FERMI | TIMEX Tracking ultrafast charge transfer in thermite reactions with femtosecond EUV pulses Understanding how energetic materials ignite and evolve at the nanoscale is crucial for developing safer and more efficient technologies in welding, propulsion, and energy conversion. Thermite systems – composed of a metal and a metal oxide – are key examples of such materials. They undergo extremely fast and exothermic redox reactions, but the initial steps of charge transfer at ultrafast timescales remain poorly understood. In this study, researchers used FERMI’s unique capabilities to investigate these early steps by focusing femtosecond extreme ultraviolet (EUV) pulses on a nanostructured thermite bilayer composed of aluminium and hematite (iron oxide). By combining a visible laser pump with EUV transient absorption spectroscopy at the Fe M2,3 and Al L2,3 edges, they monitored the electron dynamics between the two materials with element specificity and ultrafast temporal resolution. The results reveal that, upon excitation, hematite exhibits the rapid formation of localized charge carriers known as small polarons. These are signaled by a characteristic shift in the absorption spectrum that occurs within the first hundreds of femtoseconds. Small polarons are quasiparticles resulting from the strong coupling of excited charge carriers with the surrounding lattice. In hematite, this self-trapping mechanism is driven by interactions with optical phonons, which cause a local lattice distortion and the stabilization of the charge in a localized state. The formation of small polarons is essential in determining charge transport properties in iron oxides and plays a crucial role in photocatalysis and redox reactions. Interestingly, this spectroscopic signature appears weaker when hematite is deposited on aluminium compared to a reference inert parylene substrate. This difference indicates that aluminium donates electrons to hematite following the laser pulse. The extra carriers might saturate available trapping sites or alter the local electrochemical potential, influencing polaron formation and stability. The precise tracking of the zero-crossing point in the transient spectrum, a fingerprint of polaronic behavior, enabled the team to distinguish between intrinsic and extrinsic charge localization mechanisms. The study also compared the spectral evolution of hematite on aluminium with hematite on parylene, showing that the interaction with aluminium modifies the electronic response of hematite. This is interpreted as evidence of early-stage electron injection from aluminium into the iron oxide, potentially initiating a redox process at the interface. The shift in the transient absorption spectra is consistent with the formation of Fe2+ states, a hallmark of hematite reduction. In parallel, the measurements at the aluminium edge show a progressive decrease in absorption, consistent with a rapid temperature increase above 3000 K. This temperature is high enough to melt the aluminium layer and initiate the redox reaction. Structural changes such as expansion, nanoparticle formation, and partial disintegration of the layers are inferred from the transient signals and suggest that mixing at the interface is enhanced. These observations highlight the role of thermal and mechanical effects in facilitating chemical reactivity in nanostructured thermites. Further analysis indicates that the reaction proceeds primarily in the region near the interface, within a limited volume where heat, pressure, and charge flow are highest. The absence of additional spectral shifts at longer times suggests that the reaction may not propagate significantly beyond this interfacial region during the observation window. However, the early electronic reorganization observed in both layers confirms that charge transfer precedes atomic motion and is the true trigger of the redox process. The data also indicate that the local electronic and structural rearrangements take place within the first few hundred femtoseconds, while mass transport and further reactivity continue on longer timescales up to hundreds of picoseconds. These findings imply that electron transfer from aluminium to hematite happens before significant atomic diffusion, possibly initiating the chemical reaction at the interface. A smaller spectral shift observed in hematite when supported on aluminium, compared to the parylene case, supports this interpretation and hints at the formation of reduced Fe2+ species in the early stages of the reaction. This work marks the first chemically resolved, femtosecond-resolved observation of a nanoscale thermite redox system. It establishes a novel methodology to access ultrafast charge dynamics in reactive interfaces and demonstrates the capabilities of EUV pump-probe spectroscopy in revealing fast and element-specific transformations in energetic materials. This approach could be extended to more complex geometries, such as multilayers and confined nanostructures, or used in combination with applied electric fields to further control and study charge-driven reactions. Moreover, the ability to track element-specific charge dynamics on femtosecond timescales represents a powerful diagnostic for broader applications in catalysis, battery materials, and photocatalytic systems. As the demand for efficient and controllable energy conversion processes grows, so does the need for fundamental understanding of charge flow across interfaces. The demonstrated approach opens new possibilities for tailoring reactivity in nanostructured materials and could guide the development of novel devices that exploit ultrafast charge separation and transport. Elettra Highlights 2024–2025 38
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