Matter under Extreme Transient conditions investigated with Free Electron Laser radiation at FERMI

Subject to high-intensity ultra-short light pulses, solid materials are driven into short-living states of extremely high temperatures and pressures. Creating such unique states under controlled laboratory conditions and addressing their physical properties with an adequate time resolution to follow their fast-evolving dynamics has been a primary objective of pump-and-probe studies employing the high-peak-power femtosecond optical lasers that have become of increasingly widespread availability during the last three decades. Operative since few years, free electron lasers (FELs) add - to the desired properties of high brilliance, short time duration and coherence proper of those lasers - the tunable energy up to the extreme ultraviolet (EUV) and X-ray spectral ranges which is typical of synchrotron light sources, thus offering a number of advantages on both the fronts of pumping and probing. This thesis describes the efforts that have been undertaken towards the establishment of a new experimental ground for the study of matter under extreme transient conditions at the TIMEX beamline of the free electron laser FERMI in Trieste and discusses my understanding of a number of seminal investigations. Characterization of the exotic states swiftly reached by FEL-excited metallic samples is addressed through single-shot measurements of their EUV reflectance and absorbance. In particular, we provide the first demonstration of EUV absorption near edge spectroscopy at high energy densities and with sub- 100 fs time resolution. The combination of these features is only possible at FEL sources, but makes FERMI truly unique when further considering that here it comes without a need for monochromatization thanks to its seeded nature which makes the delivered pulses almost Fourier-transform limited in bandwidth. The measured dependence on irradiation fluence of the transient absorbance and reflectance of Ti for selected wavelengths across the M2,3 absorption edge and near the plasma frequency respectively is reported, revealing an ultrafast modification of the electronic structure compatible with a single FEL pulse having turned within about 100 fs room-temperature solid-density Ti into a hot free electron gas coexisting in highly non-equilibrium conditions with a cold ion lattice. An interpretive perspective is proposed that - based on simplified, yet reasonable, Drude-like models - allows to gain information on the average free electron temperature and density and the average ionization state of the generated dense plasma. Ultra-fast high-energy-density self-transmission measurements on Al at a few photon energies between the plasma frequency and the L2,3 absorption edge are presented, evidencing for the first time a non-monotonic trend of the EUV transmission as a function of fluence, that are seen to be interpretable - within a three-channel model - as resulting from an interplay between saturation effects and absorption cross-section variations due to electron heating. Higher fluence conditions are shown to induce almost full transparency in an Al-Mg-Al stack over an entire energy region across the Mg L2,3 absorption edge. An analytical model is formulated, that tentatively permits to deduce from the data an estimate for the first ionization energy of warm dense Mg. The responses of insulator and semiconductor samples to FEL and laser excitations - from the initial non-equilibrium stage, through the subsequent relaxation dynamics, to possible phase transformations - are explored through pump-and-probe experiments. Here, FEL pulses are exploited in combination with pulses from the same infrared laser that serves to initiate the FEL emission process, which ensures a natural synchronization between the two radiation sources, - again - a unique feature of seeded FELs. Measurements of transient optical reflectivity on FEL-pumped Si3N4 samples are offered as a demonstration of the nearly jitter-free pump-probe capabilities of FERMI, thus far unbeaten among FEL facilities. The results of a systematic study of the optical response of this material to FEL irradiation for different pump wavelengths and intensities, probe polarizations, sample thicknesses and types of substrate in both reflection and transmission simultaneously are discussed, hinting at the role of interference effects at play due to the different penetration depths of the EUV FEL and infrared laser pulses. Transient EUV transmissions for Ge at its M4,5 edge within a laser pump / FEL probe configuration and for Si at its L2,3 edge within a FEL pump / FEL probe configuration yield a time and energy -resolved view on femtosecond pulse -induced melting of semiconductors. We show that, while able to follow the excitation and de-excitation dynamics of the electronic population in the valence band through below- absorption edge data, we can infer dynamical structural information through on- absorption edge data, in which we recognize the fingerprint of a partial band gap closure associated with the transition to a liquid state of increased metallic character. ---