Espressione delle potenzialità dei Laser ad elettroni liberi attraverso esperimenti innovativi

The advent of X-Ray Free Electron Lasers (FELs) has opened a new era for exploring the fundamental properties of matter. These machines are the 4th generation light sources and combine the exceptional properties of conventional lasers and synchrotrons, allowing to probe the ultra-fast dynamics of atoms and molecules in simple and complex systems at a nano-scale level. This thesis work is divided in two main branches: at first, it reviews the basic theory behind FELs and points out possible improvement of these sources. Secondly, it presents some results obtained during new and peculiar experiments performed between 2017 and 2020 in different FELs facilities while showing how further improvements are needed in order to accomplish state of art results unreachable with the current state of synchrotrons and FELs. In Chapter 1, the reader is introduced to the charged particle accelerator theory culminating with its application on one of the main component of a FEL: the undulator. In order to provide its special radiation output, a FEL light source makes use of interactions between charged particles and a self-generated electromagnetic radiation inside an undulator. To let the reader easily understand the mechanism behind the radiation generated and its properties, this thesis opens with an approach to the laws of physics that rule the dynamics of the particles. Coupling the Lorentz equation with Maxwell equations we show the collective dynamics of electrons throughout the all FEL structure in Chapter 2. In Chapter 3 we present the two main places where this work of thesis has been built: the FERMI FEL in Italy and the SwissFEL machine based at the Paul Scherrer Institute in Switzerland. Here we explore the secrets of their layouts, beamlines and main experiment performed. Once the reader mastered the fundamental properties of a FEL, we show the main results achieved during this research period at the FERMI FEL and at the SwissFEL. In Chapter 4 we present the very first successfull attempt at X-Ray Transient Grating Spectroscopy on Bismuth Germanate Oxide. In order to confirm some of the results obtained through the X-ray Transient Grating technique, a few satellite measurements and simulations were also performed on the Bismuth Germanate Oxide sample, which are presented in Chapter 5. In Chapter 6 we present what we called a Real-time visualization of ibuprofen dimer vibrations with element- and enantiomeric- selectivity: here we exploit the potential of FELs through an ultrafast soft X-Ray absorption experiment that allows the visualization and disentangling of several low-frequency and near lying vibrational modes, involving specific carbon atoms in a racemic mixture of Ibuprofen. In Chapter 7 we explore the world of soft matter: here we introduce the problematics affecting the protein structure research by showing the main re- results obtained at two synchrotron facilities. Afterward, given the results we obtained, we explore the possibility to overcome such problems by introducing the Coherent and Incoherent Diffraction Imaging techniques at FELs combined to a new approach on protein bidimensional crystallization. Furthermore, we introduce a rather similar approach to a technique aiming at Catching Conical Intersection through Electronic Coherence and Noise Correlation Spectroscopy: we explore the possibility to use the TRUECARS technique to directly visualize the conical intersection dynamics, which regulates the photo-chemestry of most molecules. Chapter 8 opens with an introduction on Resonant Inelastic X-ray Spettroscopy (RIXS). Here we show the results of the application of RIXS exploiting the Ultrafast charge trapping dynamics in Cu2O in-gap states: we investigated the charge trapping dynamics in the defect-related in-gap states of Cu2O, following photoexcitation across the band gap. Finally, the thesis culminates with the Design of a soft X-Rays RIXS spectrometer for the SwissFEL's new beamline FURKA. ​