M. A. Kamel

Excitons, electron center diffusion and adsorptivity of atomic H on LiH (0 0 1) surface: Ab initio study

Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F+, F and F− centers (α, β and γ bands), diffusion of electron centers (F+, F and F−) and adsorptivity of atomic H over the undefected and defected (F+, F and F−) surfaces of LiH using an ab initio embedded cluster method at the Hartree–Fock approximation and Moller–Plesset second-order perturbation correction. The results confirm the exclusive dependence of the exciton bands on the type of the electron center. The activation energy for bulk diffusion increases monotonically in the series F+→F→F−. Bulk and surface relaxation effects are more important for F+ than for F and F− centers. The introduction of F or F− center changes the nature of adsorption from physisorption to chemisorption. The introduction of F− center changes the nature of LiH surface from an insulating surface to a semiconducting surface. As F and F− centers are introduced, the HOMO and LUMO levels of the substrate shift to higher energies and the band gaps become narrower. These changes in the electronic structure make charge transfer between adsorbate and substrate energy levels and spin pairing with F center more facile in the course of adsorbate–substrate interactions.

Properties of F+, F and F− electron centers and adsorptivity of atomic H on LiF and NaH isoelectronic crystals: an ab initio study

Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F + , F and F − centers (α, β and γ bands), diffusion of electron centers (F + , F and F − ) and adsorptivity of atomic H over the defect free and defect containing surfaces of LiF and NaH isoelectronic crystals using an ab initio embedded cluster model at the second order Moller–Plesset perturbation level. The LiF and NaH clusters were embedded in simulated Coulomb fields that closely approximate the Madelung potentials of the host crystals. The isoelectronic LiF and NaH clusters in crystals were found to exhibit distinct differences in the title properties. The defect free and F + band gaps and exciton bands of LiF were significantly greater than those of NaH. The LiF crystal was more sensitive to the relaxation effects than NaH. The activation energy barriers to the electron center diffusion hops in LiF were always greater than those in NaH. The H atom adsorbs more strongly on the defect free, F + and F − surfaces of NaH relative to LiF. The reported changes in band structure due to surface imperfection explain the dramatic increase of atomic H adsorption over F and F − surfaces of LiF and NaH as well as the preferred stability of atomic H over the defect free and F + defect containing surfaces of NaH in the course of adsorbate substrate interactions. The reported differences in properties are possibly attributed to the differences in the lattice interionic interactions and the extended charge distribution of the hydride anion.

M CENTER DIFFUSION, EXCITONS AND ADSORPTIVITY OF ATOMIC H AND He ON LiH (001) SURFACE: ab initio STUDY

An attempt has been made to examine the energetic properties of M center diffusion, excitons near M2+, M+, M, M- and M2- centers and adsorptivity of atomic H and He over defect free and defect containing surfaces of LiH using an ab initio embedded cluster method at the Hatrtree–Fock approximation and Moller–Plesset second order perturbation correction. The results confirm the following, (1) the calculated barriers to diffusion of M center in its lowest triplet excited state is always greater than those in its singlet ground state; (2) the triplet M center is not produced directly by optical processes, but, indirectly by thermal diffusion; (3) the exclusive dependence of exciton bands and the nonexclusive dependence of band gaps on the defect charge; (4) surface relaxation is not more important than bulk relaxation; (5) the M center changes the nature of H adsorption from physical adsorption to chemical adsorption; (6) bulk or surface M2- changes the nature of LiH from an insulator to a semiconductor; (7) as M center is introduced, the HOMO and LUMO levels of the substrate shift to higher energies and band gaps become narrower. This change in the electronic structure makes charge transfer between adsorbate and substrate levels and spin pairing with atomic H more facile.

Many body expansion and ion diffusion in LiH crystal

A finite LiH lattice whose Madelung potential in the central region closely approximates the Madelung potential in the host crystal is constructed. Hartree-Fock calculations were then carried out on lithium hydride clusters both within the crystal and as isolated species. The many body expansion terms and the probability of ionic motions which result in diffusion are examined and relaxation around diffused ions is taken into account. Calculations confirm that the many body expansion terms in the crystal environment are both convergent and smaller than for the isolated clusters. The two-ion rotation mechanism is the most probable. The barrier height for cation diffusion is less than for anion diffusion suggesting easier transfer of cations rather than anions within the LiH crystal. The results are correlated with those reported previously on LiF crystal.

Potential energy curves of H and H− interactions with He

Abstract Potential energy curves of H and H − interactions with He have been computed using a quadratic configuration interaction method and universal even-tempered basis sets. The effects of bond function augmentation are considered. Crossing radius and total cross section were calculated to be 2.527 a 0 and ∼ 5.6 × 10 −16 cm 2 in closer agreement with the experimental determination of 1.997 a 0 –2.387 a 0 and ∼ 3.5–5.0 × 10 −16 cm 2 than the previously reported ab initio estimates. Bond function augmentation enhances the agreement with experiment as far as crossing radius and total electron detachment cross sections are concerned. Potential curves were fitted to analytical potentials to decompose the threshold energy at the crossing radius into its major components.

An ab initio approach to many-body energies and Be2+V- dipoles in LiH crystal and clusters

A finite LiH crystal whose Coulomb potential in the central region closely approximates the Madelung potential in the unit cell of the host crystal is constructed. The beryllium ion is then introduced to initiate the Be2+V- dipole and to examine the perfect and defect properties of LiH clusters both within the crystal and as isolated species. These include the convergence properties of many-body energies, the defect formation mechanism and hydride ion migration. Crystal field and overlap effects are examined. Lattice relaxation around defect sites is allowed and the optimal relaxation mode is assigned. Dipole aggregates, the cluster-lattice interaction, defect formation energies, the free rotation of the Be2+V- dipole in two perpendicular planes, the energy of rotation of the cation vacancy around Be2+ and the tendency of Be2+ to associate with the cation vacancy are examined and explained in relation to their scientific and technological importance.

Bulk dislocation-U defect interaction, surface excitons and adsorptivity of atomic H on dislocated surfaces of LiH crystal: ab initio calculations

An ab initio embedded cluster method was used to examine the bulk dislocation-U defect interaction, surface excitons and the adsorptivity of atomic H on dislocated surfaces of LiH using the Hartree-Fock approximation and the second-order Moller-Plesset perturbation correction. In the LiH crystal bulk, the results confirm: (1) U1 and U2 centres make dislocations more facile, (2) dislocation processes do not reduce the ionic conductivity of highly populated edge centred hydride interstitials and (3) the dislocation-U defect interaction increases monotonically in the series face→volume→edge centred interstitial structures. On LiH crystal surfaces the results confirm: (1) the exclusive dependence of band gaps and exciton bands on dislocation, (2) the strongest adsorption of atomic H on a surface is associated with X-dislocations, (3) dislocations are unable to change the nature of physical adsorption to chemical adsorption and (4) the mobility of atomic H over the Z-dislocated surface is more facile than that over the X-dislocated surface. As X-surface dislocation proceeds, the HOMO and LUMO levels of the substrate shift to higher energies and the band gap becomes narrower. This change in the electronic structure suggests that charge transfer from the X-dislocated surface is more facile in the course of adsorbate-substrate interaction.