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Abstract X-ray-induced damage is one of the key topics in radiation chemistry. Substantial damage is attributed to low-energy electrons and radicals emerging from direct inner-shell photoionization or produced by subsequent processes. We apply multi-electron coincidence spectroscopy to X-ray-irradiated aqueous solutions of inorganic ions to investigate the production of low-energy electrons (LEEs) in a predicted cascade of intermolecular charge- and energy-transfer processes, namely electron-transfer-mediated decay (ETMD) and interatomic/intermolecular Coulombic decay (ICD). An advanced coincidence technique allows us to identify several LEE-producing steps during the decay of 1s vacancies in solvated Mg2+ ions, which escaped observation in previous non-coincident experiments. We provide strong evidence for the predicted recovering of the ion’s initial state. In natural environments the recovering of the ion’s initial state is expected to cause inorganic ions to be radiation-damage hot spots, repeatedly producing destructive particles under continuous irradiation.

Energy and charge transfer processes like Interatomic Coulombic Decay (ICD) play an important role in the relaxation of excited atoms or molecules in dense media such as biological tissue. Here, we present a method to experimentally determine the site-specific efficiency of the ICD process, which is in quantitative agreement with theoretical calculations.

High-resolution absorption, dispersed fluorescence emission, and photoionization cross sections are presented for gas-phase adamantane excited by synchrotron radiation in the exciting-photon energy range of 6–30 eV. Relative and absolute absorption cross sections of so-far unmatched resolution of down to 0.27 cm ^−1 line width in the region from 6.4–28 eV are shown along with newly discovered vibronic substructures around the HOMO–LUMO transition. Absorption line positions are provided with very high accuracy and listed in tabular form to be used as spectral fingerprints for the detection of adamantane in interstellar media, where its column density may be determined via the absolute cross sections. The fluorescence emission lies in the ultraviolet range from 190–250 nm and is excited starting at the HOMO–LUMO transition at 6.49 eV, which corresponds to the highest fluorescence emission energy. Hitherto unreported fluorescence in the same spectral range and relative photoionization cross sections in the exciting-photon energy range up to 30 eV are also presented along with lifetime measurements for differentiation of the involved electronic states.

We report on the experimental observation of the direct decay of a core vacancy in van der Waals clusters by emission of a fast electron from a neighboring atom. The process can be regarded as an interatomic Coulombic decay of core holes (core-level ICD). We identify it unambiguously by electron-electron and electron-electron-photon coincidence spectroscopy of the decay of 2p vacancies in Ar clusters. While several earlier works reported the absence of this channel, we find core-level ICD to be of considerable significance and quantify the branching ratio of this nonlocal electron emission to conventional local Auger decay as (0.8±0.2)%. Our results are supported by calculations on smaller clusters and show a reasonable agreement. This report on a successfully performed electron-electron-photon coincidence experiment provides a perspective for explorations of matter exposed to ionizing radiation. The observed core-level ICD is proposed to be of general importance for studies on charge redistribution after core-level photoionization where van der Waals clusters are often used as prototype systems.

We directly observe radiative charge transfer (RCT) in Ne clusters by dispersed vacuum-ultraviolet photon detection. The doubly ionized Ne ^2+ – ${{\rm{N}}{\rm{e}}}_{n-1}$ initial states of RCT are populated after resonant 1s–3p photoexcitation or 1s photoionization of Ne _n clusters with $\langle n\rangle \approx \,2800$ . These states relax further producing Ne ^+ –Ne ^+ – ${{\rm{N}}{\rm{e}}}_{n-2}$ final states, and the RCT photon is emitted. Ab initio calculations assign the observed RCT signal to the ${}^{}{{\rm{N}}{\rm{e}}}^{2+}(2{{\rm{p}}}^{-2}{[}^{1}{\rm{D}}])\mbox{--}{{\rm{N}}{\rm{e}}}_{n-1}$ initial state, while transitions from other possible initial states are proposed to be quenched by competing relaxation processes. The present results are in agreement with the commonly discussed scenario, where the doubly ionized atom in a noble gas cluster forms a dimer which dissipates its vibrational energy on a picosecond timescale. Our study complements the picture of the RCT process in weakly bound clusters, providing information which is inaccessible by charged particle detection techniques.

Vacuum ultraviolet (VUV) fluorescence emission from neon clusters of different sizes is investigated after excitation with photons of energies between 34 and 49 eV, i.e. near and far below the Ne 2s-electron photoionization threshold. Undispersed VUV fluorescence (<120 nm) excitation functions in the Ne 2s-regime display a series of distinct features attributed to 2s → np resonant cluster excitations with subsequent cluster-specific decays. Features connected with resonant interatomic Coulombic decay are visible for all cluster sizes. For larger clusters they appear to be less prominent due to additional emissions. This emission has a threshold energy of 35.8 eV and increases with exciting-photon energy. It also increases with increasing cluster size and is interpreted as being caused by inelastically scattered 2p-photoelectrons (photoelectron impact induced luminescence). [ABSTRACT FROM AUTHOR]