Alan J. Bennett

Electronic structure of defect centers in SiO2

Abstract Molecular orbital calculations based on the Extended Huckel Theory have been performed for clusters of two and eight SiO 2 molecules. We report here the results for an ordered sutructure in which periodic boundary conditions are imposed to saturte the peripheral bonds which otherwise cause extraneous localized states to appear. We find an energy gap of ∼13 eV as compared to the experimental value of ∼11 eV. The removal of an oxygen results in several levels in the energy gap which move upon displacement of the two neighboring silicons. The levels appear to account for the observed ultraviolet optical absorption band and are also a possible origin for the positively charged slow surface states present near a SiO 2 -Si interface. The addition of either atomic or molecular hydrogen to a perfect crystal is predicted to yield preferentially a double hydrogen-single oxygen center. If ambient oxygen atoms are present, a two hydroxl group center is then most favorable.

A molecular orbital approach to chemisorption: I. Atomic hydrogen on graphite

Abstract We examine the use of Extended Huckel Theory, a semi-empirical molecular orbital approach, in treating chemisorption. The calculations include all valence electrons and overlap integrals. Adsorbate energy levels are self consistently determined. The substrate is represented by a relatively small number of atoms which is shown to be adequate for obtaining semi-quantitative results. The small representation permits investigation of adsorbate interactions with both ideal and imperfect surfaces. The binding energy, binding sites, and barrier to surface mobility exhibited by atomic hydrogen on a graphite basal surface are obtained. The experimentally observed formation of CH4 on adsorption of hydrogen is also considered. Calculations on electrophilic adsorbates reveal the necessity for a more completely self consistent treatment in which all energy levels are adjusted.

Effect of Adsorbate–Adsorbate Interactions on the Net Charge of an Alkali Atom on a Metal Substrate

A previous calculation of the effective charge of an isolated alkali atom on a metal substrate is generalized, in the low‐coverage limit, to include interactions between adsorbed atoms. The long‐range Coulomb field traditionally invoked to explain the decrease in net charge with increasing coverage is incorporated into our self‐consistent procedure. The complex nature of the dependence of this decrease on coverage is emphasized. Short‐range interactions due to the overlap of adsorbate atomic orbitals are included by allowing for both monatomic and diatomic adsorbate species. The effect of the long‐range Coulomb and overlap interactions is illustrated by numerical results for K adsorbates on a (100) W surface at various coverages. Several factors combine to cause a similar net charge per nucleus on the two species.

The effect of applied electric fields on chemisorption

Abstract A simple model is used to study the effects of applied electric fields on the neutral chemisorption bond at a metal-vacuum interface. The adsorbate-substrate complex is represented by a model diatomic molecule in an electric field which varies spatially due to substrate screening. The field induced changes in binding energy, equilibrium position and vibrational frequency of the adsorbate are calculated. The latter changes are due to the dependence of the effective field coefficients on the adsorbate distance from the surface. Measurable effects, which reflect the bond characteristics, are predicted for experimentally attainable fields.

Image-like potentials at semiconductor and electrolyte surfaces

Abstract The potential felt by a charged particle near an electrolyte (or intrinsic semiconductor) — vacuum interface is calculated. The Poisson-Boltzmann equation is used to describe the mobile charge in both types of substrate together with a classical dielectric description of the aqueous medium (or filled valence band). For charge-surface separations greater than or equal to the inverse screening length in the substrate, a shifted image potential is obtained. The shift distance depends on the surface potential, screening length and dielectric constant of the material. The potential due to the compressibility of any charged molecular adsorbate layer (or conventional surface states) present is estimated and found to be small for a wide range of parameters.

Proposed investigation of the density of electronic states in ionic solutions by tunneling at oxide coated electrodes

Summary An electrochemical tunneling experiment and its analysis are suggested in order to explore the effective electronic densities of states of ions in solution. The use of oxide coated electrodes permits an investigation over a broad energy range, since the width of the barrier may be adjusted to avoid mass transport limitations even when the charge transfer occurs from peaks in the density of states. Simultaneous measurements of the current and its derivative with respect to bias are shown to yield important temperature averaged information.