Ultrafast electronic and lattice dynamics in Al/Fe2O3 composite probed with the FERMI free electron laser

The aim of the thesis is that of investigating ultrafast dynamics in Al/Fe2O3 composite using time-resolved and chemically selective spectroscopic techniques. The experiments discussed in the thesis were performed at the EIS-TIMEX and EIS-TIMER beamlines at the FERMI free electron laser (FEL) and consisted in time-resolved absorption spectroscopy and four wave mixing (FWM) measurements with ultrashort extreme ultraviolet (EUV) pulses. TIMEX is an instrument developed to investigate matter in extreme conditions with a pump-probe approach. The beamline is equipped with a robust pump-probe setup that exploits the seed laser for users (SLU) and the FEL radiation, providing a time resolution of ∼ 100 fs with a jitter of just a few fs (< 10 fs). An introduction of FEL radiation and a description of FERMI and of the aforementioned beamlines is followed by a discussion of the main project: the sudy of the ultrfast dynamics of the heterogeneous solid-state redox reaction between Al and Fe2O3. The innovation of this work consist in using EUV pulses to investigate the reaction dynamics between two (heterogeneous) reaction partners, while, so far, spectroscopic investigations in this wavelength range have been restricted to intramolecular process in the gas phase. Al/Fe2O3 is the prototypical composition of energetic materials that, upon activation, results in the commonly known thermite reaction. A lot of research has been conducted on the family of thermite reactions, because they have several applications in different fields, ranging from propellants, explosives and pyrotechnics to industrial waste treatment and material synthesis. The experiment was performed at the TIMEX end-station, photoexciting the sample with the laser at 785 nm to initiate the reaction and probing the process with chemical selectivity at the Fe M2,3 edge absorption edge and at the Al L2,3 edge absorption edge. The experiments conducted during my stay at FERMI proves the capability of EUV transient absorption spectroscopy, which is sensitive to the variations of the atomic species oxidation state, to investigate chemical reaction dynamics. A blue shift of the isosbestic point in the transient absorption trace of Fe2O3 has been observed and it has been associated with the formation of a small polaron in Fe2O3. We have compared the behaviour of the prototypical thermite mixture in the transient absorption traces with bare hematite deposited on parylene. We attributed the substrate dependent spectral divergences to the injection of electrons from Al to Fe2O3, supporting this interpretation with the simulations of the static absorption spectra of Fe2O3 and one of the expected reaction intermediates, FeO. The calculations were performed by our CALTECH collaborators. Then, a selection of experiments performed at FERMI will be shortly treated. In these projects the staff of the beamline was largely engaged for the development and the post-processing. The experimental techniques that were explored are of interest for this thesis because they differ from the conventional TG and they have the capability to be employed for investigating chemical problems. In conclusion, the main topic of my PhD project has been that of moving the first steps towards the investigation of heterogeneous solid-state chemical reaction with the FERMI FEL light source. Although we did not observe all the steps of the reaction, we were able to monitor chemically relevant dynamics, i.e. small polaron formation and electron transfer process. The work conducted in this project shows that chemically-selective and time-resolved spectroscopy is a valuable tool for studying chemical reactions. Additionally, I had the chance to actively participate in the development of novel EUV based techniques beyond conventional TG at the TIMER beamline.