In this work we deal with the relaxation of the 2p core hole in transition metals via LVV Auger decay, a specific aspect in itself, though contributing a piece of information for a more general understanding of charge redistribution in the final states of both primary photoemission and following de-excitation process. In the past, the analysis of the process was performed mainly by high energy photoemission spectroscopy (PS) experiments, either standard PS [1], [2] or coincidence spectroscopy [3], [4]. We have carried out a study of the Coster Kronig (CK) decay in Fe, Ni and NiO by PS using synchrotron radiation. In particular, we have measured the branching ratio β between the direct L2VV and L2L3V→L′3VVV CK transitions. β has been determined as a function of the photon energy in the proximity of the L2 edges. For all samples β becomes independent upon the photon energy already at few eV above threshold. Nonetheless, the asymptotic value is found to be larger in the metals than in the insulator, this reflecting the different localization of the electronic states involved in the cascade process. Capability and limitations of the method used for the quantitative evaluation of β are also discussed.

We performed an extended X-ray absorption fine structure (EXAFS) investigation of ultrathin Fe films grown in situ on Ni(100) surface. From the first shell analysis of the EXAFS oscillations we obtained values of the bond length, of the effective coordination number and of the Debye–Waller factor that are compatible with a model where there is coexistence of face centred cubic phase (with tetragonal distortion) and of body centred cubic (bcc) phase in the films. We also performed theoretical simulations of the EXAFS data including signals from higher order shells with multiple scattering effects. From these simulations we obtained that the experimental data can be well reproduced with a linear combination of spectra, one reproducing the bulk bcc Fe structure and one obtained by a face centred tetragonal (fct) lattice structure with an interlayer distance d=1.9 Å. In the model the relative weights of the two spectra change with an increasing value for the weight corresponding to the bcc phase. At 20 ML, the weight for the fct phase (30–40%) is still considerable.

The atomic geometry and growth mechanism of Fe films (0.5-20 monolayers (ML)) epitaxially grown on Ni(001) have been investigated by a multitechnique approach with the twofold aim of assessing the reliability of quick structural techniques (primary-beam diffraction modulated electron emission (PDMEE), secondary electron imaging (SEI)), suitable for on-line monitoring of film growth, and studying the structural evolution of the Fe/Ni system. To this end, samples were analysed by PDMEE and SEI together with synchrotron radiation photoelectron diffraction (PD), and extended x-ray absorption fine structure (EXAFS). Results show the effectiveness of the approach by quick techniques. In the early stage of growth, Fe arranges in a strained fcc(001) structure. Transition to the bce phase occurs through nucleation of bcc(110) domains with the bce < 111 > //fcc < 110 > orientation. Quantitative analysis based on PD and EXAFS data is also presented. (C) 2000 Elsevier Science B.V. All rights reserved.

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0-14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fee to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the bcc[111]parallel to fcc[110] in-plane orientation. Interlayer spacings of 2.05 +/- 0.068 Angstrom, 2.01 +/- 0.03 Angstrom, and 1.85 +/- 0.03 Angstrom were found in the bcc region, between bcc and fee layers and in the fee region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was 2.49 +/- 0.02 Angstrom, in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of 2.04 +/- 0.04 Angstrom and in plane n-n distance of 2.47 +/- 0.01 Angstrom. At 14 ML the PD pattern collected over a 94 degrees azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the bcc[111]parallel to fcc[110] alignment.

The feasibility of angle resolved APECS (Auger Photo-Electron Coincidence Spectroscopy) on solids is demonstrated with an experiment performed at the ALOISA (ELETTRA, Trieste) beamline on the L3M45M45 Auger transition of the Cu(111) surface. This beamline, with its multicoincidence detection system based on several individual electron analysers, was designed in order to perform such experiment with good efficiency.The correlation effects displayed by the measured angular distribution are not explained by the two step model, that conversely is commonly accepted for non coincidence experiments performed at the same energy and on the same orbital.

We have investigated the atomic geometry of Fe films on a Ni(001) crystal in the thickness range 0–25ML with the aim of following the transition from the fcc pseudomorphic structure to the equilibrium bcc phase. The structural techniques used are primary-beam diffraction modulated electron emission (PDMEE) and low-energy electron diffraction (LEED). For film thicknesses up to 5ML the films are fcc, strained in the direction of film growth. For higher Fe coverages, the films begin a transition to the bulk bcc structure which is complete after 14ML of Fe coverage. We have obtained evidence for intermixing between Fe and Ni in the first 3ML. The orientational relationship between the bcc phase and the underlying fcc substrate has also been investigated, finding a bcc (110) phase with four possible domains, each with one of the 〈111〉 directions parallel to an fcc 〈110〉 direction.

The new beamline ALOISA, now operational at the Elettra Synchrotron, is designed for surface studies by means of several experimental techniques: surface x-ray diffraction and reflectivity, photoemission spectroscopy, photoelectron diffraction, e−-Auger coincidence spectroscopy. A new monochromator has been specifically designed and realized for this multipurpose beamline: it makes use of a channel-cut Si crystal dispersive element for the 3–8 keV range and of a plane mirror-plane grating element for the 200–2000 eV range. Both dispersive elements share the same optical system. In the low energy range (200–900 eV) the spectral resolving power exceeds 5000 while maintaining a throughput higher than 1010 photons/s/200 mA/0.02% BW. In the case of the N2 1s→π* and Ne 1s→3p transitions, the extremely high signal-to-noise ratio of the absorption spectra allowed a very accurate determination of the corresponding natural linewidth (116±2 and 250±10 eV, respectively). Moreover, the vibrational structure of the CO–oxygen 1s→π* transition has been fully resolved. In the high energy range, the measured flux exceeds 1010 photons/s/200 mA up to 6.5 keV with a resolving power of ∼7500.

The new beamline ALOISA is now working at the Elettra synchrotron facility. It is equipped with a monochromator of new design to cover the 200 eV - 8 KeV energy range: the monochromator has two interchangeable dispersive systems, one for the low energies up to 2 KeV using reflection gratings and another with crystals for the higher energies. The monochromatic beam is finally re-focused from the fixed exit slit to inside the experimental chamber by a toroidal mirror. In this paper the results of the first resolution measurements are reported for the low energy section of the monochromator: a resolving power higher than 5000, with peaks of 10000 in the 400 - 500 eV region, is obtained while using all the optical elements at full illumination.

This work reports a successful electron-electron coincidence experiment performed in grazing-angle reflection geometry. (e,2e) measurements with a 300-eV impact energy have been carried out on a clean highly oriented pyrolytic graphite surface and the feasibility of binding-energy spectroscopy with quasimomentum discrimination has been established. Evidence is given that direct impact ionization of the valence electrons is the dominant ionization mechanism in the highly asymmetric kinematics used here.

The possibility of using the reflection grazing angle (e,2e) technique as a binding energy and/or momentum spectroscopy of surface states rests on the accurate knowledge of the ionisation process. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geometry: a single inelastic collision at large momentum transfer or a double collision (elastic plus inelastic). In this paper are presented the results of (e,2e) experiments that allow to elucidate the ionisation mechanism at intermediate energies (300 eV) and asymmetric kinematics. The measurements, performed on highly oriented pyrolitic graphite, also show that an overall energy resolution as good as 1.2 eV can be achieved.

The possibility of using the grazing angle (e,2e) technique as a binding energy and/or momentum spectroscopy of surface states rests on the accurate knowledge of the ionisation mechanism and on the capability of achieving a sufficiently good energy resolution. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geometry: a single inelastic collision at large momentum transfer or a double collision (elastic plus inelastic). In this paper are presented the results of new (e,2e) experiments that allow to elucidate the ionisation mechanism at intermediate energies (300 eV) and asymmetric kinematics. The measurements, performed on highly oriented pyrolitic graphite, also show that an overall energy resolution as good as 1.2 eV can be achieved.