H.-J. Freund

Hydroxy1 driven reconstruction of the polar NiO(111) surface

Abstract We have studied the reconstruction of the polar NiO(111) surface predicted recently by D. Wolf with low energy electron diffraction. Thin NiO films (10–20 A) are used as substrates. As prepared, the films with a p(1 × 1) NiO(111) structure are covered with hydroxyl groups, which may be removed through a simple heat treatment. As the hydroxyl groups are desorbed, the surface reconstructs, exhibiting a diffuse p(2 × 2) structure. Readsorption of water onto the reconstructed surface lifts the reconstruction and again leads to the formation of the p(1 × 1) hydroxyl covered surface.

The structure of Pt-aggregates on a supported thin aluminum oxide film in comparison with unsupported alumina: a transmission electron microscopy study

Abstract Platinum has been deposited from the vapor onto a sample consisting of a thin aluminum oxide film grown on a NiAl(110) substrate wedge. At the tip of the wedge small γ-alumina crystallites are formed which have also been platinum decorated. The size distribution of the Pt aggregates on the thin aluminum oxide film is almost identical to the one found for the support-free alumina crystallites. It is also identical to the size distribution observed with STM under UHV conditions in earlier experiments [Th. Bertrams et al., Surf. Sci. 331–333 (1995) 1515]. The present structural analysis reveals that after Pt decoration the structure of the substrate remains unchanged. The Pt forms flat, epitaxial islands on both substrates. The nearest neighbor distances within the platinum aggregates have been determined for the supported oxide as a function of particle size. For large particles with radii greater than 20 A the nearest neighbor distance of bulk Pt is observed. In the case of the smallest particles with radii close to 10 A a contraction of up to 10% is detected. The contraction of the interatomic distance appears not to be isotropic. The observed Moire patterns comply with the assumption that the substrate lattice underneath the Pt aggregates does not change.

Electronic surface state of NiO (100)

Abstract The electronic structure of the (100) surface of NiO has been studied using EELS (electron energy loss spectroscopy) and ab initio calculations. In addition to the previously documented bulk excitations of NiO, two new states at energies of 0.57 and 1.62 eV have been found. These states are attributed to d—d transitions of the nickel surface ions. As expected for surface states, they are affected by the interaction with an adsorbate, i.e. adsorption of NO leads to a shift to higher energy. Ab initio cluster calculations show that electronic structure of the surface is considerably different from that of the bulk which is a result of the lower symmetry of the crystal field at the surface (O 2 →C 4v . The nature of the observed surface states has been identified by a comparison of the experimental data with theoretical results.

Influence of alkali co-adsorption on the adsorption and reaction of CO2 on Pd(111)

The Pd(111) surface has been extensively used as a model substrate to study CO oxidation. The reverse reaction, CO2 dissociation, however, has not been investigated to a comparable extend on this surface. We report angle-resolved photoemission studies on the system CO2 (+Na)/Pd(111). On a clean Pd(111) surface at 85 K no CO2 adsorption takes place. Upon small precoverages of Na, adsorption of CO2 occurs at T = 85 K. Via an analysis of the angular dependence of the photoelectron spectra we obtain information about the orientation of the adsorbed molecules. Upon annealing the surface to 125 K, CO2 reacts to form co-adsorbed CO and O. For a thick Na film on Pd(111) we also find CO2 adsorption but the reaction of the adsorbed species occurs along different reaction channels, ending at adsorbed carbonate species.

Dynamical studies of UV‐laser‐induced NO‐desorption from the polar NiO(111) versus the nonpolar NiO(100) surfaces

We have studied the UV‐laser‐induced desorption of NO adsorbed on an epitaxial film of NiO(111) grown on Ni(111). The desorbing molecules were detected state selectively via a resonance enhanced ionization technique [REMPI(1+1)] using the A 2Σ(v’=0,1,2)←X 2Π(v‘=0,1,2) transition as intermediate state. Our results are compared with our experiments on NO desorption from NiO(100). The similarities and differences of the results due to the different surface structure of the polar NiO(111) and the non polar NiO(100) are discussed. For both surfaces we observe bimodal velocity flux distributions independent of the rovibrational state. Due to a rotational temperature of about 400 K and a vibrational temperature of 1800 K thermal processes can be ruled out. The wavelength dependence of the desorption cross section strongly correlates with the electronic structure of the NiO indicating a surface mediated excitation process. The spin orientation in the NO molecules influences the life time of the excited state depe...

Lifetimes of electronically excited states of molecules on oxide versus metal surfaces

Abstract The optically forbidden 1 Σ + → 3 Π transition of CO adsorbed on an epitaxially grown Al 2 O 3 (111) surface has been studied by means of electron energy loss spectroscopy. On this surface several CO adsorption states exist with adsorption enthalpies ranging from 88 (multilayer) to 170 meV. For the first time even in the (sub) monolayer range the vibrational fine structure of the energy loss peaks due to the 1 Π + →a 3 Π electronic transition of adsorbed CO could be clearly resolved. The half widths of the loss peaks have been used to estimate the lifetimes of the excited states.

On the electronic structure of the coadsorbate system CO + O (2×1)/Pd(111): A precursor for CO2 formation☆

We report results of an angle resolved photoelectron spectroscopy (ARUPS) study using synchrotron radiation from the BESSY storage ring of the CO + O (2×1)/Pd(111) coadsorbate system in comparison with the CO(√3 × √3)R30°/Pd(111) adsorbate. The band structures of the CO induced 5σ1π and 4σ levels for both structures are observed and favourably compared with the proposed structure models as well as tight-binding model calculations. Light polarization dependent measurements as well as the angle dependence of the 4σ shape resonance indicate that the CO molecular axis in the pure CO adsorbate is perpendicular to the Pd(111) surface, while it appears slightly (<10°) inclined with respect to the surface normal in the coadsorbate. We show by comparison with other pure CO adsorbates using the measured E versus k‖ dispersions that the 4σ wave function in the coadsorbate is very similar to the pure adsorbate. It is shown that this is different with respect to other, namely CO + K, coadsorbates.

Photoinduced processes on alkali covered surfaces: NO desorption from KCr2O3(0001)

Abstract We have studied the ultraviolet laser induced desorption of NO Cr 2 O 3 (0001) , K Cr 2 O 3 (0001) and the coadsorbate system NO/K/Cr 2 O 3 (0001) using resonance enhanced multiphoton ionization spectroscopy for state selective detection of the desorbing species after excitation with nanosecond laser pulses. The goal of our experiments was to study the influence of surface electronic modifications via alkali adsorption on the photodynamics of a simple molecule. The photochemistry of the isolated and the coadsorbate systems is strongly dependent on the coverage of the diverse components. In this paper we shall mainly focus on data for the low coverage regime of potassium. From the two adsorbate species of NO, a chemisorbed and a physisorbed species, we present data of the chemisorbed species. The velocity distributions show a strong dependence on the excitation energy which we interpret on the basis of electron energy loss spectra as being due to surface charge transfer states. This is corroborated with our coadsorption experiments with low coverages of potassium which alter the velocity distributions.

Lateral Interaction in Ordered Hydrocarbon Overlayers: C-H Band Dispersion of Adsorbed Benzene

We have determined the dispersion of a C-H–derived band (2a1g) in an ordered (√7 × √7)R 19.1° benzene overlayer on Os(0001) via angle-resolved photoelectron spectroscopy. The maximum dispersion of about 0.4 eV can be explained on the basis of tight-binding calculations on a free unsupported benzene overlayer.

Adsorption, thermal and photochemical reactions of NO on clean and oxygen precovered Ni(100) surfaces

We have studied the adsorption, thermal and photochemical reactions of NO adsorbed on clean Ni(100), epitaxially grown NiO, and Ni(100) precovered with chemisorbed oxygen. The electronic and geometric structure of the substrate surfaces and the adsorbed NO molecules were investigated by electron spectroscopic techniques, i.e. HREELS, NEXAFS and LEED, whereas the thermal and photochemical properties of the adsorbate layer were probed using TPD and laser induced desorption, respectively.