Physics
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Item Multiadatom effects in the chemisorption energy of ordered overlayers(American Physical Society, 1977) Einstein, Theodore L.The indirect interaction energy per adatom is evaluated for a c(2x2) overlayer in a model system, and is found to be very well approximated by (twice) the next-nearest-neighbor pair interaction energy, verifying the validity of earlier work. Three adatom (nonpairwise) interaction energies are also considered; while insignificant for total interaction energies, they are comparable to more-distant (viz., third-) neighbor pair interactions.Item Indirect Interaction between Adatoms on a Tight-Binding Solid(American Physical Society, 1973) Einstein, Theodore L.; Schrieffer, J. R.The indirect interaction between adatom pairs on the (100) surface of a simple-cubic tight-binding solid is investigated within a molecular-orbital approach. A general scheme for calculating the surface-density-of-states change and the interaction energy of one and two single-level adatoms is presented, and contact (and a correction) is made with Grimley's formulation. The method permits binding above surface atoms, at bridge sites, or at centered positions, and yields interaction energy as a function of band filling, adatom energy level, and a general hopping potential V between an adatom and the nearest surface atom(s).Item Statistical Mechanics of a Simple Model of a Displacive Ferroelectric(American Physical Society, 1973) Cohen, Michael; Einstein, Theodore L.Item Short-chain model of chemisorption: Exact and approximate results(American Physical Society, 1975) Einstein, Theodore L.The binding energy of an adatom to a chain consisting of three atoms is obtained exactly by computer as a function of adatom Coulomb repulsion, adatom-substrate hopping, and substrate bandwidth. Three simple approximations are also plotted: (i) weak-binding limit, in which the binding energy is given by the expectation value of the adatom-bulk hopping Hamiltonian calculated in second-order perturbation theory; (ii) rebonded surface complex, in which the adatom forms a diatomic molecule with its nearest neighbor in the chain, and the dimer rebonds perturbatively to the indented chain; and (iii) Hartree-Fock, both restricted and unrestricted. The first two schemes can be joined smoothly by hand, and the resulting curve is much better than Hartree-Fock. The physics of all three cases is carefully studied. An appendix treats the surface diatom case of just a single bulk atom. Here the exact solution and unrestricted Hartree-Fock can be performed analytically. Comparison is also given with Brenig and Schönhammer's solution based on Green's-function formalism with matrix self-energy corrections.Item Changes in density of states caused by chemisorption(American Physical Society, 1975) Einstein, Theodore L.The process of chemisorption is studied via the change in density of states when an adatom with a single level Ea bonds by a hopping parameter V to the (100) surface of an s-band simple cubic crystal. As V increases, the bond changes from a perturbative regime to the formation of a surface complex. Investigations of local and layer-summed show this dimerlike structure to be well localized near the bond. Steric effects (binding-site symmetry) are easily introduced. A damping parameter can be added to simulate decay effects of surface probes. Application of the model to photoemission angular-averaged and angular-resolved difference spectra for light gases on tungsten gives understanding of the bonding peak near the bottom of the band, of the depletion of states near the Fermi energy, and of the slight shifting of the energy-momentum-conserving peak.