D. Norman

Surface relaxation of SrTiO3(001)

Surface X-ray diffraction has been used to examine the 300 K structure of SrTiO3(001)1×1 with a termination of 78% TiO and 22% SrO. The data indicate that a lateral ferroelectric distortion is absent on both terminations, consistent with a recent theoretical calculation. Although the negligible Ti atom relaxation found on the TiO termination is consistent with medium-energy ion scattering data, it differs from recent density functional theory calculations by 0.13 and 0.16 A. In contrast, the Sr displacement towards the SrO surface of 0.22±0.07 A is in good agreement with theory.

Structural models for the interaction of oxygen with Al(111) and A1 implied by photoemission and surface exafs

Abstract New results are reported for the Extended X-ray Absorption Fine Structure (EXAFS) of oxygen on Al(111) at submonolayer coverages. The O-Al bond lengths are derived for both the chemisorbed phase characterized by a 1.4 eV chemical shift of the Al 2p level and the oxide-like phase formed upon heating and exhibiting a 2.7 eV shift. Comparison of the derived bond length of 1.75 ± 0.03 A for the submonolayer oxide-like phase with those in amorphous alumina (1.85 – 1.88 A) and bulk corundum (α − Al 2 O 3 ) (1.92 A) which all exhibit the same chemical shift leads to structural models for the progressive oxide formation. We show that for a predominantly ionic bond changes in bond length and coordination number are directly related. Such considerations lead to the conclusion that the short (1.76 ± 0.03 A) bond in the chemisorbed phase must exhibit considerable covalent character.

Incorporation of sulphur into the (111) surface of nickel

Abstract With the aim of monitoring coverage-dependent adsorbate coordination, we have studied the polarisation-dependent S K-edge SEXAFS of p(2 × 2)S and ( 5 3 × 2) S overlayers on Ni(111). The data indicate S bonding to the three-fold hollow sites in the p(2 × 2)S phase, with a SiNi bond distance of 2.23 ± 0.02 A. In the higher coverage, ( 5 3 × 2) S phase the S-Ni distance is 2.27 ± 0.02 A. The Ni (111)(5 3 × 2) S results are compared with those expected on the basis of the three proposed structural models of this surface. The data are consistent with only one of these models, in which S occupies rectangular hollow sites in a pseudo-Ni(100)c(2 × 2)S overlayer.

A PSID SEXAFS study of H2O adsorption on Si(100)

Abstract The technique of photon stimulated ion desorption (PSID) surface EXAFS (SEXAFS) has been applied in a study of the Si (100) H 2 O surface. Information on the Si surface geometry is derived by comparison of the H + ion yield Si K-edge SEXAFS data with theoretical models of the Si(100) surface. Within the constraints of a single dimer model geometry and hydrogen bonding to the upper atom of an asymmetric dimer, the data favour both the symmetric model of Levine and the asymmetric model of Chadi.

A surface exafs study of the Vanadium/Si(111) interface

Surface EXAFS spectra recorded above the K-absorption edge of the refractory metal vanadium for submonolayer coverages on the Si(111)7 × 7 surface are presented. Analysis of spectra recorded after depositions at 300 K. gives a VSi bond length of 2.57 ± 0.03 A. By comparison with possible sites on an unreconstructed Si(111) surface the data most favour the six-fold interstitial site between the first and second Si layers. No evidence was found for V cluster formation at submonolayer coverages. The results therefore indicate some degree of reactivity/ intermixing at the room temperature VSi interface, unlike previous reports. SEXAFS spectra recorded for a ∼ 2 monolayer V overlayer following annealing at 825 K for 5 min indicated the formation of the disilicide, VSi2, as previously reported.

The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction

The ‘clock’ reconstruction of the Ni(100)(2×2)p4g–N system has been measured using surface X-ray diffraction. An in-plane displacement of the surface layer Ni atoms of dxy=0.30±0.01 A was found. This value is smaller than that measured for this system using photoelectron diffraction (PED) and surface extended X-ray absorption fine structure (SEXAFS). Possible reasons for this are discussed.

Partial densities of states in amorphous alloys from resonant photoemission and Cooper minima

Abstract Synchrotron radiation has been used to investigate resonant photo- emission in amorphous metal systems Cu x Zr 100− x , Cu x Ti 100− x and Fe x Zr 100− x . Constant initial state spectra reveal resonant enhancement of Zr 4 d -like conduction band states near the Fermi level in Cu 45 Zr 55 and 3 d -like Ti states in Cu 33 Ti 67 at np → nd phonon eexcitation energies; this is in contrast to the absence of resonance effects in a lower lying “filled” Cu 3 d -like band. Observation of a Cooper minimum in the photoemission cross section in Cu 45 Zr 55 gives further support to the importance of Zr 4 d states near the Fermi edge. In contrast both conduction band features in Fe 90 Zr 10 show Fe 3 p → 3 d resonant enhancement with some evidence of Zr 4 p → 4 d resonance. This is consistent with Cu 45 Zr 55 and Cu 33 Ti 67 exhibiting split band behaviour, while the d electrons in Fe 90 Zr 10 appear to form a common band.

OPERATION AND OXIDATION OF THERMIONIC DISPENSER CATHODES STUDIED BY HIGH RESOLUTION PHOTOEMISSION

Thermionic dispenser cathodes are practical devices whose action is dominated by their surface properties. At their operating temperature of 1450 K a near-monolayer barium oxide coating in dynamic equilibrium maintains a low work function. We have used high resolution synchrotron radiation x-ray photoelectron spectroscopy (SRXPS) to help elucidate the mechanism of operation and poisoning (by oxidation) of two types of cathode, one made from a pure tungsten matrix (B type) and the other coated with an osmium/ tungsten alloy (CD type). SRXPS measurements have been made on cathodes both at room temperature and at their operating temperature. Comparison between these confirms that room temperature experiments are representative of operational cathodes, with no significant change in the chemical state of the surface species. For the B type cathode, the core level binding energies are consistent with the presence of a single layer of barium oxide at the surface, while for the CD type, the Ba is shifted to a low...

Partial densities of states in amorphous alloys from resonant photoemission: CuTi, CuZr, CuHf

Abstract Synchrotron radiation has been used to investigate resonant photoemission in the amorphous metal systems CuTi, CuZr and CuHf. Constant initial state spectra reveal resonant enhancement at n p → n d photon excitation energies of conduction band states near the Fermi level; these are Ti3d-like states in CuTi, Zr4d-like states in CuZr, and Hf5d-like states in CuHf. In contrast there is an absence of such resonance effects in a lower lying “filled” Cu3d-like band. The results are consistent with CuTi, CuZr and CuHf all exhibiting split band behaviour, with little hybridisation between Cu3d and transition metal n d states.

Resonant photoemission studies of amorphous Ca based alloys

Abstract Photoemission from a synchrotron source along with in situ resistivity measurement is used to study conduction hand states in amorphous Ca x Al 1− x and Ca x Mg 1− y alloys. In the mid-composition range CaAl shows an anomalously broad Fermi edge, while CaMg behaves normally at all compositions. The 3p → 3d giant resonance in Ca leads to resonant photoemission from the conduction band of Ca metal but it greatly decreases in strength with increasing Al concentration.