V. Dudr

Interfacial reconstruction in the system Pb/Ag(1 1 0)

Abstract We have examined the Pb/Ag(1xa01xa00) adsorption system using low energy electron diffraction (LEED) and synchrotron radiation X-ray photoemission spectroscopy (SRXPS). At a temperature of 120 K, layer-by-layer growth occurs and no superstructure is observed by LEED. On heating a two monolayer film to 230 K, or on deposition of two monolayers at 300 K temperature, a c(2xa0×xa04) superstructure is observed. We interpret this as a reconstruction involving a (1xa0×xa02) Ag substrate, and a buckled hexagonal overlayer of Pb. The reconstruction is unusual as it is induced at a solid–solid interface, rather than by an adsorbate at the solid–vacuum interface. Core level shifts of the Pb 5d and 4f and Ag 3d levels are also reported.

Phase-sensitive lock-in imaging of surface densities of states

A new way of imaging the local density of states has been devised through a combination of the constant-height scanning tunnelling microscopy operational mode and lock-in techniques. We have obtained current images simultaneously with real space dynamical conductance maps (d I/d V) for energies around the Fermi level, on the Si(111)-(7 × 7) surface. We reconstructed the normalized dynamical conductance spectra—(d I/d V)/(I/V). Since the (d I/d V)/(I/V) curves are closely related to the local densities of states, we compared their sum over the unit cell to photoelectron spectra and theoretical calculations. We find that the results are in good agreement. Consequently, the extent of localization of surface electronic states at lattice positions was determined.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys

The reaction of oxygen at low pressure with the Sn/Pd(110) system has been examined by photoelectron spectroscopy using synchrotron radiation. The c(2 × 2) and (3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers (ML) Sn coverage and a 1.75xa0ML Sn overlayer on the Pd(110) surface after flashing to 470xa0K were studied. The Sn 4d core level is strongly affected by O(2) adsorption while the Pd 3d core level shows very little change other than a decrease in intensity. Starting with a 10xa0L dose of oxygen, prominent changes in the spectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4d core levels indicates that oxidation proceeds with the formation of well-defined states of Sn, which were identified as a Pd-Sn-O interface layer, SnO and SnO(2) oxides. The valence band spectra confirm this assignment. The Sn(2+) and Sn(4+) component signals originate from the topmost surface layer, i.e.xa0tin atoms in more highly oxidized states constitute the topmost surface layer on top of the Pd-Sn-O interface. The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxide formation; the Sn-O phase formation is accompanied by Pd-Sn bond dissociation.

Interaction of oxygen with Au/Ti(0001) surface alloys studied by photoelectron spectroscopy

The interaction of oxygen with gold adsorbed on Ti(0001) was studied by synchrotron radiation photoelectron spectroscopy. Two kinds of surfaces were explored: as-deposited 0.38, 1.16 and 1.85 monolayer (ML) thick Au overlayers on the Ti(0001) surface, and the same samples after thermal treatment, which resulted in the formation of Au-Ti intermetallic surfaces. The Ti 3p core level was strongly affected by reaction with oxygen, while the Au 4f core level showed only minor changes other than a decrease in intensity. The Ti 3p peak was fitted with several components which were identified as Ti atoms in different oxidation states, namely TiO, Ti(2)O(3), TiO(2) and Ti-OH. Titanium oxide phase formation is accompanied by Au-Ti bond dissociation and outward diffusion of Ti. The presence of an Au-Ti intermetallic phase on the Ti(0001) surface promotes oxidation of the Ti atoms.