A. J. Maxwell

Metal-oxide interaction for metal clusters on a metal-supported thin alumina film

Abstract The interaction between deposited metal clusters and a thin model alumina film grown on NiAl(110) have been studied using X-ray absorption spectroscopy (XAS) and core and valence photoelectron spectroscopy. A lower limit for the fundamental gap of the supported alumina film is determined, and found to be slightly lower than that of alumina surfaces. O 1s XAS shows that new states appear in the fundamental gap upon metal deposition. Al 2p X-ray photoelectron spectra from the alumina film are also sensitive to metal deposition, whereas spectra from Al atoms at the substrate–oxide interface appear unaffected. The present data demonstrate the existence of gap states in the pristine film, and we discuss the effects of these states for the properties of this film as a model oxide substrate.

Interaction of CO with Pd clusters supported on a thin alumina film

The adsorption of CO on Pd particles supported on a thin alumina film has been studied employing high resolution x‐ray photoelectron spectroscopy (XPS) and x‐ray absorption spectroscopy (XAS), and of special interest was the CO–Pd interaction as a function of island size and CO coverage. CO saturation at 90 K leads to an overlayer characterized by a rather weak CO–Pd hybridization as manifested by the core ionized and core excited states. The interaction strength gradually increases with island size. Desorption of parts of the overlayer results in CO more strongly interacting with the Pd islands. A comparison between the XPS and XAS energies yields a behavior indistinguishable from metallic systems for islands larger than 15 A, i.e., the XPS binding energy appears near the x‐ray absorption onset. For the smallest islands (5 A), a CO coverage dependent reversal of the XPS–XAS energy relation was observed, indicating a drastic change in the screening ability of the CO–Pd complex.

C 1s ionisation potential and energy referencing for solid C60 films on metal surfaces

We present a measurement of the C Is ionisation potential (IP) for solid C-60, as well as a reliable determination of the highest occupied molecular orbital (HOMO) IP. The difference between the gas phase and solid state measurements for the HOMO agrees w

Resonant photoemission of solid C60

Abstract Resonant photoemission spectra of thick films of C 60 on a Mo(110) substrate are presented. The data show distinct resonances of the three lowest binding energy photoemission structures, at each of the first four peaks in the C 1s absorption spectrum, thereby indicating significant localization of the excited electron for all of the absorption transitions below the ionization limit. The importance of matrix element effects for an accurate understanding of the intensity variations is stressed.

Vibronic coupling in the photoemission bands of condensed C60

We present photoelectron spectra for the highest occupied molecular orbital (HOMO) of C60 in the gas phase and a two-dimensional (2D) condensed layer. For low temperatures the similarity in the spectra is striking and places a strict upper limit of 0.07 eV on the HOMO electronic bandwidth for the 2D solid, which is a fraction of the theoretically predicted value. This is shown to be consistent with expectations of a polaronic model for the bands and suggests the need fora reassessment of the band dispersions in 3D C60.

Adsorption of C60 on Al(111) studied with scanning tunnelling microscopy

Scanning tunnelling microscopy has been used to study the adsorption and interactions of C60 on Al(111) at room temperature. Initially, C60 forms chains of molecules on the lower terraces of step edges, which indicates that the molecules have a high mobility even at room temperature. When the coverage is increased, the C60 molecules form a close-packed overlayer of (23 × 23)R30° symmetry with respect to the substrate surface which is compressed by 1% in all directions compared with the (111) plane of fullerite. Voltage-dependent intramolecular structure is observed, indicating that the adsorbate-substrate bonding is strong enough to prevent the molecules from rotating, and that C60 prefers to have a six-membered ring facing the surface. The (23 × 23)R30° overlayer structure is metastable, irreversibly transforming to (6 × 6) symmetry when heated above 490 K.

A photoemission and XAS study of oxygen coadsorbed with a (2x2) layer of K on graphite

We have studied the coadsorption of oxygen with a (2 × 2) monolayer of K on graphite. At least three different adsorption phases for oxygen have been found. Different spectroscopic techniques have been used in order to identify the different oxygen species adsorbed on the surface. For low oxygen coverage, the molecules dissociate and form a K2O species. At higher oxygen doses, K2O2 and KO2 are present on the surface up to saturation coverage. XAS yields an O2 bond length of about 1.39 A for the KO2 in this phase. This species is found to form an ionic complex which is weakly bound to the graphite.