D.R. Warburton

The electronic structure of SrTiO3 from a direct-transition analysis of angle-resolved photoemission data

Abstract Angle-resolved photoemission spectra of SrTiO3(100) have been recorded in the range 18eV⩽ hν⩽ 50eV. The results have been interpreted using a direct transition model and a free-electron like final state, yielding empirical Δ-line dispersion relations. While in overall agreement with available band structure calculations, significant discrepancies are revealed. In addition, resonance photoemission above the Ti 3p threshold has been used to identify regions of Ti 3d mixing in the mainly 0 2p valence bands, the results being consistent with theoretical models.

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.

On the dominance of an indirect mechanism for photon stimulated ion desorption from SrTiO3(100)H2O

Abstract The adsorption of H 2 O on fractured SrTiO 3 (100) has been studied using photoemission and photon stimulated ion desorption (PSID). The photoemission results are, by comparison with data obtained on an Ar + bombarded/annealed surface, consistent with dissociative H 2 O adsorption. PSID spectra recorded above the SrL 1 and TiK edges indicate that the dominant mechanism for ion desorption is X-ray induced electron stimulated desorption.

NEXAFS fingerprinting of TiO2(110)-SO2

Abstract Polarisation-dependent S K-edge NEXAFS has been used to monitor the progression of the thermally-activated reaction between TiO 2 (110) and SO 2 .SO 2 chemisorbs at 105 K, reacting to form a surface sulphate-like species as the substrate is heated to ⩾ 135 K. This is the final reaction product, having a coverage of 0.1 ML up to at least 450 K. A sulphite-like species is identified as an intermediate in this reaction, having a maximum concentration of ∼ 0.2 ML at 155 K. Structural models are suggested in which SO 2 initially bonds to Ti sites, subsequently forming complex anion-like species by incorporation into the raised row of bridging O atoms.

Determination of sulphur coordination to the two-fold hollow site of Ni(110) using polarisation-dependent sexafs

We have used the technique of surface EXAFS (SEXAFS) to study the structure of the Ni(110)c(2×2)S adsorption phase. The SEXAFS measurements employed the Auger-yield mode, monitoring S KLL emission above the S K-edge in both near-normal and grazing incidence geometries. The data show that S lies in the rectangular hollow site, bonding to one Ni atom in the second layer and to four atoms in the surface plane, with bond distances of 2.23±0.04 and 2.31±0.02 A, respectively. This indicates a first-nickel-layer expansion of ≈12% from the bulk value, in broad agreement with the results of other techniques.

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.

Transition-metal monoxides: band or Mott insulators? Angle-resolved photoemission results for CoO

Evidence of itinerant band-like behaviour of the Co 3d states in paramagnetic CoO has been observed in CoO(001) angle-resolved photoemission spectra. Two d bands are identified, which disperse 0.4 eV and 1.7 eV between Gamma and X. In broad terms the results are consistent with a recently proposed model of the electronic structure in which the insulating gap arises largely from an exchange potential perturbation of the one-electron band structure.

TiO2(100) structure-reactivity relationship

The influence of point defects (oxygen vacancies) and steps on the reactivity of TiO2(100) has been studied with LEED and soft X-ray photo-emission using H2O as a model adsorbate. An O termination of the TiO2(100)1*1 surface is deduced by comparison of surface core-level shift data for this surface and TiO2(001); ordering of O vacancies produced in the surface layer by annealing gives rise to a TiO2(100)1*5 reconstruction. The interaction of H2O with 1*1 and 1*5 TiO2(100) surfaces in the range 120 to 450 K is almost identical, indicating that the ordered oxygen vacancies of the 1*5 surface do not play a major role in the surface chemistry. A comparison of planar and 3 degrees vicinal TiO2(100)1*1 reactivity shows that steps also play a spectator role. Although unreactive at room temperature, H2O adsorbs molecularly at 120 K. Thermally induced dissociation occurs on annealing, to form Ti-O-H species with a bond angle of approximately 90 degrees .

Evidence of itinerant 3d-electron character in the angle-resolved photoemission spectra of CoO

Evidence of itinerant (band-like) behaviour of the Co 3d states in paramagnetic CoO has been adduced from CoO(001) angle-resolved photo emission spectra. Two d bands are identified, which disperse 0.4 eV and 1.7 eV between Γ and X. In broad terms the results are consistent with a recently proposed model of the electronic structure in which the insulating gap arises largely from an exchange potential perturbation of the one-electron band structure.

PSID SEXAFS Studies of Silicon Surface Chemistry

Photon stimulated ion desorption (PSID) surface EXAFS (SEXAFS) showed early promise, being successfully applied to the Mo(100)/0 system [1]. The technique offers a means to determine the coordination geometry of a substrate atom to which the desorbed adsorbate ion was bonded. Information regarding the adsorbate coordination geometry can also be obtained, although electron and fluorescence yield techniques can usually be used for this purpose if a majority surface species is the subject of interest. The surface specific nature of the PSID yield is unique amongst probes of the surface absorption coefficient. This conveys the possibility of using PSID SEXAFS to determine the coordination geometry about a substrate atom bonded to hydrogen or, for instance, the bond geometry of -OH on an oxide surface. In addition, the bond geometry of minority surface species can be probed, as in the Mo(100)/0 study. Clearly, PSID SEXAFS is potentially a very powerful tool in surface structure determination.