Experimental Realization of Auger Decay in the Field of a Positive Elementary Charge.

Auger electron spectroscopy is an omnipresent experimental tool in many fields of fundamental research and applied science. The determination of the kinetic energies of the Auger electrons yields information about the element emitting the electron and its chemical environment at the time of emission. Here, we present an experimental approach to determine Auger spectra for emitter sites in the vicinity of a positive elementary charge based on electron-electron-electron and electron-electron-photon coincidence spectroscopy. We observe a characteristic redshift of the Auger spectrum caused by the Coulomb interaction with the charged environment. Our results are relevant for the interpretation of Auger spectra of extended systems like large molecules, clusters, liquids, and solids, in particular in high-intensity radiation fields which are nowadays routinely available, e.g., at x-ray free-electron laser facilities. The effect has been widely ignored in the literature so far, and some interpretations of Auger spectra from clusters might need to be revisited.

X-ray radiation damage cycle of solvated inorganic ions.

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.

Photoelectron circular dichroism of O 1s-photoelectrons of uniaxially oriented trifluoromethyloxirane: energy dependence and sensitivity to molecular configuration.

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane (TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincident electron and fragment ion detection using cold target recoil ion momentum spectroscopy. The corresponding calculations were performed by means of the single center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare the results for the differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl group by the trifluoromethyl group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in the literature.

Suppression of X-ray-Induced Radiation Damage to Biomolecules in Aqueous Environments by Immediate Intermolecular Decay of Inner-Shell Vacancies.

The predominant reason for the damaging power of high-energy radiation is multiple ionization of a molecule, either direct or via the decay of highly excited intermediates, as, e.g., in the case of X-ray irradiation. Consequently, the molecule is irreparably damaged by the subsequent fragmentation in a Coulomb explosion. In an aqueous environment, however, it has been observed that irradiated molecules may be saved from fragmentation presumably by charge and energy dissipation mechanisms. Here, we show that the protective effect of the environment sets in even earlier than hitherto expected, namely immediately after single inner-shell ionization. By combining coincidence measurements of the fragmentation of X-ray-irradiated microsolvated pyrimidine molecules with theoretical calculations, we identify direct intermolecular electronic decay as the protective mechanism, outrunning the usually dominant Auger decay. Our results demonstrate that such processes play a key role in charge delocalization and have to be considered in investigations and models on high-energy radiation damage in realistic environments.