Ultrafast nonthermal processes in FEL irradiated solids

by Nikita Medvedev (CFEL, DESY)

Thursday, May 23, 2013 from to (Europe/Berlin)
at AER19 / 3.11
In the seminar recent theoretical studies on the ultrafast electronic and atomic kinetics after FEL irradiation will be presented. The first part of the talk consists of the theoretical foundation of the newly developed experimental techniques allowing measuring the single shot FEL pulse duration and jitter with a few-femtosecond precision. Our study demonstrates the limits of such technique and allows finding optimum parameters which can be used in experiments. It is based on the ultrafast electron excitation and relaxation in the excited solid and, therefore, requires quantitative understanding of the femtosecond non-equilibrium electron kinetics. The Monte Carlo (MC) code is applied to model the electron kinetics, which includes the primary photo-ionization, secondary elastic and inelastic scattering of electrons, Auger-decays of deep-shell holes, and electron-hole recombination. With the calculated transient electron distribution, the transient change of the optical properties (reflection and transmittance of the visible light) of the material is estimated in a good agreement with experimental observations.
In the second part, I will present the recently developed theoretical approach for studying both, electrons and atoms of a solid under FEL irradiation. The hybrid model is based on an efficient combination of the (1) MC method for treating photoabsorption, high-energy-electron and core-hole excitation, kinetics and relaxation processes; (2) low energy electrons within the valence and conduction bands of the material are treated with a temperature equation; (3) the atomic motion is followed with the Molecular Dynamics method on the evolving potential energy surface; (4) the potential energy surface is calculated with help of the time-dependent tight-binding Hamiltonian. The electron band structure is also obtained with this method, allowing tracing its evolution in time within the irradiated sample. 
This combination of methods in one hybrid model allows investigation of non-equilibrium structural changes within materials after X-ray femtosecond laser pulse irradiation. Our analysis performed for diamond irradiated with photon energies from 24 eV up to 10 keV predicted for the first time in this wide wavelength regime the nonthermal graphitization. This phase transition is induced by the changes of the interatomic potential after the ultrafast electron excitations. It occurs within a few tens of femtoseconds, well before the heating stimulated by electron-phonon coupling starts to play a role. The calculated damage threshold fluence for different photon wavelengths agrees very well with the experimental data.