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 sp3 and sp3s*. The sp3 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.
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.
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.
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).
We model numerically the interaction of an ultrashort VUV laser pulse (FWHM = 10 fs, photon energy of 100 eV) with liquid water. The incident laser photons interact with water by ionizing water molecules and creating free electrons. These excited electrons are elastically scattered by water molecules and are able to produce secondary electrons via ionization. To track each free electron and its collisions event by event, we use the Monte Carlo method similar to (N. Medvedev and B. Rethfeld, Transient dynamics of the electronic subsystem of semiconductors irradiated with an ultrashort vacuum ultraviolet laser pulse, New Journal of Physics, Vol. 12, p. 073037 (2010)). This approach allows us to describe the transient non-equilibrium behaviour of excited electrons on femtosecond time scales. We present transient electron energy distributions and a time resolved energy transfer, i.e.: the changing kinetic energy of excited electrons, the increase of the energy of holes, and excitation of water molecules via elastic collisions. We compare results obtained with different models for the energy levels in liquid water: either assuming dense water vapour or an amorphous semiconductor with a band gap.
In solids under irradiation with femtosecond laser pulses, photoabsorption produces a strongly nonequilibrium highly
energetic electrons gas. We study theoretically the ionization of the electronic subsystem of either a semiconductor
(silicon) or a metal (aluminum) target, exposed to an ultra-short laser pulse (pulse duration ~10 fs) of VUV-XUV
photons. We developed a numerical simulation technique, based on the classical Monte-Carlo method, to obtain transient
distributions of electrons within conduction band. We extend the Monte-Carlo method in order to take into account
quantum effects such as the electronic band structure, Pauli's exclusion principle for electrons in the conduction band and
for holes within the valence band (for semiconductors), and free-free electron scattering (for metals).
In the presented work, the temporal distribution of the energy density of excited and ionized electrons were calculated.
The transient dynamics of electrons is discussed regarding the differences between semiconductors and metals. It is
demonstrated that for the case of semiconductors, since a part of the energy is spent to overcome ionization potentials,
the final kinetic energy of free electrons at the end of the laser pulse is much less than the total energy provided by the
laser pulse. In contrast, for metals all the energy is present as kinetic energy in the electronic subsystem, unless the
photon energy is greater that an ionization potential of a deep atomic shell. In the latter case, a part of the energy is
shortly kept by deep-shell holes, and is released back to the electrons by Auger-processes on femtosecond timescales.
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.
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.