Electron spectra of photoexcited Ne clusters are shown to display a signal at low kinetic energies that is neither present in the Ne monomer nor at photon energies below the inner-valence 2s threshold. These findings are strong evidence for the existence of interatomic Coulombic decays (ICD), a mechanism that was recently predicted theoretically [Phys. Rev. Lett. 79, 4778 (1997)]. In ICD, an inner-valence hole state in a weakly bonded system can undergo ultrafast relaxation due to energy transfer to a neighboring atom, followed by electron emission from this neighboring site. DOI: 10.1103/PhysRevLett.90.203401 PACS numbers: 36.40.Mr, 33.80.Eh, 34.30.+h, 82.33.Fg The nonradiative decay of a core hole vacancy in an excited atom by electron emission is a well known process in spectroscopy, commonly denoted as Auger decay. Traditionally, it is assumed that in extended aggregates -molecules or bulk matter-electrons only at the excited site actively take part in the decay and that the environment of the initially excited atom only modifies the Auger energy spectrum via its influence on the energy levels [1][2][3]. In contrast to that, for weakly bound aggregates, such as van der Waals clusters and hydrogen bonded systems, a radiationless decay mechanism has been predicted, which is possible only by electron emission from neighboring sites of the vacancy [4][5][6]. The final states populated in this so-called ''interatomic Coulombic decay'' (ICD) thus have two positive charges distributed at two different atoms of the system [7]. This lowers their total energy by shielding the Coulomb repulsion of the final state holes. For a lot of systems, it is just this shift in final state energy which makes the decay energetically possible. Where present, ICD should be an ultrafast relaxation pathway, which proceeds on time scales of 1-100 fs. It is therefore expected that it dominates over competing channels, like radiative decay or relaxation involving the nuclear dynamics.Although recently an indirect indication of the relevance of interatomic transitions in the core vacancy decay of molecules has been found [8], an experimental verification of the effect is missing. The major reason for this is probably the low kinetic energy of the electrons emitted by ICD, which is typically a few eV. The unambiguous identification of these electrons would be the only distinct proof for the process. However, the region close to zero kinetic energy in the electron spectrum of a bulk material exposed to vacuum ultraviolet radiation is dominated by inelastically scattered photoelectrons from the valence band. The same is true for clusters, but due to the limited spatial dimensions of the system short range effects like ICD make a relatively higher contribution to the signal close to zero kinetic energy.In this Letter we present direct experimental evidence for ICD in the photoexcited electron spectra of small Ne clusters.In the following, we first introduce the ICD mechanism in some detail. Excited electronic states can decay in two ways, ...

Valence and C1s core level photoelectron spectra of gaseous alanine have been recorded with synchrotron radiation. Using ab initio Green's Function calculations of the vertical outer valence ionization energies and CMS-XR calculations of the orbital ionization cross-sections, it is possible to account well for the features of both the new hν ) 92 eV valence photoelectron spectrum and also its differences with an earlier hν ) 21.2 eV spectrum. Good agreement may be achieved by considering just the contribution of a single molecular conformation. This agrees with previous experimental findings, but conflicts with calculations which suggest that a range of molecular conformations would coexist in an equilibrium sample. A study of the valence photoelectron spectrum of the amino acid threonine complements that of alanine, but unlike the latter is limited by the effects of thermal decomposition of the sample. The C1s core level spectrum of alanine is reported and its peaks are assigned to ionization of the three C atoms in the molecule. A fourth minor peak that is observed is tentatively assigned to a peptide CONH 2 linkage which may be formed between alanine monomers.

Inner-shell photoelectron spectra of the N 1s level in N 2 have been measured with sufficient resolution to resolve the splitting between the gerade and ungerade components. The selective enhancement of the gerade component on the N 2 σ shape resonance is clearly seen, confirming that the resonant behavior is mainly caused by the σ u channel. The splitting of the two components is found to be 97(3) meV.