New capabilities at X-ray free-electron laser facilities allow the generation of two-colour femtosecond X-ray pulses, opening the possibility of performing ultrafast studies of X-ray-induced phenomena. Particularly, the experimental realization of hetero-site-specific X-ray-pump/X-ray-probe spectroscopy is of special interest, in which an X-ray pump pulse is absorbed at one site within a molecule and an X-ray probe pulse follows the X-ray-induced dynamics at another site within the same molecule. Here we show experimental evidence of a hetero-site pump-probe signal. By using two-colour 10-fs X-ray pulses, we are able to observe the femtosecond time dependence for the formation of F ions during the fragmentation of XeF₂ molecules following X-ray absorption at the Xe site.

Water oxidation is the key half reaction in artificial photosynthesis. An absence of detailed mechanistic insight impedes design of new catalysts that are more reactive and more robust. A proposed paradigm leading to enhanced reactivity is the existence of oxyl radical intermediates capable of rapid water activation, but there is a dearth of experimental validation. Here, we show the radicaloid nature of an intermediate reactive toward formation of the O-O bond by assessing the spin density on the oxyl group by Electron Paramagnetic Resonance (EPR). In the study, an¹⁷O-labeled form of a highly oxidized, short-lived intermediate in the catalytic cycle of the water oxidation catalystcis,cis-[(2,2-bipyridine)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺was investigated. It contains Ru centers in oxidation states [4,5], has at least one Ru^V= O unit, and shows |A_xx| = 60G¹⁷O hyperfine splittings (hfs) consistent with the high spin density of a radicaloid. Destabilization of π-bonding in thed³Ru^V= O fragment is responsible for the high spin density on the oxygen and its high reactivity.

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window covering (284–543 eV) attosecond soft x-ray pulses that can simultaneously access electronic and lattice parameters via dispersive x-ray absorption fine-structure (XAFS) spectroscopy. We validate this approach with an identification of the ${\sigma }^{*}$ and ${\pi }^{*}$ orbital contributions to the density of states in graphite simultaneously with its lattice’s four characteristic bonding distances. This work demonstrates the potential of dispersive XAFS, in combination with attosecond pulses, as a powerful investigative tool that is equally applicable to gas, liquid, and condensed phase.