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Abstract: Understanding X-ray induced dynamics is relevant for a broad field of applications, ranging from X-ray imaging of materials, to medical application of X-rays. Due to the large amount of energy imparted by an absorbed X-ray photon, the triggered relaxation processes typically take place on ultrafast time scales. Water molecules in the gas phase have been studied at the Maloja endstation using an X-ray pump X-ray probe scheme. The pump pulse was tuned to 390eV, which leads to valence (1b1, 3a1, 1b2) and inner-valence (2a1) ionization of the molecules, but favors innervalence ionization. Upon ionization of the two outermost orbitals, we expect stable cations, while ionization of the 1b2 and 2a1 orbitals is expected to result in dissociation. We probe the cations and dissociation products using X-ray absorption spectroscopy (XAS) by scanning the photon energy across the oxygen K-edge. The charge-state specific ion yields are detected with an ion time-of-flight (TOF) spectrometer. The resulting time-resolved ion-yield XAS spectra track the electronic structure of the fragments produced upon ionization of water as they undergo further dissociation. In this way, ion TOF spectroscopy captures both the initial excitation and the subsequent fragmentation pathways in a single measurement.1[2] The figure .png)
shows a 2D difference map of the evolution in time (y-axis) and probe photon energy (x-axis) of the O+ ion yield. The blue negative features correspond to the ground-state bleach of the H2O resonances at 534-540eV, while two strong positive signals can be observed arising on different timescales. The one around 532eV is assigned to O2. While the other shifts towards lower energies and contains resonances tentatively assigned to H2O+ and OH. Insights into water fragmentation obtained from the spectroscopic data, complemented by simulations performed by our theory collaborators, will be presented. These results provide a benchmark for the subsequent analysis of water clusters recorded during the same beamtime.
Furthermore, we compare X-ray induced dynamics in ethanol molecules and clusters, studied with an X-ray pump NIR probe scheme, to understand how the environment affects the relaxation processes. The pump pulse, tuned above the oxygen K-edge, doubly ionizes ethanol. In isolated molecules, intramolecular hydrogen mobility gives rise to a roaming H2 molecule, one of the channels for H3+ production [3], which we track with time-resolved ion yields. In clusters, both intramolecular and intermolecular proton transfer occur, producing characteristic fragments such as H3O+. Further analysis will allow us to infer the timescales of the relaxation processes in the clusters. This experiment deepens our understanding of the interplay between local and non-local relaxation processes and their consequences for the fragmentation dynamics. 1 M. N. Piancastelli et al., Phys. Rev. A 59.1 (Jan. 1999), pp. 300–306. [2] Julius Schwarz et al., Phys. Chem. Chem. Phys. 24 (38 2022), pp.
[3] Ngai et al. Sci. Rep. 15, 3201 (2025) 23119–23127.