leading to a large number of active configurations. Their number can be so large that solving the set of respective evolution equations becomes computationally inefficient and another modeling approach should be used instead. To circumvent this complexity, the commonly used continuum models employ a superconfiguration scheme. Here, we propose an alternative approach which still uses ``true'' atomic configurations but limits their number by restricting the sample relaxation to the predominant relaxation paths. We test its reliability, performing respective calculations for a bulk material consisting of light atoms and comparing the results with a full calculation including all relaxation paths. Prospective application for heavy elements is discussed.

We present the results of Monte-Carlo simulations of excitation and subsequent relaxation of the electronic subsystem of quartz in the close vicinity of the swift heavy ion (SHI) trajectory. Comparison between numerical results and experiments detecting X-ray K-shell emission of silicon atoms (KαL n transitions) from quartz aerogel irradiated with SHIs demonstrated that the adequate description of the relaxation kinetics of the excited electronic subsystem of a solid in a SHI track cannot be possible without taking into account fast inter-atomic Auger processes which are absent in gaseous targets.

Characterization of a free-electron laser (FEL) pulse can be done with a pump–probe scheme, using an FEL pump and a visible light probe on an optically transparent solid-state target. With such experimental scheme, pulse duration can be monitored on a shot-to-shot basis. It relies on the changes in optical properties induced by the FEL excitation of electrons. Here we analyze effects of different cross sections used in the modeling of electron kinetics. XCASCADE, a Monte Carlo toolkit for modeling x-ray-induced electron cascades (N. Medvedev, Appl. Phys. B 118 (2015) 417), is used for this purpose. Two different cross sections are compared: atomic BEB model vs complex-dielectric function formalism that accounts for collective effects in solids. It is shown that for photon and electron energies above a few tens of eV, the both models coincide very closely. For lower energies in the VUV regime, the difference in the cross sections become more significant, nevertheless producing qualitatively similar electron kinetics and increase in the density of excited electrons.

Monte Carlo code TREKIS is applied to trace kinetics of excitation of the electron subsystem of ZnO and MgO after an impact of a swift heavy ion (SHI). The event-by-event simulations describe excitation of the electron subsystems by a penetrating SHI, spatial spreading of generated electrons and secondary electron cascades. Application of the complex dielectric function (CDF) formalism for calculation of the cross sections of charged particle interaction with a solid target allows to consider collective response of the target to perturbation, which arises from the spatial and temporal correlations in the target electrons ensemble. The method of CDF reconstruction from the experimental optical data is applied. Electron inelastic mean free paths calculated within the CDF formalism are in very good agreement with NIST database. SHI energy losses agree well with those from SRIM and CasP codes. The radial distributions of valence holes, core holes and delocalized electrons as well as their energy densities in SHI tracks are calculated. The analysis of these distributions is presented.

In-situ spectroscopy is applied to analyze the spatial distribution of point defects ($F$-color centers) formed in the nanometric vicinities of the trajectories of different swift heavy ions (from C to U) decelerated in LiF crystals in the electronic stopping regime. The formation of these defects results from the decay of self-trapped excitons appearing due to self-trapping and further relaxation of valence holes generated during beam induced ionization processes. A Monte Carlo model is applied to estimate the initial radial distribution of valence holes in the ion tracks by the time the ionization cascades are finished. It is demonstrated that there exists a large difference between this and the radial distribution of point defects detected in the experiments. This difference is described by fast and long-range diffusion of valence holes before their self-trapping. The effect illustrates the fundamental importance of the relaxation kinetics of the valence band on structure transformations in tracks. The average diffusion coefficient of holes ${D}_{h}$ and their conversion efficiencies into color centers \ensuremath{\beta} in relaxing tracks of different ions in LiF are estimated (${D}_{h}$ = 0.016--1.4 cm${}^{2}$/s, \ensuremath{\beta} = 0.01--0.04).

International audience; Recently, few-femtosecond pulses have become available at hard X-ray free-electron lasers. Coupled with the available sub-10 fs optical pulses, investigations into few-femtosecond dynamics are not far off. However, achieving sufficient synchronization between optical lasers and X-ray pulses continues to be challenging. We report a 'measure-and-sort' approach, which achieves sub-10 fs root-mean-squared (r.m.s.) error measurement at hard X-ray FELs, far beyond the 100-200 fs r.m.s. jitter limitations. This timing diagnostic, now routinely available at the Linac Coherent Light Source (LCLS), is based on ultrafast free-carrier generation in optically transparent materials. Correlation between two independent measurements enables unambiguous demonstration of ~6 fs r.m.s. error in reporting the optical/X-ray delay, with single shot error suggesting the possibility of reaching few-femtosecond resolution.

Part of Focus on High Energy Density Physics. In this paper, we present a novel theoretical approach, which allows the study of nonequilibrium dynamics of both electrons and atoms/ions within free-electron laser excited semiconductors at femtosecond time scales. The approach consists of the Monte-Carlo method treating photoabsorption, high-energy-electron and core-hole kinetics and relaxation processes. Low-energy electrons localized within the valence and conduction bands of the target are treated with a temperature equation, including source terms, defined by the exchange of energy and particles with high-energy electrons and atoms. We follow the atomic motion with the molecular dynamics method on the changing potential energy surface. The changes of the potential energy surface and of the electron band structure are calculated at each time step with the help of the tight-binding method. Such a combination of methods enables investigation of nonequilibrium structural changes within materials under extreme ultraviolet (XUV) femtosecond irradiation. Our analysis performed for diamond irradiated with an XUV femtosecond laser pulse predicts for the first time in this wavelength regime the nonthermal phase transition from diamond to graphite. Similar to the case of visible light irradiation, this transition takes place within a few tens of femtoseconds and is caused by changes of the interatomic potential induced by ultrafast electronic excitations. It thus occurs well before the heating stimulated by electron–phonon coupling starts to play a role. This allows us to conclude that this transition is nonthermal and represents a general mechanism of the response of solids to ultrafast electron excitations.

We present results of single-shot resonant magnetic scattering experiments of $\mathrm{Co}/\mathrm{Pt}$ multilayer systems using 100 fs long ultraintense pulses from an extreme ultraviolet (XUV) free-electron laser. An x-ray-induced breakdown of the resonant magnetic scattering channel during the pulse duration is observed at fluences of $5\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$. Simultaneously, the speckle contrast of the high-fluence scattering pattern is significantly reduced. We performed simulations of the nonequilibrium evolution of the $\mathrm{Co}/\mathrm{Pt}$ multilayer system during the XUV pulse duration. We find that the electronic state of the sample is strongly perturbed during the first few femtoseconds of exposure leading to an ultrafast quenching of the resonant magnetic scattering mechanism.

Nature Communications 4, 1731 (2013). doi:10.1038/ncomms2754, The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations., Published by Nature Publishing Group, London

Effects of applications of two different models of atomic ionizations in MC simulations describing excitation of the electronic subsystem of an insulator in tracks of swift heavy ions decelerated in the electronic stopping regime are investigated. The first mechanism is based on binary collisions of a fast projectile with atomic electrons considered as independent. The second one assumes redistribution of the energy transferred to an atom from an ion between the atomic electrons before autoionization of the excited atom. It is demonstrated that an appreciable difference occurs only in the kinetics of the most energetic electrons appeared during the ionization. This difference affects only a small fraction of the excess electronic energy in the far periphery of a track.

We describe theoretically the interaction of an ultrashort VUV-XUV laser pulse (FWHM = 10fs, photon energy of 100eV) with liquid water. Incident photons ionize water molecules and create free electrons. These excited electrons interact via elastic collisions with other water molecules and produce secondary electrons due to impact ionization. To track each free electron and its collisions event by event, we use the Monte Carlo method. This approach allows us to describe the non-equilibrium behaviour of electrons in irradiated water on femtosecond timescales. As results we present the transient electron particle- and energy-distributions. Furthermore, we exhibit a time resolved description of the total amount of electrons and we also show the corresponding energy redistribution: change in the kinetic energy of excited electrons, increase of the energy of holes, and energizing of water molecules via elastic collisions.

Physical review letters 107(16), 165003(2011). doi:10.1103/PhysRevLett.107.165003, The femtosecond dynamics of the electrons in aluminum after an intense extreme ultraviolet pulse is investigated by Monte Carlo simulations. Transient distributions of the conduction band electrons show an almost thermalized, low-energy part and a high-energy tail. Constructing emission spectra from these data, we find excellent agreement with measurements. The radiative decay mainly reflects the colder part of the distribution, whereas the highly excited electrons dominate the bremsstrahlung spectrum. For the latter, we also find good agreement between predicted and measured energy scales., Published by APS, College Park, Md.

Physical review / B 84, 214105 (2011). doi:10.1103/PhysRevB.84.214105, Published by APS, College Park, Md.

A Monte Carlo approach was applied for simulations of the early stage (first tens of femtosecond) of kinetics of the electronic subsystem of silica $({\text{SiO}}_{2})$ in tracks of swift heavy ions (SHIs) decelerated in the electronic stopping regime. At the first step multiple ionizations of target atoms by a projectile (${\text{Ca}}^{+19}$, $E=11.4\text{ }\text{MeV}/\text{amu}$) were described that gave the initial spatial distributions of free electrons having different momenta as well as distributions of holes in different atomic shells. Spatial propagation of fast electrons results in secondary ionizations of target atoms as well as in energy transfer to the lattice at times much shorter than the times of atomic oscillations (phonons). The well detected front of excitation in the electronic and ionic subsystems is formed due to this propagation which cannot be described by models based on diffusion mechanisms (e.g., parabolic equations of heat diffusion). At times $\ensuremath{\sim}10\text{ }\text{fs}$ after the projectile passage, about $\ensuremath{\sim}0.1\mathrm{%}$ of the energy is already transferred to the lattice. About 63% of the energy deposited by the ion is accumulated in holes at these times. Calculated distributions of these holes through the atomic shells are in excellent agreement with the spectroscopy experiments. Comparison with these experiments demonstrated also that relaxation of the electronic subsystem in SHI tracks in solids cannot be described adequately without taking into account intra-atomic and interatomic Auger (Knotek-Feibelman) processes.

When ultrashort laser pulses irradiate a solid, photoabsorption by electrons in conduction band produces nonequilibrium highly energetic free electrons gas. We study the ionization and excitation of the electronic subsystem in a semiconductor and a metal (solid silicon and aluminum, respectively). The irradiating femtosecond laser pulse has a duration of 10 fs and a photon energy of h-bar {omega} = 38 eV. The classical Monte Carlo method is extended to take into account the electronic band structure and Pauli's principle for electrons excited to the conduction band. In the case of semiconductors this applies to the holes as well. Conduction band electrons and valence band holes induce secondary excitation and ionization processes which we simulate event by event. We discuss the transient electron dynamics with respect to the differences between semiconductors and metals. For metals the electronic distribution is split up into two branches: a low energy distribution as a slightly distorted Fermi-distribution and a long high energy tail. For the case of semiconductors it is split into two parts by the band gap. To thermalize, these excited electronic subsystems need longer times than the characteristic pulse duration. Therefore, the analysis of experimental data with femtosecond lasers must be basedmore » on non-equilibrium concepts.« less

We investigate theoretically the interaction of a semiconductor with an ultrashort high-intensity VUV laser pulse produced by new light source FLASH at DESY in Hamburg. Applying numerical simulations of excitations and ionization of electronic subsystem within a solid silicon target, irradiated with femtosecond laser pulse (25 fs, photon energy of 38 eV), the transient distribution of electrons within conduction band is obtained. The Monte Carlo method (ATMC) was extended in order to take into account the electronic band structure and Pauli's principle for electrons excited into the conduction band. Secondary excitation and ionization processes were included and simulated event by event as well. In the presented work the temporal distribution of the density of excited and ionized electrons, the energy of these electrons and their energy distribution function were calculated. It is demonstrated that due to the fact that part of the energy is spent to overcome ionization potentials, the final kinetic energy of free electrons is much less than the total energy provided by the laser pulse. We introduce the concept of an 'effective energy gap' for collective electronic excitation, which can be applied to estimate the free electron density after high-intensity VUV laser pulse. The effective energy gap depends on properties of the material as well as on the laser pulse.

Journal of physics / B 46(16), 164009(2013). doi:10.1088/0953-4075/46/16/164009, Here we propose a scheme for probing the outer-shell atomic energy levels within a laser-created nanoplasma, using photoelectron spectroscopy data obtained from the irradiation of the nanoplasma with an ultraintense 'probing' pulse from a soft x-ray free-electron laser. The proposed method can then detect shifts of outer-shell energy levels of an atom or an ion within a plasma, due to the effect of charged plasma environment on atomic potentials, known as the 'plasma screening effect'. Various theoretical models exist that estimate the magnitude of the screening effect. However, the first experimental data that can verify theoretical models have become available only recently (Vinko et al 2012 Nature 482 59). They were obtained with a hard x-ray-based experimental method which uses the information encoded in fluorescence spectra and is, therefore, restricted to deep atomic shells. Below, we show that our photoelectron spectroscopy method of probing a nanoplasma with a destructive, high-intensity soft x-ray pulse that brings the irradiated system to the regime of 'massively parallel' ionization (Gnodtke et al 2012 Phys. Rev. Lett. 108 175003) enables access to the information on the energy level (and ultimately energy level shift) of the valence orbital. In particular, the result of such a photoelectron spectroscopy experiment could help to clarify the discrepancy between the ion abundances in nanoplasmas observed during the recent high-harmonic-generation and free-electron-laser experiments with intense soft x-ray pulses., Published by IOP Publ., Bristol

We present a new model describing the irradiation of semiconductors with ultrashort laser pulses. Based on the earlier developed multiple rate equation [Rethfeld, Phys. Rev. Lett. 92, 187401 (2004)], the model additionally includes the interaction of electrons with the phononic subsystem of the lattice and allows for the direct determination of the conditions for crystal damage. In contrast to commonly used approaches based on the thermodynamic description of the material, our model is applicable to nonequilibrium electronic conditions. Accounting for the dynamic changes in the optical properties of the target (i.e., reflectivity, photoabsorption coefficients), the developed model allows for a comprehensive evaluation of the damage by tracing the changes in the optical parameters, lattice heating and subsequent melting. Our model effectively describes the dynamics of the electronic subsystem and lattice heating and the results are in very good agreement with experimental measurements on the transient refl...

The irradiation of semiconductors with ultrashort laser pulses causes excitation of electrons from bound states (the valence band and deep atomic shells) to the conduction band, which produces non-equilibrium highly energetic free electrons. We apply a Monte-Carlo simulation technique to study the ionization and excitation of the electronic subsystem of a solid silicon target irradiated with a femtosecond laser pulse, obtaining the transient distribution of electrons within the conduction band. We take into account the electronic band structure and Pauli's principle for excited electrons. Secondary excitation and ionization processes induced by electrons in the conduction band and holes in the valence band were also included and simulated event by event. The temporal distributions of the density and the energy of excited electrons are calculated and discussed. We demonstrate that, due to the energy used to overcome the ionization potential, the final kinetic energy of the free electrons is significantly less than the total energy provided by the laser pulse. We extend the concept of an 'effective energy gap' for multiple electronic excitations, which can be applied to estimate the free-electron density after high-intensity vacuum ultraviolet (VUV) laser pulse irradiation. The effective energy gap depends on both the properties of the material and the laser pulse parameters. The concept provides a fundamental understanding of the experimentally accessible pair creation energy measured in the limit of long times.