Because ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table 2,3 , these results could have implications for our understanding of ionization damage in living tissues. NPHYS-2009-06-00979a 2 Electronic vacancy states can be produced in matter by ionizing radiation, such as X-ray photons or fast charged particles. When a state with a high electronic excitation energy has been produced by impact of such particles, electron correlation can cause the ejection of electrons. Auger decay is the best known representative of this class of secondary processes that is more generally termed autoionization. In other words, the mechanism is a concerted transition in which a single hole in an inner shell is replaced by two vacancies in the outer valence shells of two adjacent molecules, and a free electron. This decay channel was termed Intermolecular (Interatomic, in the case of atomic clusters) Coulombic Decay and was subsequently observed in rare gas clusters 4-7 .The process is shown schematically in Fig. 1. A resonant variant of ICD, which may take place after photoexcitation into an unoccupied orbital, has also been discussed [7][8][9] . In the present paper, we consider ICD of inner valence vacancy states, for which case the ejected electrons have a low kinetic energy.On the basis of energetic considerations, ICD can take place whenever the binding energy of the ionized state lies above the double ionization threshold of the corresponding cluster or liquid. This prerequisite for ICD is fulfilled in hydrogen-bonded systems 2,10 , but so far the process has not been seen. Calculations of the energy spectrum of electrons ejected by ICD of small water clusters give a hint as to why it has escaped observation: A broad, rather unstructured distribution of energies is expected, which peaks at zero eV 10 . Ifwe consider an experiment with a conventional electron energy analyser on a bulk or liquid NPHYS-2009-06-00979a 3 sample, an electron spectrum with this shape can hardly be distinguished from the "universal curve" 1 for secondary electrons (Fig. 2). In this respect our work differs from earlier experiments, which were either restricted to dimers 5-7 , or dealt with simpler cases where an ICD feature appears from simple electron kinetic energy spectra 4,8,9 . Producing primary electrons of a well-defined energy by photoionization and detecting them in coincidence with the ICD electron has allowed us to overcome the aforementioned problem. Here, we demonstrate that ICD follows the photoionization of medium-sized water clusters and show that -above the corresponding photoionization threshold -ICD electrons make an important contribution to the low kinetic energy spectrum.In our experiment, a jet of water clusters with a mean size 〈N〉 of 40 or 200 was used.Such clusters are believed to form amorphous structures, which resemble the hydrogenbonded network of liquid water rather than that of crystalline ice 11 . Inner valence vacancies were p...

Electron-electron coincidence spectra of Ar-Kr clusters after photoionization have been measured. An electron with the kinetic energy range from 0 to approximately 1 eV is found in coincidence with the Ar 3s cluster photoelectron. The low kinetic energy electron can be attributed to an Ar + Kr+ + Kr+ final state which forms after electron transfer mediated decay. This autoionization mechanism results from a concerted transition involving three different atoms in a van der Waals cluster; it was predicted theoretically, but hitherto not observed.

In this article, a newly constructed electron spectrometer of the magnetic bottle type is described. The instrument is part of an apparatus for measuring the electron spectra of free clusters using synchrotron radiation. Argon and helium outer valence photoelectron spectra have been recorded in order to investigate the characteristic features of the spectrometer. The energy resolution (E/∆E) has been found to be approx. 30. Using electrostatic retardation of the electrons, it can be increased to at least 110. The transmission as a function of kinetic energy is flat, and is not impaired much by retardation with up to 80 % of the initial kinetic energy. We have measured a detection efficiency of most probably 0.6 +0.05 −0.1 , but at least of 0.4. Results from testing the alignment of the magnet, and from trajectory simulations, are also discussed.

Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, i.e., the provision of knobs with which to tune properties, stoichiometric variation and surface decoration are two effective approaches that have been followed. In angleresolved photoelectron spectroscopy (ARPES) experiments, both approaches are seen to lead to electronic band-structure changes. Most importantly, such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases. The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of super-band-gap light a topological insulator surface has been exposed to under typical ARPES experimental conditions. Our results show that this third knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous and direct competition with the latter. The data clearly point towards surface photovoltage and photoinduced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary, and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a detailed framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature, and photon exposure. These knobs can be used in conjunction to fine tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.