Franco Ricca

CO adsorption on MgO crystals: Hartree-fock calculations for regular adlayers on a (001) lattice plane

The regular adsorption of CO on a MgO(001) monolayer has been investigated at an “ab initio Hartree-Fock crystalline-orbital level, with a basis set containing 9 atomic orbitals per atom. Both vertical bindings to Mg (through oxygen and carbon) have been considered and the C-O as well as the Mg-CO or Mg-OC distances have been optimized. At the 1/2 surface coverage (one CO molecule every two surface Mg ions) adsorption energies of 4.13 (carbon binding) and 3.29 (oxygen binding) kcal/mol were obtained, to be compared with an experimental adsorption heat of 3.6 kcal/mol. Similar energies were obtained for the 1/4 coverage, whereas the 1/1 adsorption is less favoured, due to the repulsion between the CO molecules. The charge redistributions in the CO molecule and in the substrate are considered, as well as the changes occurring in the density of states for the interacting systems.

States of Helium Atoms Adsorbed on Krypton and Xenon Crystals

The quantum‐mechanical state of a He atom absorbed on the (100) face of Kr and Xe crystals has been studied. The variational function for the adsorbed atom has been constructed using a linear combination of the Bloch sums of local functions corresponding to suitable three‐dimensional anharmonic oscillators. The probability distribution for the coordinates of the He atom has been described by a set of isodensity curves similar to those normally employed when studying atomic or molecular orbitals. The gap between the energy bands corresponding to the fundamental and first excited state implies that, at the lowest experimental temperatures, the surface mobility must be interpreted in terms of the fundamental state. For the fundamental state, the total energy is higher than the potential barrier to surface migration in the case of Kr and slightly lower in the case of Xe.

Regular chemisorption of hydrogen on graphite in the crystalline orbital NDO approximation

A Hartree–Fock treatment of the regular chemisorption of hydrogen on a graphite monolayer in a crystalline orbital NOO approximation is presented. The computational problems concerning the full exploitation of the point symmetry of the lattice, as well as the integration procedures are the same as encountered in a corresponding ab initio treatment. Three different cases have been considered, corresponding to the chemisorption of one hydrogen atom per unit cell: (A) above alternate carbon atoms; (B) above the center of a C–C bond; (C) at the center of a carbon ring. Bound states are found to occur only for cases A and B. The electronic features of the adsorbent–adsorbate system are appreciably differentiated. Such differences are discussed by using energy band structures and total and partial density of states spectra.

Statistical analysis and model determination for thermal desorption spectra: Nitrogen on tungsten

Abstract Thermal desorption spectra of nitrogen chemisorbed on tungsten have been examined in order to discriminate between two simple alternative kinetic models for the desorption process. Both these models consider the β 2 desorption as a second order process, while for the β 1 peak they assume first order and second order kinetics, respectively. The maximum likelihood method has been used for fitting the spectra and the resulting kinetic parameters have been discussed through the analysis of the correlation matrix. The strict correlation which exists between activation energies and frequency factors prevents effective discrimination and should diminish confidence in too simple and rigid models.

LCAO SCF crystalline orbital method in the CNDO approximation

Abstract Point group transformation of the LCAO CO basis functions is applied to confine the Roothaan equations within the irreducible part of the Brillouin zone. The general treatment is then translated into the CNDO scheme and a preliminary application to the graphite monolayer is considered.

Embedded versus non-embedded-cluster treatment of chemisorbed systems: A model computation

Abstract The chemisorption of carbon monoxide on the basal plane of graphite has been considered in a CNDO 2 approximation using an isolated- and an embedded-cluster approach. It is shown that conformational data practically coincide in the two approaches, whereas chemisorption energies and electronic structure data exhibit relevant discrepancies.

Regular phases of atomic hydrogen chemisorbed on graphite

The crystalline orbital-NDO method is applied to regular chemisorption of atomic hydrogen on a graphite monolayer and further features of this method are discussed, especially as concerning truncation in direct lattice sums. Particular attention is given to the balance of Coulomb terms at relatively short distances. The surface phases allowed by the graphite symmetry for hydrogen atoms chemisorbed at equivalent positions in the hydrogen/carbon atomic ratios 12, 11 and 32 are discussed in terms of adsorption energy and electron charge distribution, duly taking into account the substrate relaxation. The greatest adsorption energy is shown by the 11 phase, with formation of typical CH chemical bonds. An even greater stability is shown by a phase having the same overall composition in which the graphite monolayer is attacked on both sides by atomic hydrogen, giving rise to a “chair” structure of carbon rings similar to that in cyclohexane. Such a phase could be of interest for inclusion and exfoliation processes possibly occurring in graphite demolition.

Ab-mitio Hartree-Fock investigation of the surface features of LiH slabs of different thickness

Abstract Slabs from one to five layers thick have been studied by using the CRYSTAL ab-initio program, in order to test their suitability to simulate the (001) surface of a semi-infinite LiH crystal. Surface and relaxation energies have been evaluated, as well as ionic charges, orbital populations, dipole and quadrupole moments perpendicular to the surface, energy bands and density-of-states spectra. The three-layer slab appears to be adequate for a reasonable approach to problems concerning surface processes.

Multiple expansion of molecular charge distribution

A method is given for generating multipole moments of any order from electron densities defined by monodeterminantal wavefunctions. The convergence of the multipolar expansion for the electrostatic field due to a given molecule is discussed for different segmentations of the total charge, and the application of the above method to reactivity studies is considered. Maps for the electrostatic potential and its gradient are given for pyrimidine, together with the corresponding force lines.

The adsorption of SiO molecules on MgO surfaces as a model for the silicon lever atomic force microscope (AFM)

Abstract Computer simulation of a 1 :4 SiO monolayer adsorbed on a MgO monatomic slab has been performed by using the CRYSTAL “ab initio” program. The Mg 2+ ions have been identified as the preferred adsorption sites. Adsorption energies, geometries and electron densities have been evaluated. Their dependence on the interlayer distance has been considered for investigating the behaviour of the AFM with microfabricated silicon lever in the pressure region of 10 −8 N. The AFM topographical images are strongly dependent on the chemical nature of the tip.