S Iacobucci

Electron–electron coincidence spectroscopies at surfaces

Abstract In the past 20 years, a steadily increasing number of electron–electron coincidence experiments on atoms and molecules have contributed to a deeper understanding of electron–electron correlation effects. In more recent years this technique has been extended to the study of solid surfaces. This class of one photon IN two electrons OUT experiments will be discussed with an emphasis on grazing incidence geometry, that is expected to be particularly suited for studying surfaces. The crucial question of which is the dominant mechanism that leads to ejection of pairs of electron from the surface will be addressed. It will be shown that, depending on the kinematics chosen, the correlated behaviour of the pairs of electrons detected might be singled out from independent particle one.

An angular resolved electron energy loss investigation of highly oriented pyrolitic graphite electronic structure

Abstract Angular resolved electron energy loss measurements on highly oriented pyrolitic graphite have been performed in reflection kinematics and at grazing incidence angles. Because of the selected kinematics it has been possible to change, almost independently, either the momentum transfer parallel to the surface ( q ∥ ) or the angle between the momentum transfer and the graphite c axis ( q ^ c ). The present measurements have proved to be very sensitive to the surface structure of the sample and have allowed the assignment of the prominent loss at about 6 eV to a π → π * excitation mainly localized perpendicular to the c axis. Possible contributions from surface localized states have also been detected. This transition shows negligible dispersion in q ∥ and its energy centroid appears to be shifted with respect to previous surface and bulk measurements. Furthermore, the measured cross-sections are brought to the same relative scale for each grazing angle, and the loss and scattering functions are derived from data taken in reflection kinematics. The relevance of the kinematic prefactor in deriving transition energies from loss spectra is discussed with an emphasis on the role of q ∥ zeroes.

L2L3V Coster–Kronig decay in Fe: the near-edge region

Abstract We have investigated the Auger emission in Fe for photon energies varying across the 2p edges. The probability of the L 2 L 3 V Coster–Kronig (CK) decay process has been measured by photoemission spectroscopy using synchrotron radiation. The branching ratio between the process L 2 → L 3 ′ VV (CK preceded) and the direct Auger decay of the L 2 hole has been determined as a function of the photon energy near the 2p 1/2 -edge (717–735 eV): this branching ratio is found to rapidly converge to an asymptotic value of about 0.83, which favorably compares with the Auger-photoemission coincidence spectroscopy results.

Contribution of Surface Plasmon Decay to Secondary Electron Emission from an Al Surface

Secondary electron spectra emitted from polycrystalline Al have been measured in coincidence with 500 eV electrons reflected after undergoing a given energy loss. Contributions from bulk- and surface-plasmon decay have been observed, as well as contributions from LVV Auger electrons and electron-electron scattering.

Fully resolved kinematics of grazing-incidence (e,2e) experiments

The capability of grazing-incidence (e,2e) spetroscopy to map the spectral momentum density ρ(e,q) of the valence electronic states in surfaces was demonstrated by a pioneering experiment in which both the band structure (binding energy, e, versus momentum, q) and the momentum density, ρ, were obtained [S. Rioual et al., Phys. Rev. B 57 (1998) 2545]. (e,2e) experiments reveal, in coincidence, after energy and angle selection, the two final electrons generated by the impact of a primary electron on the surface. The momentum resolution achieved by the first experiment was limited because of the integration over the wide solid angle accepted by the ejected-electrons analyzer. This limitation has been overcome in the present work by upgrading the ejected-electrons analyzer with a position-sensitive detector (PSD). The PSD allows us to measure the azimuth angle of the ejected electrons, and hence to collect in a parallel mode (e,2e) spectra that are fully differential in momentum. Preliminary data relative to the ionization of the π-band of highly oriented pyrolitic graphite (HOPG) show that a momentum resolution better than 0.2 A -1 has been achieved without degrading the overall spectrometer luminosity.

REFLECTION (E,2E) EXPERIMENTS : A NOVEL SURFACE SPECTROSCOPY

Abstract 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.

Electronic structure of H:GaP(110)

Abstract The H:GaP(110) system was studied by EELS and AES. H induced losses were found in the 8, 12 and 18 eV region respectively. The PL 2.3 Auger lineshape was recorded as a function of emitted electron take-off angle. This method was proven to be able to identify structures in the electronic density of states at the surface both in the clean and H exposed surface. Two H induced states. located at about 2 and 4.5 eV below the valence band respectively, have been singled out.

ELECTRON-ENERGY LOSS AND AUGER-SPECTROSCOPY OF THE YB-GAP(110) INTERFACE

Abstract The Yb GaP(110) interface formation was studied by EELS (electron energy loss spectroscopy) and AES (Auger electron spectroscopy). The EELS was performed in the 2–27 eV loss range at different coverages ranging from 0.5 to 20 ML (monolayer) with a primary beam energy of 100 eV. Losses typical of the interface were found at ∼ 35 and 6 eV. The EEL data are characterized by the persistence of the 20 eV loss associated with the Ga 3d level. The AES P L2,3VV data were taken at 1 and 2 ML as a function of the take-off angle. Lineshape changes were observed both versus coverage and versus angle, allowing insight into the bonding at the P site versus coverage and depth. A model of the morphology of the interface during the growth process is presented characterized by Yb P compounds formation and Ga segregation at the surface.

SATELLITE STRUCTURES OF THE NICKEL L3 VV AUGER SPECTRUM IN NICKEL SILICIDES - A SYNCHROTRON RADIATION INVESTIGATION

We report the first synchrotron radiation investigation of the satellite structures of the Ni L3M4, 5M4, 5 Auger spectrum in the Ni compounds Ni2Si and NiSi2. The line shape of the satellites is extracted for both compounds for a set of increasing ionizing energies; the procedure used to generate the difference spectra is described. A comparision of the two sets of satellites with each other and with results reported in the literature for the case of pure Ni allows us to identify the changes in the line shape due to the different chemical environment. Possible mechanisms leading to the formation of these structures are discussed.