The high temperature vibrational entropy of (001) surfaces of ionic crystals—Effects of relaxation and rumpling

The high temperature vibrational surface entropy of (001) alkali halide surfaces expressed in terms of entropy per surface atom is much smaller than for the same metal surface if relaxation is neglected. Plausible relaxation and rumpling tends to reduce the surface entropies still further.     

Bound state resonances and related interaction potential for atomic hydrogen on NaCl(001)

Bound state resonances in the coherent elastic scattering of a monoenergetic H atom beam (48.5 meV) on a NaCl(001) surface are measured and analysed, consistently yielding three bound state energies. These energy levels fit into a Morse potential (well depth D = 20.8 meV) and range parameter k = 9.3 nm^?1) which compares well to those of similar interaction systems (as H/NaF and H/KCl) but differs from results reported recently in literature.     

On the electronic potential and ionic relaxation at metal surfaces

Some results, obtained by a simple method which gives the main effect of the ions on the surface potential of metals, are advanced. A general expression is given for the change in the surface dipole barrier. The ionic relaxation of (110) and (111) faces of Al are obtained self-consistently, tending to support one of the two current interpretations of LEED diagrams.     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

Vibrations at 3C-SiC(001)-(32) surfaces

Si-terminated 3C-SiC(001) surfaces with () and ()reconstructions were investigated by high-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and Auger electron spectroscopy. The surfaces were prepared by subsequent annealing steps after cleaning by heating in a Si flux. At ()-reconstructed surfaces, the HREELS intensity increases while the widths of the loss lines decrease with proceeding preparation. Eventually, weak loss structures at 380 and 700 cm^-1 are detected besides the strong Fuchs-Kliewer phonon loss peaks. They are attributed to surface-localized vibrations, i.e., to stretching modes of on-top Si dimers and of C-Si-C groups, respectively. The weak features vanish after exposure to atomic deuterium, but reappear after subsequent annealing. At () reconstructed surfaces the HREELS lines are broadened and no surface-localized modes were resolved. Received 18 January 1999     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

High energy electron diffraction at Si(001) surfaces

Elastic scattering of 10 keV electrons at Si(001) surfaces at grazing incidence was investigated. The intensity of the specularly reflected elastically scattered electrons as a function of the angle of incidence I_el(γ) was measured for different azimuthal angles and was compared with calculations using the dynamical diffraction theory. It turned out that the contribution of the elastically scattered electrons to the total intensity of the reflections strongly decreases with decreasing angle of incidence. Exciting the reflection (008) the elastic contribution is around 30%, decreasing to about 12% in the case of the reflection (004). In the calculations multiple beam effects, absorption, a smooth variation of the potential at the surface and a reduction of the topmost interlayer spacing were taken into account. There is satisfactory agreement between the structures of experimental and calculated intensity curves, I_el(γ) indicating a slight compression of the surface lattice to be probable. Quantitative agreement, however, for absolute intensities was not obtained.     

Ordering and segregation at (001) surfaces of Cu3Au

The interplay of surface ordering and segregation at the (001) surface of Cu₃Au is studied. The calculations are carried out using the tetrahedron approximation of the cluster variation method. The almost continuous disordering observed by low-energy electron diffraction and the Au enrichment observed by low-energy ion scattering spectroscopy can be consistently and accurately reproduced by our model.     

Dynamic ion pairs in the adsorption of isolated water molecules on alkaline-earth oxide (001) surfaces

Low coverage water adsorption on alkaline-earth oxides is studied by first principles methods and ab initio molecular dynamic simulations. On CaO and BaO(001), water dissociation is thermodynamically favored in contrast with the molecular adsorption found for MgO(001). On CaO(001), the barrier for water splitting is very small leading to a tight ion pair H-OH that exhibits a dynamic character with the hydroxyl group able to visit four equivalent adsorption sites while remaining attached to the proton. In contrast, ion pair separation is endothermic by 0.8 eV. These results are common to other basic surfaces such as BaO(001) and have important implications in the chemistry of partially hydroxylated oxide surfaces.     

Very low energy electron reflection at Cu(001) surfaces

Measurements of the coefficient of elastic reflection of very low energy electrons at Cu(001), Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N surfaces are reported. The measurements refer to normally-incident electrons with kinetic energies E in the range 0.5–22 eV. The elastic reflection coefficient R_el was determined from separate observations of the total reflection coefficient and of the energy distribution of reflected electrons. For Cu(001) R_el is 0.55 at E = 0.5 eV and drops monotonically to 0.03 with increasing E, the maximum slope being at E = 3 eV. Theoretical calculations of R_el are reported. The reflection amplitude of the substrate crystal was parameterized using existing results of accurate band structure calculations, and the surface scattering matrix was evaluated for assumed surface scattering potentials. It is shown that to fit the observed R_el it is necessary to take account of both the image potential and the extension of the imaginary part of the crystal scattering potential into vacuum. From the fit, the range of the imaginary potential is 1.0 Å. For Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N the values of R_el at E = 0.5 eV were 0.35 and 0.15, respectively. The effect of adsorption in reducing R_el is especially marked for E < 2 eV. Adsorption of either O or N results in an additional peak in R_el near E = 12 eV.     

Lattice relaxation at na(100) and na(110) surfaces

The surface relaxation of the first interlayer spacing for both the (110) and (100) faces of Na has been calculated selfconsistently using the density functional formalism and a new density matrix method which obviates the use of wavefunctions. For the Na(110) surface we find no relaxation of the first interlayer spacing in agreement with LEED studies, while for the Na(100) surface we predict a 2% reduction. Force constants for the movement of the last atomic layer are also determined.     

Surface energy and relaxation effects at (111) semiconductor surfaces

A local orbital method is used to calculate the relaxed position of the unreconstructed (111) surface layer of atoms in a silicon or diamond crystal. The separation of the outermost layers decreases to about 40% of its bulk value and the surface energy is found to be about 1.0 eV per surface atom for silicon and 1.6 eV for carbon.