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.

The pulse duration, and, more generally, the temporal intensity profile of free-electron laser (FEL)\ud pulses, is of utmost importance for exploring the new perspectives offered by FELs; it is a nontrivial\ud experimental parameter that needs to be characterized. We measured the pulse shape of an extreme\ud ultraviolet externally seeded FEL operating in high-gain harmonic generation mode. Two different methods\ud based on the cross-correlation of the FEL pulses with an external optical laser were used. The two methods,\ud one capable of single-shot performance, may both be implemented as online diagnostics in FEL facilities.\ud The measurements were carried out at the seeded FEL facility FERMI. The FEL temporal pulse\ud characteristics were measured and studied in a range of FEL wavelengths and machine settings, and they\ud were compared to the predictions of a theoretical model. The measurements allowed a direct observation of\ud the pulse lengthening and splitting at saturation, in agreement with the proposed theory

Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.