Adsorbate-adsorbate interactions and the ordering of organic monolayers on metal surfaces

To study the ordering of molecules adsorbed on single-crystal substrates, a molecular cross-section (MCS) is defined, which measures the surface area occupied by each molecule. With this MCS, a two-dimensional packing coefficient C_2D is then defined for ordered arrays of adsorbed molecules. Values and trends for MCS and C_2D are discussed for known surface structures, especially for benzene adsorbed on metal surfaces. The packing is found to be generally less dense at surfaces than one would expect from comparison with packing in three-dimensional organic crystals. The Van der Waals packing energy and the repulsive dipole-dipole energy are also computed to study this issue. The lack of close-packing is attributed to the need to respect structural coincidence with the substrate and/or co-adsorption of small molecules like CO. These concepts are then applied to the prediction of the long-range order that a monolayer of adsorbed molecules may adopt: thereby possible adsorption structures can be defined, restricting the number of possibilities in a further structural determination.     

Nonstationary processes in an electric field during coadsorption of gold and alkali metals on tungsten

The processes induced by a strong electric field in the “tungsten-gold-alkali metal” adsorption systems are studied. Field desorption of K⁺ and Cs⁺ ions initiates nonstationary processes involving ion current pulsations and periodic displacements of desorption regions under constant experimental conditions. The observed effects are explained as resulting from the processes of (i) formation of an alkali-metal-gold film exhibiting properties of a wide-band-gap semiconductor, (ii) breakdown and restoration of this film in an electric field, and (iii) accumulation and relaxation of the electric charge.     

The electron-ion interactions in metals

The electron-ion interactions are evaluated exactly over the actual shape of the atomic polyhedron, instead of approximating it by a sphere, by making use of simple coordinate axes transformations and lattice symmetry in the case of f.c.c. and b.c.c. structures. It is shown that there are several alternative ways of expressing the interference factor,S(q) and the different expressions given by Sharan and others and Bross and Bohn are just two of these equivalent expressions. By comparing these expressions in the symmetry directions with those obtained under spherical approximation their apparent differences are discussed.     

Adatom dipole moments on metals and their interactions

An expression is derived for the dipole moment μ, associated with an arbitrary adatom on an arbitrary metal surface, in terms of the differential work done in moving the adatom from the interior of the metal to its actual position in the presence and absence, respectively, of small, asymptotically uniform electric field on the exterior of the metal. With the aid of this expression it is shown that the non-oscillatory part of the interaction energy of two dipoles μ_A and μ_B, situated on the surface and separated by a large distance v, is given by U_AB = 2μ_Aμ_Bv^?3. This exceeds by a factor of 2 the interaction energy of two dipoles in the same relative configuration but in a vacuum.     

Electron-phonon interactions in transition metals

The tight-binding method is used to calculate 〈I²〉, the Fermi surface average of the squared electron-phonon coupling constant for 4d b.c.c.—transition metals and alloys. When nonorthogonality effects are properly included, our results for 〈I²〉 agree very well with empirical values. Moreover, the variation of 〈I²〉 can be understood in simple physical terms.     

The electron-ion interactions in tetragonal metals

The electron-ion interactions are evaluated exactly over the actual shape of the atomic polyhedron, instead of approximating it by a sphere or an ellipsoid, by making use of simple co-ordinate axes transformations and lattice symmetry in the case of fct and bct structures. It is shown that there are several alternative ways of expressing the interference factor,S(q), one of which was obtained by Sharan and others in the case of indium. By comparing these expressions with the latter, with those of corresponding cubic structures as well as with those obtained under spherical approximation respectively, the crystallographic equivalence and stability of tetragonal structures as well as the validity of Wigner-Seitz approximation are discussed.     

Electron-ion interactions in close packed metals

The electron-ion interactions are evaluated exactly over the actual shape of the atomic polyhedron by making use of simple co-ordinate axes transformations and lattice symmetry in the case of hcp and ccp structures. It is shown that the expressions for the interference factor,S(q,t) of hcp structure are complex while those of ccp structure are real, even when both atomic arrangements are referred to the same orthorhombic co-ordinate axes, and in each case, lattice atom contributions could be distinguished from basis atom contributions toS(q,t). By comparing these expressions with each other as well as with those obtained by approximating their atomic polyhedra by an ellipsoid of equivalent volume, apparent differences between interference factors of hcp and ccp structures, validity of Wigner-Seitz approximation for a diatomic lattice and the manner in which the electron-ion interactions contribute to the different modes of vibration in a hexagonal lattice are discussed.     

Quadrupole interactions in metals and alloys

The contributions of the perturbed charge density of conduction electrons due to quadrupole moments of impurity paramagnetic ions and nuclei to the electric field gradient at the nuclei in metals and alloys are analyzed. The indirect multipole interactions of nuclei in metals via the mediation of conduction electrons is investigated. The influence of these interactions on the NMR parameters is studied.     

Carbon monoxide and hydrogen coadsorption on polycrystalline platinum

The coadsorption of carbon monoxide and hydrogen was studied on a polycrystalline platinum foil. In a comparison with single crystal surfaces, the desorption kinetics of the individual species most closely resemble those on Pt(110). In coadsorption there is competition between CO and H₂. Carbon monoxide completely blocks hydrogen adsorption, but on a hydrogen-saturated surface only two-thirds of the carbon nomoxide adsorption is blocked. Shifts in peak temperatures for hydrogen desorption indicate that repulsive interactions between adsorbed CO and H₂ are important, and lead to segregated islands of the two adsorbates. Under some conditions there is also a new low temperature desorption peak for hydrogen which indicates that there are regions on the surface where carbon monoxide and hydrogen are intermixed.     

Anisotropy of electron-phonon interactions in alkali metals

Electron-phonon scattering in the solid alkalis is distinguished from that in most other metals by a combination of three circumstances: The phonon spectra and structure factors are very anisotropic, the Fermi surface in the reduced zone is simply connected and virtually spherical and important large momentum transfers (0.7<(q/2k _F)<1.0) fall within the first large peaks of the phonon structure factors. Anisotropy of microscopic contributions to the macroscopic coefficients is controlled by and is quite sensitive to values of electron-ion matrix elements at large momentum transfer, and can be explored by a realistic yet relatively simple theoretical calculation. A brief summary is presented of such calculations, for the all alkalies, of mean free paths, thermoelectric powers, and electron-phonon mass enhancements. The results show marked anisotropy only for lithium, are consonant with experimental low field Hall coefficients and in addition indicate strong anisotropy in the mass enhancement for lithium.     

Impurity interactions and pseudo-molecule formation in metals

It is shown that the concept of molecular orbitals is a useful one also in describing interactions between impurities in metals. The conclusion is based on first principles, self-consistent calculations for H₂ in jellium. The dissociation energy is primarily determined by the position of the Fermi energy relative to an antibonding molecular resonance. In a dense metal like A? this is completely filled and the dissociation energy is an order of magnitude smaller than for free H₂.     

NMR: hyperfine interactions in oxides and metals

A NMR characterisation is given of various polymorphs of TiO₂ (anatase, rutile and brookite), Ti₂O₃, perovskites CaTiO₃ and BaTiO₃, FeTiO₃, TiB₂, titanium metal, the titanium aluminides Ti₃Al, TiAl, TiAl₂, TiAl₃, and TiAg. Values of chemical or Knight shift, nuclear quadrupole coupling constant and asymmetry parameter were derived from the (1/2, −1/2) powder lineshapes. For TiB₂, titanium metal, TiAl, and TiAl₃, where ±(1/2, 3/2), and higher satellite transitions were observed, a value for the axial component of the Knight shift was obtained.     

Nature, strength, and consequences of indirect adsorbate interactions on metals

Pattern selection, occurring when a nonuniform state of a nonlinear dissipative system propagates into an initially unstable, homogeneous basic state is reconsidered by application of the causality principle. In particular, the nonlinear marginal stability criterion that determines the selection of a nonlinear front solution is replaced by an exact general necessary condition that has never been considered before. The demonstration is based on the causal signaling problem derived in the context of plasma physics.     

Influence of Coulomb interactions on the phase diagram of quasi-one dimensional metals

The influence of only partially screened Coulomb interactions on the phase diagram and the order parameter of quasi-one dimensional metals is investigated. Using a standard microscopic model, the free energy functional is derived by means of the heat kernel method. It is assumed that the Peierls gap and the mismatch are small compared to the band width and the reciprocal lattice vectors, respectively. Furthermore we neglect interchain to intrachain hopping elements. The resulting mean field phase diagram and the properties of the order parameter are discussed. We show in particular that the Coulomb forces are responsible for:a) a first-order transition between the incommensu-rate and the commensurate phase;b) only small deviations of the order parameter from a single plane wave over the entire incommensurate phase; andc) the approximate temperature independence of the wavelength of the modulation throughout the incommensurate phase. The possible relevance of these results for quasi-one dimensional systems exhibiting nonlinear conduction is pointed out.     

The coadsorption of hydrogen and potassium on Ir(111)

The adsorption of gas-phase atomic hydrogen on potassium-precovered Ir(111) surfaces was investigated. Even very low coverages of potassium adatoms strongly inhibit the dissociative adsorption of molecular hydrogen. However, using gas-phase atomic hydrogen allows us to overcome the activation barrier for dissociative hydrogen adsorption. In addition, abstraction of hydrogen adatoms by impinging atomic hydrogen occurs. The probabilities and cross-sections for both reactions and the maximum number of hydrogen adsorption sites are derived and compared to data obtained on other surfaces. Furthermore, a kinetic isotope effect in the desorption of hydrogen and deuterium was observed. Implications of these results with respect to the potassium-hydrogen interaction are discussed.     

The coadsorption of I and Cl on Pt(111)

Using LEED and AES, the coadsorption of iodine and chlorine on Pt(111) was studied. Five surface structures were observed: (1 × 1), (√3 × √3)R30°, (√7 × √7)R±19.1°, (3 × 3) and c(2 × 4). Fractional monolayer coverages for both Cl and I are assigned to these structures. The importance of the adatom-adatom steric interactions is discussed.