J.A.D. Matthew

Transition rates for interatomic Auger processes

Abstract Theoretical estimates in the LCAO approximation reveal that matrix elements for interatomic Auger processes are very strongly energy dependent, but that for very low energy Auger transitions they may be comparable with the competing intra-atomic Auger processes. Though inter-atomic Auger mechanisms will not produce appreciable lifetime broadening, they may radically change the low energy Auger spectrum, for example, of metallic elements in the early stages of oxidation, and may mainly control the transition rates of low energy Auger processes in ionic crystals.

Background intensity determination in AES/XPS

Abstract Some methods for deconvolution of the background intensities of inelastically scattered electrons from observed AES/XPS spectra are discussed. It is found that a frequently used empirical deconvolution method introduces significant inaccuracies. The Tougaard deconvolution formula is generalised to include elastic scattering effects, whose effects are considered for a range of electron transport models.

Electronic structure of the surface F centre in ionic crystals

Theoretical calculations for the Fs centre (electron trapped at a surface anion vacancy) on the (001) surface of ionic crystals of the NaCl structure are presented. The one electron Schrodinger equation is solved numerically for the ground state and crystal field split excited state for various crystal potentials. The differences between the point ion potential and model potentials incorporating core repulsion are investigated, and surface relaxation, rumpling and electronic polarisation are also considered. Hyperfine constants for the ground state of MgO are in reasonable accord with experiment, but calculated transition energies do not seem to correspond to those observed in reflectance measurements, in accordance with Hartree-Fock cluster calculations on the same system.

Quasi-atomic fine structure in the Auger spectra of solid silver and indium

Abstract High resolution M 4 , 5 N 4,5 N 4,5 Auger spectra of Ag and In reveal fine structure, which may be interpreted in terms of spin-orbit splitting of the initial state and multiplet structure in the final state. Interaction between d holes in Ag is an important factor in determining the shape of the Auger spectrum, and so the Auger profile is not related in any simple way to the one-particle density of states.

Infrared reflectance and magnetorefractive effects in metal–insulator CoFe–Al2O3 granular films

The infrared reflectance of (Co50Fe50)x–(Al2O3)1−x granular films which exhibit tunneling magnetoresistance has been measured in the wavelength range 2–25 μm for compositions both above and below the critical percolation volume fraction. The modifications of the Al2O3 Restrahlen reflection region are well accounted for in an effective medium model. In addition, frequency dependent changes of the reflection have been observed under the application of magnetic fields up to 12 kOe. At the longitudinal optic reflection edge around 9 μm distinctive magnetic signatures are found below and above the percolation threshold φc at x=0.17.

Electron energy loss spectrum of cyanogen on Pt (100)

Abstract Electron energy loss spectra for adsorbed cyanogen on Pt(100) are presented, and discussed in terms of possible models suggested by other techniques. Adsorbate induced loss features are found at 11 and 14 eV, and these are associated with levels below the Fermi level as observed in ultra-violet photoemission. As in the case of CO adsorption losses in the adsorbed phase occur at energies significantly larger than in the gas phase, indicating an upward shift of the final 2π ∗ state due to mixing with metal orbitais. Thermal desorption studies of C 2 N 2 on Pt clearly resolve α and β phases, but there is some controversy over whether the β phase involves mainly single CN units bonding to the metal, whether C 2 N 2 is molecularly adsorbed, or whether paracyanogenlike structures form at the surface. The electron spectroscopic evidence is examined, and is shown on balance to support adsorption in some molecular form.

Valence excitations of rare earths by electron energy loss spectroscopy

Abstract Electron energy loss spectroscopy in reflection mode has been used to probe low energy excitations ( Δ E Δ E Δ E Δ E

An experimental study of bonding and crystal structure modifications in MoSi2 and MoSi2+xAl (x=10 to 40 at% Al) via Auger parameter shifts and charge transfer calculations

Abstract The alloying behaviours of as-cast MoSi2 and MoSi2+xAl alloys have been studied using high-energy XPS with Cr Kβ radiation. The charge transfer occurring upon alloying was calculated using the variations in the Auger parameters of Mo, Si and Al between alloyed and unalloyed conditions and the linear potential model of Thomas and Weightman and the non-linear potential model of Cole, Gregory and Weightman. In MoSi2 there was a significant increase in the Auger parameter of Si, while the shift in the Auger parameter of Mo was negligible. The charge transfer towards the Si atoms was close to zero and is smaller compared to theoretical calculations. It is concluded that the atomic bonding between Mo and Si is of a covalent p–d character. In MoSi2+xAl alloys, similar observations were made for Mo and Si, while the Auger parameter of Al was reduced. Donation of electronic charge by Al atoms is possible; covalent bonds of Al with Mo are formed. The plasmon loss structures of the Si 1s and Al 1s peaks showed reduced intensity in the alloys relative to the pure metals. This was attributed to more strongly bound valence electrons. The opposite was the case for the Mo 2p3/2 peak. The substitution of Si by Al atoms is confirmed, in agreement with previous studies.

Auger spectra of CO, C2H4, C2N2 and C6H6 adsorbed on Pt(111)

Abstract Electron excited carbon KVV Auger spectra of CO, C 2 H 4 , C 2 N 2 and C 6 H 6 adsorbed on Pt(111) are compared. By estimating the effective Coulomb interaction between the final-state holes it is possible to associate some features with transitions observed in free molecule spectra, but others must involve at least one electron with energy within the conduction band of the metal. Such “cross-transitions” are associated with strong 2π* character of filled states in the presence of a core hole in molecules such as CO.

Emissivity — a remote sensor of giant magnetoresistance

The link between emissivity in the mid-to-far-infrared regime and electrical conductivity is used to probe magnetoresistance. A direct relationship between the change in radiated flux and giant magnetoresistance is confirmed by experiment. The potential for spatially resolved measurements is also considered.