Showing papers by "Ronan McGrath published in 1992"

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

Abstract: 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.

The photoelectron bandstructure of molybdenum disulphide

Abstract: An experimental electronic bandstructure of molybdenum disulphide has been determined from angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements: polar-angle-dependent ARUPS at a photon energy of 21.2 eV allows the authors to obtain an approximate plot of dispersion parallel to the basal face while photon-energy-dependent ARUPS at normal emission gives a plot of dispersion perpendicular to this face. The experimental bandstructure is compared with three calculations and, while there is a general agreement, specific aspects are better reproduced when there is self-consistency in the calculations. An analysis difficulty reported earlier for polar angle ARUPS for this material can now be understood as arising from incorrect labelling of symmetry directions in the experiment.

Origin of the x-ray-absorption fine structure in photon-stimulated ion desorption from Si-adsorbate systems.

Abstract: There is no general agreement on whether photon-stimulated ion desorption (PSID) extended and near-edge x-ray-absorption fine structure (EXAFS and NEXAFS) can provide surface structural information. To address this question, we have monitored the ${\mathrm{H}}^{+}$-ion yield from Si(100)2\ifmmode\times\else\texttimes\fi{}1-H, Si(111)7\ifmmode\times\else\texttimes\fi{}7-${\mathrm{H}}_{2}$O, and Si(111)7\ifmmode\times\else\texttimes\fi{}7-H at the substrate Si K edge and the ${\mathrm{Cl}}^{+}$-ion yield for the systems Si(100)2\ifmmode\times\else\texttimes\fi{}1-Cl and Si(111)7\ifmmode\times\else\texttimes\fi{}7-Cl at both the substrate Si K edge and adsorbate Cl K edge. Moreover, we reassess our previously published Si K-edge ${\mathrm{H}}^{+}$-ion yield data from the Si(100)2\ifmmode\times\else\texttimes\fi{}1-${\mathrm{H}}_{2}$O system. The EXAFS in the ion yield spectra is analyzed with a view to clarifying the physical processes deter- mining the ion yield. At the substrate Si K edge, the similarity of near-neighbor distances and coordination numbers to bulk values in all the systems studied indicates that the ion yield at this edge is dominated by x-ray-induced electron-stimulated desorption. This is corroborated by NEXAFS data at the same absorption edge. For Si(100)2\ifmmode\times\else\texttimes\fi{}1-Cl and Si(111)7\ifmmode\times\else\texttimes\fi{}7-Cl the ${\mathrm{Cl}}^{+}$-ion yield at the Cl K edge produces EXAFS and NEXAFS spectra identical to those recorded using Auger-electron yield, indicating the likely desorption mechanism to be a Knotek-Feibelman intra-atomic Auger process. The implications of these findings for PSID EXAFS, and NEXAFS are discussed.