Direct Observation of Ultrafast Hydrogen Bond Restructuring Upon Electron Photoinjection from Catalytic Nanoparticles.

Metallic nanoparticles have emerged as excellent catalytic drivers due to their enhanced surface coverage of active sites, electronic and structural tunability down to the atomic level, and plasmonic resonances allowing for tailored photocatalysis. Beyond their importance in catalysis, metallic nanoparticles also enable controlled studies of ultrafast energy deposition where charge , heat transfer, and chemical reactivity are strongly coupled in far from equilibrium environments. Here we report with femtosecond-resolution MeV electron diffraction the direct measurement of the ultrafast structural response of liquid water and change in hydrogen bond structure due to electron injection from photoexcited solvated platinum nanoparticles. Through comparisons with molecular dynamics simmulations, we can extract the contributions from heat transfer and electron solvation on the change in the local water structure. We observe a non-thermal response from the water molecules on 0.5-1 ps timescale, indicated by the increase in short range oxygen-oxygen coordination and hydrogen bonding. Longtime change(>10ps) in the water structure is consistent with high density electron injection rather than pure thermal disorder, indicating initial charge transfer as the primary mechanism for change in water coordination. Our measurements and comparisons to simulations provide critical insight into the structural rearrangement of liquid water driven by charge-transfer processes at nanoscale interfaces, a long-standing challenge for the development of nano catalysis and for understanding how intense electronic excitation alters interfacial chemistry.