Plasmon single- and multi-quantum excitation in free metal clusters as seen by photoelectron spectroscopy.

Plasmons are investigated in free nanoscale Na, Mg, and K metal clusters using synchrotron radiation-based x-ray photoelectron spectroscopy. The core levels for which the response from bulk and surface atoms can be resolved are probed over an extended binding energy range to include the plasmon loss features. In all species the features due to fundamental plasmons are identified, and in Na and K also those due to either the first order plasmon overtones or sequential plasmon excitation are observed. These features are discussed in view of earlier results for planar macroscopic samples and free clusters of the same materials

Probing aqueous ions with non-local Auger relaxation

Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na+, Mg2+, and Al3+ ions. We unambiguously identify the ICD contribution to the K-edge Auger spectrum. The different strength of the ion-water interactions is manifested by varying intensities of the respective signals: the ICD signal intensity is greatest for the Al3+ case, weaker for Mg2+, and absent for weakly-solvent-bound Na+. With the assistance of ab initio calculations and molecular dynamics simulations, we provide a microscopic understanding of the non-local decay processes. We assign the ICD signals to decay processes ending in two-hole states, delocalized between the central ion and neighbouring water. Importantly, these processes are shown to be highly selective with respect to the promoted water solvent ionization channels. Furthermore, using a core-hole-clock analysis, the associated ICD timescales are estimated to be around 76 fs for Mg2+ and 34 fs for Al3+. Building on these results, we argue that Auger and ICD spectroscopy represents a unique tool for the exploration of intra- and inter-molecular structure in the liquid phase, simultaneously providing both structural and electronic information

Time of flight photoelectron momentum microscopy with 80 500 MHz photon sources electron optical pulse picker or bandpass pre filter

The small time gaps of synchrotron radiation in conventional multi bunch mode 100 500 MHz or laser based sources with high pulse rate 80 MHz are prohibitive for time of flight ToF based photoelectron spectroscopy. Detectors with time resolution in the 100 ps range yield only 20 100 resolved time slices within the small time gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows electron optical pulse picking with any desired repetition period. Aberration free momentum distributions have been recorded at reduced pulse periods of 5 MHz at MAX II and 1.25 MHz at BESSY II . The approach is compared with two alternative solutions a bandpass pre filter here a hemispherical analyzer or a parasitic four bunch island orbit pulse train, coexisting with the multi bunch pattern on the main orbit. Chopping in the time domain or bandpass pre selection in the energy domain can both enable efficient ToF spectroscopy and photoelectron momentum microscopy at 100 500 MHz synchrotrons, highly repetitive lasers or cavity enhanced high harmonic sources. The high photon flux of a UV laser 80 MHz, lt;1 meV bandwidth facilitates momentum microscopy with an energy resolution of 4.2 meV and an analyzed region of interest ROI down to lt;800 nm. In this novel approach to sub m ARPES the ROI is defined by a small field aperture in an intermediate Gaussian image, regardless of the size of the photon spo

Ag-oxide signature in Ag 3d photoelectron spectra : A study on free nanoparticles

Over decades the Ag 3d-level binding energy has been puzzling researchers with its unusual sign and value in silver oxides. For the absolute majority of metals, the metal-to-oxide binding energy shifts are positive and depend significantly on the oxidation state, while in Ag-oxides the oxide shift was time after time reported negative, small, and close for the two very different Ag(I) and Ag(III) oxidation states. In the current work, a photoelectron spectroscopy (PES) investigation on the in -situ created free nanoparticles simultaneously containing both metallic silver and silver-oxide parts provided the grounds to reconsider the old consensus on the Ag-oxide shifts. The Ag 3d energies for the metallic and the oxide parts established in the current experimental work allowed estimating a approximate to 1.2 eV positive shift for the realized oxidation state. This estimate was made possible by using a beam of free nanoparticles with finely controlled composition. The PES experiments on such a beam allowed for a continuous and fast renewal of the poorly conducting sample and for a reliable and accurate calibration relative to vacuum. The constant oxide shift observed at several different oxidation conditions, as well as the relatively narrow and symmetric oxide peaks, point to one dominating oxidation state being present in the particles

Charge delocalization dynamics of ammonia in different hydrogen bonding environments: free clusters and in liquid water solution

Valence and core level photoelectron spectra and Auger electron spectra of ammonia in pure clusters have been measured. The Auger electron spectra of gas-phase ammonia, pure ammonia clusters and ammonia in aqueous solution are compared and interpreted via ab initio calculations of the Auger spectrum of the ammonia monomer and dimer. The calculations reveal that the final two-hole valence states can be delocalized over both ammonia molecules. Features at energies pertaining to delocalized states involving one, or more, hydrogen bonding orbitals can be found in both the ammonia cluster Auger electron spectrum and in that of the liquid solvated molecule. The lower Coulombic repulsion between two delocalized valence final state holes gives higher kinetic energy of the Auger electrons which is also observed in the spectra. This decay path-specific to the condensed phase-is responsible for more than 5% of the total cluster Auger intensity. Moreover, this interpretation is also applicable to the solid phase since the same features have been observed, but not assigned, in the Auger spectrum of solid ammonia

A dose dependence study of O-2 adsorbed on large Ar clusters

An investigation of the behavior of O-2 molecules in and on O-2-doped large ( similar or equal to 8000) Ar host clusters has been performed by means of core and valence photoelectron spectroscopy. Data from pure O-2 and Ar clusters, as well as from O-2-doped Ar clusters, are presented. The experimental data together with calculations of the binding energy shifts of oxygen molecular ions in and on the surface of a large host Ar cluster show that the diffusion behavior has a strong dependence on the doping pressure. We conclude that the oxygen molecules in the doped Ar host do not partake in band formation, since there is clear vibrational resolution in the spectral features stemming from screened O-2(+) ions. This implies that valence photoelectron spectroscopy can be used to determine the geometrical structure of this and certain, similar, cluster systems. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3148883

Preferential site occupancy observed in coexpanded argon-krypton clusters

Free heterogeneous argon-krypton clusters have been produced by coexpansion and investigated by means of x-ray photoelectron spectroscopy. By examining cluster surface and bulk binding energy shifts, relative intensities, and peak widths, we show that in the mixed argon-krypton clusters the krypton atoms favor the bulk and argon atoms are pushed to the surface. Furthermore, we show that krypton atoms in the surface layer occupy high-coordination sites and that heterogeneous argon-krypton clusters produced by coexpansion show the same surface structure as argon host clusters doped with krypton. These observations are supported by site-dependent calculations of chemical shifts

The far from equilibrium structure of argon clusters doped with krypton or xenon

Heterogeneous clusters created by doping Ar host clusters with Kr or Xe are shown to have radically different structures from the mixed clusters of the same type created by co-expansion of Ar-Kr or Ar-Xe gas mixtures. In contrast to the co-expansion case, the doped mixed clusters can be produced with Kr or Xe on the surface and Ar in the bulk. With the doping technique it is thus possible to control the surface composition of a specific cluster. A study of the cluster properties as a function of the doping pressure is also reported for the case of Ar clusters doped with Xe. The clusters have been studied by means of synchrotron radiation based X-ray photoelectron spectroscopy

Preferential site occupancy of krypton atoms on free argon-cluster surfaces

Argon clusters have been doped with krypton atoms in a pick-up setup and investigated by means of ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). The width of the krypton surface feature in the XPS spectra from mixed krypton/argon clusters has been studied and found to be narrower than in the case of homogeneous krypton clusters. By considering known spectral broadening mechanisms of the cluster features and the electron binding energy shift of the cluster surface feature relative to the atomic signal, we conclude that krypton ad-atoms preferentially occupy high-coordination surface sites on the argon host-cluster. (c) 2006 American Institute of Physics