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...

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The K-shell excitation spectra of the hydrides water, ammonia, and methane have been measured in photoabsorption experiments using synchrotron radiation in combination with a high-resolution monochromator. For the case of methane, in particular, a wealth of spectral detail is observed which was not accessible in previous studies. The measured excitation energies and relative intensities compare well with values calculated using a complete second-order approximation for the polarization propagator. In order to determine the extent of admixing of valence excitations (i.e., transitions into virtual 0 orbitals) to the Rydberg manifolds, the X-H bond lengths have been varied in the calculations. In the case of H20, the two lowest-energy bands are due to the 0 1s-4a&/3s and 0 1s-2b2/3p transitions and have strong valence character; their width indicates that both excitations are dissociative. The NH3 and ND3 spectra are also broad which is not only due to possible dissociation but also to unresolved vibrational fine structure (v2 mode) and a Jahn-Teller instability. Valence character is concentrated in the lowest excited state in the Rydberg ns manifold, but is distributed more uniformly over the np(e) manifold. The weak dipole-forbidden C 1s -3s ( a & ) transition in CH4 and CD4 is accompanied by vibrational structure due to the v4 mode, indicating that it derives its intensity from vibronic coupling with the C 1s-3p(t2) transition. The structure on the latter band is extremely complicated due to Jahn-Teller coupling and cannot be assigned at present, as is the case for the Rydberg transitions at higher energies. The higher np Rydberg excitations contain considerable valence character. PACS number(s): 33.20.Rm 35.20. -i I. INTRQDUCTIDNE-shell excitation of the CH4, NH3, and H20 molecules has been studied in the past using both electronenergy-loss spectroscopy (EELS) [1 -4] and x-ray absorption spectroscopy [5 -9). Although the spectral resolution obtained with the latter technique has generally been rather modest and could not match that of EELS, recent advances in grazing-incidence monochromator design [10 -14] have resulted in a dramatic improvement of resolution with the "optical" approach. This has been shown in a number of high-resolution K-shell excitation spectra of small molecules reported recently (e.g. , ). In the present paper we describe measurements of the photoabsorption spectrum of gas phase H20, NH3, and CH4 in the near-E-edge region at a resolution sufficiently high to reveal directly the spectral line shape. To substantiate the assignments we have also calculated excitation energies and oscillator strengths using a polarization propagator method [20].In the virtual orbital spectrum of these hydrides and other saturated molecules one expects antibonding valence-type (cr ) orbitals which are the counterparts of the occupied bonding X-H orbitals. An important question is to what extent this antibonding valence character appears in the excitation spectrum. In most of the earlier E-shell studies on the h...

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