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...

Complex organic molecules (COM) such as aldehydes, ketones, carboxylic acids, esters, and amides are ubiquitous in the interstellar mediums, but traditional gas phase astrochemical models cannot explain their formation routes. By systematically exploiting on line and in situ vacuum ultraviolet photoionization coupled with reflectron time of flight mass spectrometry (PI-ReTOF-MS) and combining these data with infrared spectroscopy (FTIR), we reveal that complex organic molecules can be synthesized within interstellar ices that are condensed on interstellar grains via non-equilibrium reactions involving suprathermal hydrogen atoms at temperatures as low as 5 K. By probing for the first time specific structural isomers without their degradation (fragment-free), the incorporation of tunable vacuum ultraviolet photoionization allows for a much greater understanding of reaction mechanisms that exist in interstellar ices compared to traditional methods thus eliminating the significant gap between observational and laboratory data that existed for the last decades. With the commission of the Atacama Large Millimeter/Submillimeter Array (ALMA), the detection of more complex organic molecules in space will continue to growincluding biorelevant molecules connected to the Origins of Life themeand an understanding of these data will rely on future advances in hard core physical chemistry laboratory experiments.

Intermolecular Coulombic decay (ICD) is a ubiquitous relaxation channel of electronically excited states in weakly bound systems, ranging from dimers to liquids. As it is driven by electron correlation, it was assumed that it will dominate over more established energy loss mechanisms, for example fluorescence. Here, we use electron–electron coincidence spectroscopy to determine the efficiency of the ICD process after 2a1 ionization in water clusters. We show that this efficiency is surprisingly low for small water clusters and that it gradually increases to 40–50% for clusters with hundreds of water units. Ab initio molecular dynamics simulations reveal that proton transfer between neighboring water molecules proceeds on the same timescale as ICD and leads to a configuration in which the ICD channel is closed. This conclusion is further supported by experimental results from deuterated water. Combining experiment and theory, we infer an intrinsic ICD lifetime of 12–52 fs for small water clusters.

We present a method to utilize interatomic Coulombic decay (ICD) to retrieve information about the mean geometric structures of heteronuclear clusters. It is based on observation and modelling of competing ICD channels, which involve the same initial vacancy, but energetically different final states with vacancies in different components of the cluster. Using binary rare gas clusters of Ne and Ar as an example, we measure the relative intensity of ICD into (Ne + ) 2 and Ne + Ar + final states with spectroscopically well separated ICD peaks. We compare in detail the experimental ratios of the Ne-Ne and Ne-Ar ICD contributions and their positions and widths to values calculated for a diverse set of possible structures. We conclude that NeAr clusters exhibit a core-shell structure with an argon core surrounded by complete neon shells and, possibly, further an incomplete shell of neon atoms for the experimental conditions investigated. Our analysis allows one to differentiate between clusters of similar size and stochiometric Ar content, but different internal structure. We find evidence for ICD of Ne 2s −1 , producing Ar + vacancies in the second coordination shell of the initial site.

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.

Hydration of biomolecules and pharmaceutical compounds has a strong impact on their structure, reactivity, and function. Herein, we explore the microhydration structure around the radical cation of the widespread pharmaceutical...

Known for their stable structural and thermal properties, diamondoids and their radical cations are viable candidates as carriers for diffuse interstellar bands. 1 While previous diamondoid research has mainly focused on neutral molecules and their derivatives, little is known about their radical cations, which may form in interstellar environments by ionizing radiation. 2 We report the first experimental optical spectrum of the simplest diamondoid cation, the adamantane radical cation (C 10 H 16 + ), obtained via electronic photodissociation spectroscopy at 5 K between 310-1000 nm. The optical spectrum reveals a broad peak between 420-850 nm, assigned to the D 2 ( 2 E) ← D 0 ( 2 A 1 ) transition. This feature exhibits no vibrational structure, despite an experimental temperature below 20 K, due to lifetime broadening and/or Franck-Condon congestion. A second band system originating at 345 nm does reveal a vibrational progression and is attributed to the overlapping D 5 ( 2 A 1 )/D 6 ( 2 E) ← D 0 ( 2 A 1 ) transitions split by the Jahn-Teller effect. Comparison of the spectrum with known diffuse interstellar bands suggests that C 10 H 16 + is not likely to be a carrier. However, the strong absorption features in the UV to near IR show promise in the investigation of higher order diamondoid cations as potential candidates. 3

Interatomic coulombic decay (ICD), a radiationless transition in weakly bonded systems, such as solutes or van der Waals bound aggregates, is an effective source for electrons of low kinetic energy. So far, the ICD processes could only be probed in ultra-high vacuum by using electron and/or ion spectroscopy. Here we show that resonant ICD processes can also be detected by measuring the subsequently emitted characteristic fluorescence radiation, which makes their study in dense media possible.