R. I. R. Blyth

A NEXAFS and UPS study of thiol monolayers self-assembled on gold

We report an investigation of the nature of the chemical bond at the interface formed when a gold substrate is derivatised with a layer of functionalised octanethiol. Monolayers prepared on Au(111) under UHV conditions were studied using a combination of AES and UPS. A high sticking coefficient was evident for n-octanethiol, CH 3 (CH 2 ) 7 SH, and the prepared layer was readily damaged by incident electrons. On monolayer formation, the UPS revealed both modifications to the d-band structure of the gold and the appearance of features characteristic of an adsorbed thiol species. S K-edge NEXAFS was performed on monolayer films of bromo-octane disulphide, (Br(CH 2 ) 8 S) 2 , and octane-dithiol, SH(CH 2 ) 8 SH, prepared by adsorption from solution on Au foil. We observed chemical shifts which could be attributed to the presence of surface-bound thiolate (RS - ) and sulphonate (RSO 3 - ) species. It appears that the initially prepared S(II) undergoes facile oxidation to S(VI) as a result of photo-induced interfacial electron transfer.

The unoccupied states of a rare earth metal : Y(0001)

Normal incidence inverse photoemission spectra of Y(0001) have been measured in the photon energy range 12-32 eV. In the photon energy range 19-23 eV fluorescence from the radiative decay of a 4p core hole largely obscures the direct transitions. Outside this range a number of features were observed, which vary in intensity but show negligible dispersion with photon energy. Their positions are in good agreement with high density-of-states points along the Gamma A direction of published bandstructure calculations. A feature is observed 0.5 (+or-0.2) eV above the Fermi level, in the gap between the Gamma 4- and Gamma 3+ critical points, which may be due to either a surface state or to the high overall density of states at the Fermi level.

Photoemission Study of CO Adsorption on Gd

Angle-resolved UV photoemission spectroscopy has been used to investigate the interaction of CO with Gd films, grown on W(110). The results suggest the CO absorbs dissociatively, initially forming Gd(2)O(3), with subsequent catalytic oxidation of CO to form carbonate.

Surface structure of rare earth metals

Abstract We have performed LEED, angle-resolved UV photoemission (ARUPS) and inverse photoemission (IPE) measurements on a representative set of hcp rare earth surfaces in order to obtain basic structural information. LEED patterns from the (0001) surfaces show six-fold symmetry due to the two equally probable bulk terminations. ARUPS data show a large non-dispersing peak at ~ 10 eV binding energy, with intensity extremely sensitive to surface order, which is not reproduced in one-electron calculations. Suggestions that this many-body feature is due to an unoccupied surface state are supported by our IPE data, which appear to show such a state just above the Fermi level. LEED patterns from the (1120) surfaces of Y. Ho and Er also show six-fold symmetry, with no visible substrate spots, implying that these relatively open surfaces have collapsed to a (7 × 1) close-packed structure many layers deep. ARUPS data from the reconstructed (1120) surface of Ho are virtually indistinguishable from those obtained from Ho(0001). This suggests that the two structures are identical within the probing depth of this technique.

Core-level spectroscopy study of rare-earth metal/W(110) interface formation

The electronic structure of rare-earth metal/W(110) interfaces during the initial stages of deposition has been investigated using core-level shift spectroscopy. Three rare-earth metals (Gd, Y and Yb) have been grown on a W(110) substrate. Shifts in the W core level peaks of > 300 meV have been confirmed for the clean W(110) surface. On completion of each interface a bulk feature and only one shifted peak is observed which supports evidence that no alloying occurs. At submonolayer coverages an additional feature is seen at higher kinetic energies which is attributed to emission from W atoms under various adsorbate structures.

Gd/W(110) interface formation studied by w 4f surface core-level spectroscopy.

W 4f core-level spectra of Gd films on W(110) show components which can be identified as being due to bulk and interface W. The results suggest that the W/Gd interface is abrupt, i.e. no interface alloying occurs. At submonolayer coverages a further component is seen which is attributable to intermediate (n x 2) phases.

Utilization of Photoemission Cross-Section Effects for Monitoring Thin-Film Growth in UHV.

Given the tunability of synchrotron radiation, photoemission spectroscopy can be used to monitor the growth modes of thin films in UHV. Cross-section effects such as Cooper minima and resonant photoemission can be exploited to maximize the adsorbate signal and minimize that from the substrate. Under favourable circumstances growth can be monitored in real time at coverages of <1%. As an example the growth of Gd on W(110) is investigated.

Angle-resolved UV photoemission from Pr(0001)

We observe a number of well defined peaks in the photoemission spectra from Y(0001) over the photon energy range 25 eV<hv<60 eV. We believe that one of the features with a binding energy of 1.7 eV is due to emission from states near the upper band edge at the Γ point and that a second nearEForiginates from a surface state. We find a large peak at a binding energy of 9.6 eV which is sensitive to the quality of the surface.

Determination of the Tb A4- critical point energy from angle-resolved UV photoemission of Tb(0001)

The authors report an angle-resolved UV photoemission study of the clean (0001) surface of the rare-earth metal terbium. Normal emission spectra were recorded in the photon energy range 12-50 eV using synchrotron radiation. A number of well-defined features were observed which showed negligible dispersion with photon energy. Close similarity is found between these spectra and those from the (0001) surfaces of Y, Gd and Ho. The binding energy of the Gamma 4- critical point is determined to be 1.5(+or-0.1) eV. They suggest that the apparent shift of the Tb Delta 1 band to higher binding energy reported by previous workers was a result of the significant Fe contamination of their sample.

High d-band occupancy in alloys of Mg with some 4d and 5d transition metals: a photoemission study

Photoemission data from cosputtered films of binary alloys of Ru, Rh, Pd, Ir, Pt and Au with Mg show that the transition metal (TM) d band narrows as the TM concentration is reduced, reaching a minimum after which the intensity decreases but the width remains constant. The continuity of binding energy, spin-orbit splitting and width of the resulting d states suggests a high level of d-band occupancy in all cases. LMTO bandstructure calculations for pure TM and ordered intermetallic compounds of the form MgTM and Mg3TM also show narrowing of the d bands, with binding energies and widths which indicate that they are fully, or nearly fully occupied in all cases where the TM is dilute. The authors suggest that the additional d intensity results from a spread of the TM d wavefunctions on to the surrounding Mg sites, allowing high d-band occupancy whilst preserving charge neutrality.