Uwe Birkenheuer

The oxygen vacancy at the surface and in bulk MgO: An embedded-cluster study

The oxygen vacancy in bulk MgO and at its (001) surface have been studied by means of the EMBED program which adopts an embedded-cluster approach in the frame of the Hartree–Fock (HF) approximation [Comp. Phys. Comm. 82, 139 (1994); 96, 152 (1996)]: A spin restricted (RHF) or unrestricted (UHF) method has been used according to whether the vacancy contains an even number (F and F2+ centers) or an odd number (F+ center) of electrons. The semi-infinite crystal has been simulated with a three-layer slab. Particular attention has been given to the optimization of the basis functions which serve to describe the trapped electron(s) in the vacancy. The correlation energy was estimated using second order Mo/ller–Plesset theory. The three electronic configurations have been studied by allowing for relaxation of all neighboring ions. For the paramagnetic center, spin density data are provided and discussed with reference to results from EPR experiments and molecular cluster calculations.

Cluster embedding in an elastic polarizable environment: Density functional study of Pd atoms adsorbed at oxygen vacancies of MgO(001)

Adsorption complexes of palladium atoms on Fs, Fs+, Fs2+, and O2− centers of MgO(001) surface have been investigated with a gradient-corrected (Becke–Perdew) density functional method applied to embedded cluster models. This study presents the first application of a self-consistent hybrid quantum mechanical/molecular mechanical embedding approach where the defect-induced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regular surface. The cluster models are embedded in an elastic polarizable environment which is described at the atomistic level using a shell model treatment of ionic polarizabilities. The frontier region that separates the quantum mechanical cluster and the classical environment is represented by pseudopotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron a...

Electronic structure of benzene adsorbed on single-domain Si(001)-(2×1): A combined experimental and theoretical study

Benzene adsorption on a single-domain Si(001)-(2×1) surface has been studied by thermal desorption spectroscopy (TPD) and angle-resolved photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. Angle-resolved photoemission spectra for the saturated benzene layer exhibit well-defined polarization and azimuthal dependencies compatible with a flat-lying benzene molecule with local C2v symmetry. Based on these results two structure models are proposed. First-principles density functional cluster calculations have been performed for each of these structures. Total energy minimization and a detailed comparison of the experimental ARUPS spectra with the one-particle spectra of the model clusters leads to a 1,4-cyclohexadienelike adsorption complex with a flat-lying benzene molecule which is di-σ bonded to the two dangling bonds of a single Si–Si surface dimer. Especially, one of the unoccupied 1e2u (π*) orbitals of the free benzene molecule shifts down (by about 3 eV) and evolves into t...

CHARGE SEPARATION AND COVALENT BONDING IN METAL OXIDE SURFACES : A LOCAL DENSITY FUNCTIONAL STUDY ON THE MGO(001) SURFACE

A first principles local density functional investigation on extended, two‐dimensional periodic slab models of the MgO(001) surface is performed, using the linear combination of Gaussian‐type orbitals (LCGTO) technique as implemented in the FILMS program package. Stimulated by recent theoretical evidence for a reduced charge separation in MgO(001), a detailed analysis of the charge distribution and its influence on the electrical field above the surface is carried out. Two different methods to quantify the charge separation in the ionic substrate are employed, a local one based on the topological atom approach and a global one derived from the Madelung field of the surface near potential adsorbates. Both procedures lead to a charge separation significantly (10%–20%) below the nominal ionic value of ±2 a.u. A variational atomic orbital analysis is utilized to discuss the origin of the Mg 3s and 3p structures discernible in the crystal orbitals of the MgO slab systems. They are identified as covalent magnes...

THE ADSORPTION OF ACETYLENE ON NI(110) : AN EXPERIMENTAL AND THEORETICAL STUDY

The adsorption of acetylene on Ni(110) was investigated by temperature programmed desorption (TPD), low energy electron diffraction (LEED), angle resolved ultraviolet‐photoelectron spectroscopy (ARUPS), and near edge x‐ray absorption fine structure (NEXAFS) measurements, as well as by detailed model cluster calculations and slab model band structure calculations. By combining the experimental results and those of the cluster studies an orientation of the molecules is deduced with the C–C axis parallel to the surface and preferentially aligned along the substrate troughs ([110] azimuth) as well as with a highly coordinated adsorption site in the substrate troughs. A detailed analysis of the photoemission spectrum is given. The proposed adsorption geometry is corroborated by band structure calculations for various alternative sites and orientations that are shown to be very sensitive to the azimuthal orientation of the adsorbed molecules.

Density functional investigation of the geometric and electronic structure of ethylene adsorbed on Si(001)

A detailed first-principles density functional analysis of the geometric and electronic properties of ethylene adsorbed on the dimer reconstructed Si(001)-(2×1) surface is presented. This theoretical study was carried out in close reference to a recent angle-resolved photoemission spectroscopy investigation of the same adsorption system. Adsorbate weighted Kohn-Sham one-particle spectra are calculated and compared to the band structure derived from the angle-resolved photoemission spectra. In addition, the symmetry character of the concomitant Bloch waves is determined to yield information which can directly be related to the results of a dipole selection rule analysis of the corresponding photoemission signals. Total energy minimization of a model slab reveals a distortion of the adsorption complex at saturation coverage to local C2 symmetry involving an 11° rotation of the ethylene molecule around the surface normal and a 27° twist of the methylene groups around the CC axis. This finding is confirmed by...

On cluster embedding schemes based on orbital space partitioning

The embedding approach to the electronic structure of local perturbations in extended systems is based on the fundamental assumption that beyond a certain region around the defect, the properties of the environment are not altered by the presence of the defect. In many computational schemes the resulting subdivision of the defect system into a central and an external region is defined in terms of orbital basis functions. The fundamental embedding assumption then translates into a partitioning of matrix representations, accompanied by fixing the external region contributions to their values in the unperturbed reference system. With the help of density functional cluster-in-cluster embedding calculations we have investigated the quality of this assumption without introducing any additional approximation as usually done to arrive at a computationally feasible embedding scheme. The fundamental embedding assumption is found to cause spurious virtual orbital admixtures to the density matrix which lead to artifa...

The vibrational structure of benzene adsorbed on Si(001)

High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibrational modes of the adsorption complex, which previously was identified to exhibit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants are found to apply to different collective modes of the adsorbate. The A1 and B1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A2 and B2 modes a frozen substrate description of the adsorption complex is more appropriate.

On the adsorption site of ethylene at the Ni(110) surface : a combined experimental and theoretical study involving the unoccupied band structure

The adsorption of ethylene on Ni(110) was investigated by angle resolved inverse photoemission (ARIPE) spectroscopy as well as by detailed density functional model cluster and slab model band structure calculations to clarify the preferred adsorption site. Cluster model calculations both at the local density as well as at the gradient corrected level of theory gave a slight preference for the di-σ over the π coordinated geometry on top of the ridges, but no or, at best, a very weak binding over the troughs. A dispersionless band in the ARIPE spectra about 1.8 eV above EF is assigned to the band derived from the lowest unoccupied ethylene orbital, 1b3g(π∗). The surface state feature of the clean Ni(110) surface connecting the image state at \gG and the dyz state S2 lowered almost uniformly by 0.8 eV through the interaction with the adsorbate. For adsorption on top of the ridges in the so-called “half-bridge” position intermediate between the short bridge (di-σ) and the top site (π), the symmetry requirements imposed by the surface state band are ideally met by the second lowest unoccupied band of the adsorbate monolayer which changes its character from ethylene 4ag at Γ′ to 2b3u at Y′. A similarly strong and uniform interaction is not possible when ethylene adsorbs above the troughs. Taking all experimental and theoretical evidence together, the adsorption site in the densely packed c(2×4) C2H4Ni(110) adsorption system is identified as the half-bridge position on top of the ridges.

A Simplified Method for the Computation of Correlation Effects on the Band Structure of Semiconductors

We present a simplified computational scheme in order to calculate the effects of electron correlations on the energy bands of diamond and silicon. By adopting a quasiparticle picture we compute first the relaxation and polarization effects around an electron set into a conduction-band Wannier orbital. This is done by allowing the valence orbitals to relax within a self-consistent field (SCF) calculation. The diagonal matrix element of the Hamiltonian leads to a shift of the center of gravity of the conduction band while the off-diagonal matrix elements result in a small reduction of the conduction-electron bandwidth. This calculation is supplemented by the computation of the loss of ground-state correlations due to the blocked Wannier orbital into which the added electron has been placed. The same procedure applies to the removal of an electron, i.e., to the valence bands. But the latter have been calculated previously in some detail and previous results are used to estimate the energy gap in the two materials. The numerical data reported here show that the methods works, in principle, but that some extension of the scheme is also necessary to obtain fully satisfactory results.