D. R. Hamann

Modification of transition metal electronic structure by P, S, Cl, and Li adatoms

We have evaluated the electronic perturbations induced on a thin Rh(001) film by the adsorption of 1/4 monolayer coverages of P, S, Cl or Li atoms. The self-consistent Surface Linearized Augmented Plane Wave (SLAPW) calculations indicate that the P, S, and Cl adlayers cause only very slight work function changes, and generally give rise to perturbations in the valence charge that are small beyond the nearest neighbor Rh atoms. We attribute the latter result to the effectiveness with which the charge density is screened. In contrast, reductions induced in the Fermi level Local Density of States ( E f -LDOS), whose magnitude reflects the ability of the surface to respond to the presence of reactants, are substantial even above next nearest neighbor Rh atoms, which are 8 a.u. away. The effect is strongest for Cl and weakest for P. When Li is adsorbed, the work function is reduced by almost 2 eV. This decrease is associated with an increase in electronic charge density over most of the surface, and at the same time, an increase in the E f -LDOS. Experimental observations of CO adsorption and dissociation on transition metal surfaces indicate a “poisoning” effect by coadsorbed P, S, and Cl atoms and a “promotion” effect by coadsorbed alkali atoms. The SLAPW results for the E f -LDOS parallel these observations.

Theory of reconstruction induced subsurface strain — application to Si(100)

Abstract The stress produced by surface layer reconstruction on semiconductors can produce sizable elastic distortions of deeper layers. These are calculated using a standard bulk model of interatomic force constants for the pairing model of 2 × 1 reconstruction on the (100) surface of C, Si and Ge. Kinematic analysis for the predicted geometry with subsurface distortions is shown to give good agreement with LEED intensity data for Si. This reconciles the LEED results, for which the simple pairing model fails completely, with spectroscopic measurements, which are best fit by the pairing model.

Theory of H bonding and vibration on Pt(111)

Abstract Linearized augmented plane wave (LAPW) calculations of the structural energies of a H monolayer absorbed on a Pt(111) film predict a fcc site, a H-Pt bond length of 1.86 A, a symmetric stretch vibration frequency of 166 meV, and an asymmetric stretch frequency of 114 meV. The symmetric stretch is predicted to be weakly dipole active, with an effective charge of 0.054 e per unit cell. The results are found to be inconsistent with a nearest-neighbor spring model of the adsorbate dynamics. The site and surface charge density are compared with He diffraction results, and the vibrational frequencies are compared with electron energy loss and inelastic neutron spectra.

Electronic structure of the Cu(111) surface

Abstract Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.

Theory of H bonding and vibration on Ru(0001)

Abstract Self-consistent linear augmented plane wave total energy calculations have been carried out for a 1 × 1 H monolayer on the Ru(0001) surface. They give an HRu layer separation of 1.06 A and vibrational frequencies of 140 meV for the symmetric stretch mode and 100 meV for the asymmetric stretch mode. These compare very well with measured values of 138 and 105 meV, but reverse the original mode assignments which appeared to be consistent with a nearest neighbor harmonic force model. Our results are qualitatively inconsistent with such a model. Another simple model, the effective medium theory, also fails to account for our results in the absence of significant covalent correction terms.

Surface states of Sc(0001) and Ti(0001)

Abstract Calculated electronic properties are compared for 11-layer Sc(0001) and Ti(0001) films. Prominent surface states are found whose locations conform to expectations based on the respective bulk band structures establishing a roughly rigid band picture of the surface bands. Surface core-level shift and work function results are qualitatively explained.

Electronic structure at an abrupt GaAs–Ge interface

The potential, charge density, and interface states have been calculated for the ideal interface between intrinsic GaAs, terminated on a (100) Ga plane, and intrinsic Ge. The conduction band is found to be nearly continuous across the interface and only a small interface dipole moment is found. Even though the two crystals, Ge and GaAs, have the same crystal structure and lattice parameters (a situation which might be thought to produce an interface containing no states in the forbidden gap at the Fermi level), a simple quantum‐mechanical counting argument shows that there must be interface states at the Fermi energy for the unreconstructed interface. These states are the band‐picture equivalent of unsaturated bonds. We find that fractional occupancy of the interface bonds (each of which contains 1.75 rather than two electrons) arises via a single partially occupied band of interface states. Consideration of the electronic energy suggests that the actual interface will be reconstructed.

Theory of H bonding and vibration on close‐packed metal surfaces

Self‐consistent linearized augmented plane‐wave calculations for H monolayers adsorbed on Pt(111), Ru(0001), Cu(111), and Cu(1×1)/Ru(0001) thin slabs predict the following: the H‐atom equilibrium position on these substrates is generally in the face‐centered cubic threefold hollow at a height above the surface where the clean‐metal electron density is about 0.015 a.u. The symmetric stretch (SS) frequency lies in the range 130–165 meV, and is about 30% greater than the asymmetric stretch (AS) frequency on the same substrate. These results contradict the mode assignments of Baro et al. [A. M. Baro, H. Ibach, and H. D. Bruchman, Surf. Sci. 88, 384 (1979)] for H/Pt(111) which were based on the observation of weak dipole scattering by the low‐lying mode, and of Barteau et al. [M. A. Barteau, J. Q. Broughton, and D. Menzel, Surf. Sci. 133, 443 (1983)] for H/Ru(0001). The pairwise spring model of H–metal vibration invoked by these groups in support of their mode assignments neglects a major component of the H–me...

Modification of Cu-H bonding near a Ru(0001) surface

Abstract Surface linearized augmented plane wave (SLAPW) total energy calculations are used to study differences between Cu-H bonding on Cu(111) and on a pseudomorphic Cu monolayer on a Ru(0001) substrate. Bond lengths and vibration frequencies for H (1 × 1)/ Cu (1 × 1) Ru (0001) versus H (1 × 1) Cu (111) roughly obey an “effective medium” picture: the H atoms prefer to reside at a particular clean-surface charge density (0.013–0.015 au), and vibrate along the surface normal with a frequency, ω⊥, proportional to the local, clean-surface normal charge density gradient. This results in a predicted 20% reduction of ω⊥ on Cu (1 × 1) Ru (0001) surface relative to ω⊥ for Cu(111). At the same time, the weakening of the Cu-Cu bond when a Cu(111) layer is strained to fit pseudomorphically on Ru(0001) is partially compensated by the fact that the Cu-Ru bonding is stronger than Cu-Cu. Thus the binding energy of a H monolayer on Cu (1 × 1) Ru (0001) is calculated to be only 0.05 eV H greater than on Cu(111).

Geometric vs. electronic factor in surface electronic structure-H adsorption on Sc and Ti(0001)

Abstract By comparing outer layer local densities of H monolayers on Sc(0001) surfaces, we show that it is geometry and not electron number per atom that is the dominant influence on the surface electronic structure of these adsorption systems.