Showing papers by "Martin Neumann published in 1991"

Molecular adsorption on oxide surfaces: Electronic structure and orientation of NO on NiO(100)/Ni(100) and on NiO(100) as determined from electron spectroscopies and ab initio cluster calculations

Abstract: We have investigated the adsorption of NO on a thin NiO(100) film of several layers thickness grown on top of a Ni(100) surface in comparison with data of an in vacuo cleaved NiO(100) single crystal. The layer exhibits a high defect density. We demonstrate via application of several surface-sensitive electron-spectroscopic techniques [i.e., x-ray photoelectron spectroscopy (XPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), near-edge x-ray-absorption fine structure (NEXAFS), and high-resolution electron-energy-loss spectroscopy (HREELS)] that this layer has similar occupied (ARUPS) and unoccupied (NEXAFS) states as a bulk NiO(100) sample. In spite of its limited thickness, the band structure of the film exhibits dispersions perpendicular to the surface compatible with bulk NiO(100). It is shown that the electronic structure of the oxygen sublattice can be described in a band-structure picture while for the Ni sublattice electron localization effects lead to a breakdown of the band-structure picture. NO on NiO desorbs at 220 K. This indicates weak chemisorption. The NO coverage is close to 0.2 relative to the number of Ni surface atoms as determined by XPS.HREELS reveals that there is only one species on the surface documented by the observation of only one bond-stretching frequency. NEXAFS data on the system and a comparison with previous data on the system NO/Ni(100) indicate that the molecular axis of adsorbed NO is tilted by an angle of approximately 45\ifmmode^\circ\else\textdegree\fi{} relative to the surface normal. The N 1s XP spectra of the weakly chemisorbed species show giant satellites similar to the previously observed cases for weak chemisorption on metal surfaces. This is the first observation of an intense satellite structure for an adsorbate on an insulator surface, which shows that there must be sufficient screening channels even on an insulating surface. A theoretical assignment of the peaks is discussed. We compare the spectroscopic properties of the NO species on the thin-film oxide surface, which is likely to contain a certain number of defects, with NO adsorbed on a basically defect-free bulk oxide surface by thermal-desorption (TDS) and XP spectra. TDS and XP spectra of the bulk system are basically identical as compared with the oxide film, indicating that the majority of species adsorbed on the film is not adsorbed on defects but rather on regular NiO sites. Results of ab initio oxide cluster calculations are used to explain the bonding geometry of NO on regular NiO sites.

Reaction Field Simulations of Monatomic and Diatomic Dipolar Fluids

Abstract: Firstly, the conditions are explored for which molecular dynamics simulations yield reliable thermodynamic results when the reaction field method is applied for dipolar fluids. The influence of the system size, of the scaling method for avoiding the total energy updrift, and of the reaction field dielectric constant are studied. With appropriate conditions, the results for the Stockmayer fluid when compared to previous results show excellent agreement up to very high dipole moments. In the next step the method is applied to two-centre Lennard-Jones plus ideal axial dipole (2CLJD) molecules of elongation L = 0.505. For one state point, the influence of the molecular shape as well as of the dipolar strength on energy and pressure is studied in detail. Finally, in the project of developing a physically based equation of state for polar fluids, thermodynamic results are presented for 2CLJD fluids of elongation L = 0.505 for four different dipole moments at 29 state points in a temperature and density...

Photoemission on NiO

Abstract: The interpretation of the photoemission spectra of NiO is considered with respect to the dispersion of thed-bands. The dispersion of thed-bands can not be measured due to the formation of an exitonic state. The electronic nature of the states which are responsible for the optical gap in NiO is discussed.

Pimc simulations of solid parahydrogen

Abstract: We have evaluated the kinetic energy of the center of mass of solid parahydrogen at T=18.9 K, and four different densities, by means of path-integral Monte Carlo computer simulation, using the pairwise additive Lennard-Jones potential. The results have been compared with the available experimental neutron data of Herwig et al. [Phys. Rev. B 41, 96 (1990)]. The agreement between experiment and simulation is excellent for the kinetic energy but not as good for the pressure. There are indications that the use of a more realistic pair potential would increase the agreement with the experimental pressure data without destroying that of the kinetic energy.

Photoemission studies of BaPb1−xBixO3 and Ba1−yKyBiO3 crystals

Abstract: Potassium and lead substituted single and polycrystalline material was examined by X-ray diffraction, X-ray topography, nonresonant microwave absorption, UPS and XPS. A covalent model instead of an ionic one is proposed. We find no evidence for a mixed valency of Bi. The valency of the Bi atoms at the unequivalent sites is close to +3. No density of states at the Fermi level has been detected.

Averaged Powerspectra of the Electrocardiograms of Patients after Heart Transplantation

Abstract: Data acquisition and beat-by-beat FOURIER analysis is performed on a PC-based system, aiming at the detection of rejection episodes from electrocardiograms of heart transplant patients