No abstract

The transfer matrix of a solid described by the stacking of principal layers is obtained by an iterative procedure which takes into account 2" layers after n iterations, in contrast to usual schemes where each iteration includes just one more layer. The Green function and density of states at the surface of the corresponding semi-infinite crystal are then given by well known formulae in terms of the transfer matrix. This method, especially convenient near singularities, is applied to the calculation of the spectral as well as the total densities of states for the (100) face of molybdenum. The Slater-Koster algorithm for the calculation of tight-binding parameters is used with a basis of nine orbitals per atom (4d, 5s, 5p). Surface states and resonances are first identified and then analysed into orbital components to find their dominant symmetry. Their evolution along the main symmetry lines of the two-dimensional Brillouin zone is given explicitly. The surfacestate peak just below the Fermi level (Swanson hump) is not obtained. This is traced to the difficulty in placing an appropriate boundary condition at the surface with the tight-binding parametrisation scheme.

Theoretical models and ab initio Hartree-Fock wave functions are used to investigate the N 1s core level binding energies of N-containing calcined carbonaceous materials. Comparison of calculated and experimental values for a series of test molecules reveals that the N 1s core level shift from one compound to another is mainly originated by initial state effects. This permits a systematic study of different situations and allows establishment that three different types of nonoxidized N atoms can be present in these materials. These are "pyridinic", "pyrrolic", and "graphitic" nitrogen with binding energies of ≈399.0, ≈400.3, and ≈401-403 eV, respectively. This assignment is in very good agreement with a recent experimental X-ray photoelectron spectra on petroleum cokes and demonstrates, for the first time, that it is possible for N to exhibit rather large core level 1s energies without requiring the presence of any charge transfer from N-oxide groups. Theoretical reasons for such a behavior are also given.