We use ab initio total-energy minimization methods to study the equilibrium structure of the InP(100) surface, and in particular examine the structure of the surface as a function of In coverage (Θ). ...We find the low-coverage (
Θ = 0.25), non-dimerized (2 × 4) surface to yield the lowest energy per In atom, and therefore to possibly constitute the ground-state structure of the surface, even though it has not been observed experimentally. Nevertheless, we have found the In-dimerized surfaces at all coverages to be stable. For the fully In-covered surface, we observe the equilibrium structure to be the symmetric (2 × 1) phase, i.e., with the In dimers adopting a symmetric configuration with respect to the surface. For a coverage of
Θ = 0.75, we find a symmetric-dimer (2 × 4) reconstruction to provide a stable equilibrium configuration. The dimer bond length, we observe, is relatively insensitive to coverage, and in view of the fact that LDA calculations tend to underestimate distances a little, we predict that the experimental bond-length value should be very close to 2.8 Å. For low In coverage surface, our calculations indicate that phosphorus dimers may form.
The aim of the present paper is to contribute to the elucidation of the molecular structures obtained on a platinum surface as this surface is submitted to an anodic potential (with respect to a ...silver reference electrode) when dipped into pure 2-methyl 2-propenenitrile (methacrylonitrile). Modified surfaces are examined using X- and UV-photoelectron spectroscopies (UPS and XPS). The results evidence the formation of an ultra-thin (20–40 Å) grafted oligomer film, which is not classical polymethacrylonitrile (PMAN), as obtained through a radical or anionic mechanism: spectral characteristics argue in the sense of a cationic polymerization of methacrylonitrile through its nitrile groups, as evidenced by a lowering of the gap as well as by the UPS and XPS (N 1s region) spectra. Molecular models of the reactants and reaction intermediates are proposed for the cationic polymerization of methacrylonitrile, and show that this polymerization is about as feasible as that of acetonitrile, at least on kinetic control grounds. Two different mechanisms are nonetheless possible, leading either to a quasi conjugated poly-imine type -(N C)
n
-, or to a poly-cumulene type -(N C C)
n
- network. Theoretical consierations on reactants properties lead us to select the poly-imine way as the most plausible. Along with literature data concerning chemisorbed nitriles on platinum surfaces, a molecular model of the final state of the poly-imine reaction is then designed, comprising a three atom cluster to render the grafting site, and a dimer to render the grafted structure. A full geometry optimization is performed on the organic moiety at the Hartree-Fock (ab initio) level of theory, and a rough evaluation of the spectral footprint of the interface bond in the N 1s region is performed on the basis of Koopmans theorem with calibration on the bulk polymer peak. A preliminary 2.7 eV downward shift is predicted for N 1s interface nitrogens with respect to the polymer peak, which can be compared with the low-energy contribution, found about 2.0 eV below the polymer peak, in the experimental spectrum. The directions in which the molecular model of the interface need be improved are discussed. On the basis of the present results, as well as those obtained previously in the methacrylonitrile/nickel cathode interaction, the conclusion examines the proposition that the structure of the very interface in the final state of electropolymerization reactions is the frozen footprint of the initial stages of the interaction, and alludes to electrochemistry as a tool to monitor molecule/surface interactions.
The origin of the core-level shift of the Si 2p level of surface atoms and that of the (
n−1)s level of adsorbate metal atoms has been investigated by means of all-electron ab initio Hartree-Fock ...cluster model wave-functions. The calculated initial-state core-level shifts for both levels is much smaller than the one expected from an ionic bond where the interacting systems are charged, and seem to indicate that these results are not compatible with the formation of an ionic bond. Nevertheless, the main trends are in agreement with the experimental findings. However, a careful analysis carried out by means of the CSOV method permits an understanding of the origin of these small calculated (or experimental) core-level shifts and allows us to identify the physical mechanisms responsible for this behavior.
Ab initio SCF-MO calculations using effective core-potential basis sets are employed to evaluate ionization potentials and electron affinities for CF3I and the geometries and energies of the singlet ...and triplet states of the CF3I dimer. The calculated geometry of the single state of the dimer is in qualitative agreement with the experimental geometry for condensed phase CF3I. The calculated energy for vertical excitation from the singlet to the triplet state is 4.1 eV at the Hartree-Fock level and 4.5 eV after incorporation of correlation at the Moller-Plesset 2nd-order level, consistent with excitation by 193 nm (6.4 eV) light. The equilibrium geometry of the triplet consists essentially of a CF3I+, CF3I− ion pair, in which the CI bond distance in the anionic component has increased to 5.5Å, compared with 2.1Åin the neutral molecule. The calculated binding energy of the triplet ion pair is about 4 eV.
Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the ...Hartree-Fock level on AlSi
n
subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T
4 site resulting in Al adsorption at the T
4 site to be 5 kcal/mol more stable than at the H
3 site and considerably more stable than adsorption at the T
1 site. However, we only have confidence in this result after performing for the T
4 site a partial geometry optimization on the AlSi
5 subcluster in the AlSi
26H
24 cluster and by including appropriate correlation corrections.
The bulk and surface excitons in the crystalline LiF were theoretically studied by means of a cluster model embedded in the ion cage. The method employed is that of the self-consistent field (SCF) ...theory, where multi-center exchange potentials as well as integrals are calculated exactly within the accuracy of the basis sets. It will be shown that the bulk exciton band lies 11.3 eV above the valence band at the SCF level approximation while a correlation correction brings the band 13.4 eV above the valence band. The experimental bulk value is 13.5 eV. The SCF approximation gives the surface exciton 9.4 eV above the valence band while the correlation correction brings the band 11.5 eV above. The experimental value is 10.3 eV. The difference of the charge distributions between the surface and bulk exciton states will be also discussed.
Based on the path integral method formulated in terms of the Thouless parametrization, numerical calculations of the ensemble average of the polarizability and the second hyperpolarizability are ...carried out by means of the Monte Carlo method. An H
3
+ system is examined to elucidate the performance of the present method. We discuss the dependence of the calculated (hyper)polarizabilities on the number of the division of the partition function and the accuracy of the results obtained by the present method.
The effect of the electric field created by an external voltage at a scanning tunneling microscope interface is analyzed by means of a self-consistent LCAO approach. The theoretical method introduced ...in this paper allows us to analyze realistic tip-sample geometries without introducing spurious two-dimensional periodicities. We present results for both the lateral forces acting on different alkali atoms adsorbed on an Al(100) sample and the atomic desorption potential induced by an external voltage. The evaporation fields for Na and K are calculated for different tip-sample distances, and the results are in good agreement with the experimental evidence.
Ab initio molecular orbital theory and an atomic cluster model are used to study the structures and energetics of a boron-terminated diamond (100) surface, and hydrogen chemisorption on this surface. ...A 2×1 B–B dimer reconstruction is shown for the boron-terminated C(100) surface and the strength of B–B dimer bond is found to be stronger than that of a π bond in the CC dimer of a clean C(100)2×1 surface. Chemisorption of hydrogens on the boron-terminated C(100)2×1 surface breaks the surface B–B dimer bonds without activation energy which is in contrast to that on the clean C(100)2×1 surface. The strength of a B–H bond on the boron-terminated C(100) surface is found to be weaker than that of a C–H bond on the clean C(100) surface and so is the dehydrogenation energy.
