S. D'Addato

The localisation of 3d hole states in Fe and FeAl studied by Auger vacancy satellite spectroscopy

Abstract For Ni, Cu and Zn the occurrence of the L2L3M4,5 Coster-Kronig transitions give rise to L3M4,5-M4,5M4,5 Auger vacancy satellite transitions to low kinetic energy of the main L3M4,5M4,5 transitions. In pure Fe the intensity of the Auger vacancy satellites are negligibly small even though L2L3M4,5 Coster-Kronig occur, a fact which has been attributed to the delocalisation of the M4,5 (3d) hole state within the lifetime of the L3 hole. In this work we present Auger spectra for Fe and FeAl excited using radiation obtained on station 5U.1 of the Daresbury synchrotron. Our spectra support earlier work regarding the relative lifetime of the L3 and M4,5 hole state in Fe though our spectra are different in detail. We also show that the Fe 3d hole is significantly more localised in FeAl than in Fe.

VARIATIONS IN THE LIFETIME OF 3d HOLE STATES IN ULTRATHIN Fe FILMS GROWN ON Cu(100) DEDUCED FROM THE LMM AUGER SPECTRA OF Fe

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L2 ionisation edge, and in coincidence with the Fe 2p3/2 and Fe 2p1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M4,5 holes of L3M4,5 double hole states are localised for longer than the L3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

Adsorbate-induced de-reconstruction in the interaction of H2S with Ge(001) 2×1

The room temperature adsorption of H2S on Ge(001)2*1 has been studied using surface-extended X-ray-absorption fine structure, low-energy electron diffraction and Auger electron spectroscopy. The Ge(001)2*1-S system formed by a saturation dose followed by an anneal to remove H has a S-Ge bond length of 2.36+or-0.05 AA, with S sitting in twofold bridge sites, consistent with breaking of the Ge-Ge dimer reconstruction. Further adsorption of H2S at low or elevated temperatures is kinetically unfavourable, and it is proposed that this is consistent with the surface Ge atoms sitting in non-ideal rather than bulk-terminated positions.

An X-ray absorption fine structure study of Ge(001)(2×1)-S

The Ge(001)(2×1)-S overlayer formed by dosing clean Ge(001) with H 2 S at room temperature and annealing to remove H has been studied using surface extended X-ray absorption fine structure (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS). The results indicate that the S atoms sit in bridge sites on the surface with a bond length of 2.36±0.05 A. This is consistent with the breaking of the Ge-Ge surface dimers, and is thus an example of the phenomenon of adsorbate-induced dereconstruction. Comparison is made with a previous study of this system using angle-resolved photoemission fine structure (ARPEFS) and with experimental and theoretical studies of the related Ge(001)(1×1)-S overlayer formed by dosing with S 2

Charge transfer and redistribution in the formation of the K/GaP(110) interface: a photoelectron spectroscopy study

We present the results of a photoelectron spectroscopy study of the K/GaP(110) interface grown at T = 120 K. We have investigated the system looking at the Ga 3d and P 2p core level spectra by analysing their lineshape. This analysis shows that the transferred charge in the adatom-substrate bonding formation affects the electronic and chemical environment of both surface Ga and P atoms. The results suggest relevant differences with the intuitive picture of strong ionic localized bonding between the alkali atom and surface Ga. They are instead more consistent with a delocalized redistributed valence charge between the adsorbate and the substrate. We also conclude that the system at ML is metallic, with an increasing disorder and inhomogeneity at the highest coverage studied. The similarities and differences with the Na/GaP(110) and the other alkali metal/GaAs(110) systems are discussed.

Structural and magnetic properties of Ni/γFe/Ni multilayers

Abstract The crucial point in the characterization of magnetic interfaces is the knowledge of the morphology and the structure of the films. Ni/γ Fe/Ni multilayers were grown on three different buffer layers. Structure and morphology were investigated by X-ray diffraction and scanning tunneling microscope. Mossbauer spectroscopy was used to provide local information on both structure and magnetism of the buried Fe layers.

Coverage-dependent azimuthal alignment of SO2 on Ag(110)

Abstract The polarisation dependence of S K -edge NEXAFS has been used to study the adsorption geometry of SO 2 on Ag(110). The molecular plane is azimuthally aligned along the [001] azimuth at 1 3 monolayer coverage, but rotated from [001] by about 55° at 1 2 monolayer, with the molecular upright at both coverages.

Charge-Transfer Satellites in K L23 XAS Data for K/Si(111)-(2 × 1): Evidence for Strong Ionic Bonds

We report K L23 edge X-ray absorption spectra (XAS, both normal and grazing incidence) and photoelectron spectra (XPS) for K/Si(111)-(2 × 1) at three different coverages (0.1, 0.3 and 0.5 ML). The data show features which exhibit a very strong variation in both the energy, intensity, and line shape with incidence angle and coverage. Some of these features correspond to charge-transfer satellites and give a measure of the hybridization of the ground state. They indicate very ionic bonding.

Experimental and theoretical study of the PL2,3VV Auger lineshape of GaP(110)

Abstract A joint experimental and theoretical investigation of the PL 2.3 VV Auger transition on GaP(110) is presented. The theoretical data, calculated within a pure band-like picture, are compared to the experimental spectra. The angular momentum character of the five features constituting the lineshape is identified. Strong contributions to the experimental data from many-body effects may be ruled out due to the good overall agreement with the theoretical lineshape therefore supporting a single particle description for this Auger feature. We discuss effects such as Coulomb correlation, initial state core hole screening, self-energy corrections, that may possibly contribute to the relative energy shifts observed in one of the pp components of the compared spectra.

Dispersion effects and electron energy loss of silicon surface

A detailed calculation based on a phenomenological three-layer model of the electron energy loss spectrum of Si(111) 2×1, in the region of dangling bond excitation, was performed. The model results are discussed and compared with existing experimental results. The discussion shows the important role played by the kinematical pre-factor in determining the loss peak positions. The comparison with the experiment reinforces the thesis of interface plasmon excitation in the scattering process and indicates the need of a q‖ dependent surface dielectric constant to describe the dielectric properties of the surface layer.