One-dimensional (1D) and two-dimensional NMR exchange measurements were performed to probe the dynamics of hydrogen adsorption on highly dispersed silica-supported ruthenium particles at pressures from ${10}^{\mathrm{-}4}$ to 600 Torr and temperatures between 292 and 400 K. We found three distinct NMR resonances, ${\mathrm{\alpha }}_{\mathit{I}}$, ${\mathrm{\alpha }}_{\mathit{M}}$, and β, respresenting hydrogen interacting with the metal surface. The character of motion of the three hydrogen species was inferred from 1D and 2D NMR experiments. Also, two types of exchange processes were found: (i) a slow chemical exchange occurring on a submillisecond time scale between ${\mathrm{\alpha }}_{\mathit{M}}$ and β and (ii) a fast exchange between weakly adsorbed hydrogen and hydrogen in the gas phase, with both hydrogen species contributing to the β NMR line.

A general concept employing the dehydrogenation of hydrogen-containing organic and inorganic compounds is shown to readily affect the production of a wide variety of elemental and binary clusters. Hydrogen-containing molecules such as ${\mathrm{C}}_2$${\mathrm{H}}_4$, ${\mathrm{NH}}_3$, ${\mathrm{SiH}}_4$, ${\mathrm{H}}_2$S, or ${\mathrm{H}}_2$O, are introduced into an extremely energetic plasma generated from laser irradiation of a metal surface; this plasma dehydrogenates the molecules to produce nascent C, N, Si, S, and O atoms. These C, N, Si, S, and O atoms can react with metal atoms or ions contained in the plasma and form metallocarbohedrenes or metal-carbide, metal-nitride, metal-silicide, metal-sulfide, or metal-oxide clusters. Alternatively, individual constituents may associate to produce pure elemental clusters such as those comprised of carbon or silicon. In principle, this technique can be applied to all gaseous hydrogen-containing compounds. The results of the present study provide an alternative way (in some cases it could be the only feasible way) to generate clusters of these materials. Study of the ensuing distributions provides unique insight into the cluster-formation mechanisms involving this type of cluster source.

We have investigated the ${\mathit{L}}_{2,3}$${\mathit{M}}_{4,5}$${\mathit{M}}_{4,5}$ Auger spectra of Ag excited with bremsstrahlung. The main experimental spectra are in good agreement with atomic-multiplet-structure calculations in which the initial state is treated in jj coupling and the final state in the intermediate coupling scheme. Anomalous loss structures appear at lower energies than those of the main spectra. These losses appear to be atomic in origin and are not manifest in the corresponding spectra of In, Sn, and Sb. We demonstrate that these losses are consistent with a model of d-band spectator vacancy shake-up satellites. Coster-Kronig processes present in Cu appear to be absent here.

The electronic structure of dye/AgBr interfaces has been studied by UV photoelectron spectroscopy for several merocyanine dye films vacuum deposited on AgBr films. With these results energy diagrams of interfaces were obtained, which include the vacuum level, Fermi level, top of the valence band, and bottom of the conduction band of AgBr, and the energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the dyes. These energy diagrams were consistent with the trend in the measured abilities of these dyes for spectral sensitization in the framework of the electron-transfer mechanism. The vacuum level of AgBr was found to be significantly lowered by dye deposition. We discuss the factors determining the electronic structure, including the origin of the electric double layer at the interface, which gives rise to the shift of the vacuum level.

Hindered rotation, i.e., the libration and precession of a molecule adsorbed on a surface is demonstrated to play a role in the photoionization of adsorbates. Here we develop a theoretical model including hindered rotation and unravel the qualitative influence of this nuclear mode on photoionization spectra. We consider two typical examples. First, for physisorbed species, the effect of hindered rotation is significant placing the adsorbate at the midway between a model situation where the molecule is freely moving in the gas phase and a molecule frozen in space. Second, for the chemisorbed species the influence of hindered rotation shows up as a measurable modification of the angular distribution of electrons and our model is able to account for the experimentally observed temperature effects on the photoionization spectra. Finally, we give a recipe connecting the angle (mean angle in the quantum-mechanical sense) between the molecular axis and the surface normal and the photoionization cross section. Using experimental data this recipe will allow a better estimation of the position of the molecule on the surface.

We consider the relationship between several different formalisms for treating the multiphonon inelastic scattering of atomic projectiles from surfaces. Starting from general principles of formal scattering theory, the trajectory approximation to the scattering intensity is obtained. From the trajectory approximation, the conditions leading to the fast-collision approximation for multiquantum inelastic scattering are systematically derived.

A very fast method, cluster low-energy electron diffraction (LEED) is proposed for LEED I-V spectral analysis, in which three appproximations are introduced: the small-atom approximation, omission of the structure factors, and truncation of higher order (>2) scattering events. The method has been tested using a total of four sets of I-V spectra calculated by fully dynamic LEED for (i) the simple overlayer system, O on Ni{100}, and (ii) the reconstructed system, Cu on W{100}, and also one set of experimental data from W{100}-c(2×2)-Cu. In each case the correct structural parameters are recovered. It is suggested that for complex systems cluster LEED provides an efficient fast route to trial structures, which could be refined by automated tensor LEED.

We investigate submicron sized quench condensed Au films, annealed to 40 K. By combining time traces and characteristic magnetoresistance patterns of the conductance fluctuation we can derive a picture of the dynamics of the Au atoms. At 1.3 K a pool of about 600 out of 1×${10}^9$ film atoms are mobile and change position on a time scale of 30 s. An additional 2400 atoms participate on a much slower time scale of 20 h in the hopping. The diffusion constant is of the order of ${10}^{\mathrm{-}27}$ ${\mathrm{m}}^2$/s.

We present a theoretical study of the structural stability of ordered configurations of ${\mathrm{B}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{-}\mathit{x}}$ solid solutions with the graphite structure. The calculations were carried out self-consistently using norm-conserving pseudopotentials in the local-density approximation. The cohesive energies of a number of ordered structures at different compositions x have been subjected to a cluster expansion, which enables us to predict the cohesive energy of the random alloy at the corresponding compositions and to gain insight on the tendency of the system towards ordering. We find that, at T=0 K, some ordered configurations have a cohesive energy higher than the corresponding random distribution at the same compositions. The inspection of the cluster interactions leads moreover to the conclusion that segregation of boron in graphite is highly discouraged and long-range order is likely to occur.

The growth of Sn on Ni(100) and the structure of the c(2×2)-Sn/Ni(100) surface were studied using 500-eV ${\mathrm{Li}}^{+}$ scattering, Auger-electron spectroscopy, and low-energy electron diffraction (LEED). The polar-angle dependence of the ${\mathrm{Li}}^{+}$ scattering from Sn and Ni of the c(2×2)-Sn/Ni(100) bimetallic surface prepared at 250 K reveals that an ordered two-dimensional surface alloy is formed. However, Sn is not coplanar with the first Ni layer, but buckled above the Ni plane by 0.44±0.05 Å. These studies show that Sn and Ni mixing occurs at low temperatures. When the surface alloy prepared at 250 K was annealed to above 400 K, additional features appeared in the polar-angle dependence of ${\mathrm{Li}}^{+}$-Ni single scattering, suggesting that small surface reconstructions occur on the c(2×2) alloy surface. Further annealing to 900 K results in a LEED pattern characterized by a splitting of the half-order beams into quartets. Formation of out-of-phase domains may be responsible for this LEED pattern.

Adsorption, desorption, and positron-annihilation spectroscopy have been employed to study ethane physisorbed on grafoil. From the adsorption and desorption measurements, the interlayer and intralayer ethane-ethane and ethane-substrate interaction energies have been obtained as 9.6, 13.8, and 175 meV, respectively. Doppler broadening of the annihilation radiation has been measured as a function of coverage at 108 K. The S parameter exhibits a marked change at a coverage of 0.8. Doppler broadening measurements have also been carried out as a function of temperature in the range 10–100 K for a constant coverage of 0.9. We discuss the temperature dependence of the S parameter in terms of an order-disorder phase transition around 39 K, a solid-solid transition associated with the rotation of the ethane molecule about the C-C axis, and a solid-fluid transition.

The Coulombic fission of small doubly charged lithium and potassium clusters has been investigated around their critical size of stability ${\mathit{n}}_{\mathit{c}}$=25 and ${\mathit{n}}_{\mathit{c}}$=21, respectively. Neutral clusters are produced by the gas aggregation technique, photoionized by a pulsed laser source and mass analyzed in a tandem time-of-flight mass spectrometer. Individual fragmentation channels due to the unimolecular dissociation of size-selected parents are analyzed by using the retarding-field method. In this size range it is found that the fission has an asymmetric character. The shell effects govern the preferred output channels which include in any case the singly charged trimer. The absolute values of the fission energy barriers are estimated.

The resonant coherent interaction of an ion with an oriented crystal surface, under grazing-incidence conditions with respect to a special direction of the crystal, gives rise to electron loss to the continuum from electronic bound states of the ion. The calculations presented below predict large probabilities for electron emission due to this mechanism. The electrons are emitted with well defined energies, expressed in terms of the condition of resonance. Furthermore, the emission takes place around certain preferential directions, which are determined by both the latter condition and the symmetry of the surface lattice. Our calculations for MeV ${\mathrm{He}}^{+}$ ions scattered at a W(001) surface along the 〈100〉 direction with glancing angle of 0–2 mrad indicate a yield of emission close to 1. Using heavier projectiles, one obtains smaller yields, but still large enough to be measurable in some cases (e.g., ≊0.9 for 53 MeV ${\mathrm{B}}^{4+}$ and an angle of incidence of 1 mrad). Besides, the initial bound state is energy shifted due to the interaction with both the crystal potential and the velocity-dependent image potential. This results in a slight shift of the peaks of emission, which suggests a possible spectroscopy for analyzing the dynamical interaction of electronic bound states with solid surfaces.

We investigate the problem of which universality classes of dynamic surface critical behavior exist for semi-infinite systems whose dynamic bulk critical behavior and static surface critical behavior are representative of a given dynamic bulk universality class and a given static surface universality class. To this end systems whose bulk dynamics are described by either model B or model A of Halperin, Hohenberg, and Ma are considered, where the dynamics are allowed to be modified near the surface as follows: In the case of model B, surface terms that locally break the conservation law for the order-parameter density φ are allowed; in the case of model A, φ is assumed to be locally conserved near the surface. Semi-infinite field-theory models are constructed that are (i) compatible with the requested bulk dynamics and the requirements of causality, detailed balance, and relaxation to thermal equilibrium and (ii) minimal in the sense that no redundant or irrelevant surface terms are included. The boundary conditions to which the surface terms of the action correspond are worked out. For model B it is shown that nonconservative surface terms are relevant. Their strength can be parametrized by a coupling constant c${\mathrm{̃}}_0$≥0 whose inverse plays the role of an extrapolation length for the auxiliary (Martin-Siggia-Rose) field φ̃. Thus two distinct semi-infinite extensions of model B exist—one with c${\mathrm{̃}}_0$>0 called model ${\mathit{B}}_{\mathit{A}}$, and one with c${\mathrm{̃}}_0$=0 called model ${\mathit{B}}_{\mathit{B}}$—and each static surface universality class splits up into two dynamic surface universality classes.

The static and dynamic surface critical exponents of these dynamic surface universality classes as well as the crossover exponent associated with the nonconservative surface term are shown to be expressible in terms of static bulk and surface exponents. Model ${\mathit{B}}_{\mathit{A}}$ and its results should apply, e.g., to uniaxial ferromagnets whose rotational spin symmetry is broken in the vicinity of a surface plane. For model A the considered local modification of the dynamics near the surface is irrelevant in the renormalization-group sense and hence does not lead to new dynamic surface universality classes.

Recent helium atom scattering experiments have shown that Ar monolayers form high-order commensurate (HOC) phases on Pt(111) substrate at low temperature and that HOC-HOC transitions proceed as functions of coverage and temperature. This leads to the realization of a two-dimensional devil’s staircase. Potential calculations using effective pairwise Pt-Ar and Ar-Ar interactions and substrate-mediated Ar-Ar/Pt contributions are performed to determine the equilibrium geometries of the monolayer vs Ar coverage. The free energy calculated at finite temperature (T≤50 K) shows that five metastable hexagonal HOC structures occur within the range 31 ≤T≤46 K. When T increases in this range, the nonrotated structures appear with increasing lattice parameters 3.71, 3.78, 3.82, 3.84, and 3.89 Å. The most stable geometry is a (4×4) unit cell with 9 Ar atoms. Rotated phases exhibit the same behavior with, however, a slightly shifted temperature range and shifted values for the lattice parameter. These results are in close agreement with the experimental findings and they corroborate the devil’s staircase event for such a system.

We have grown by molecular-beam-epitaxy thin Cr(110) films on Nb(110) buffer layers on sapphire substrates. Films ranging from 50 Å to 1100 Å in thickness exhibit a single-domain structure. Using extensive x-ray characterization methods, we find a pronouced in-plane expansion of the Cr layers which relaxes as the film thickness increases, while the out-of-plane lattice spacing remains constant and bulklike over the entire thickness range. The in-plane lattice relaxation can be described by an isotropic strain relaxation model. In contrast, the out-of-plane behavior is highly anomalous, which may be due to strain density waves.

The effects of long-range macroscopic forces are used to explain the high resolution as well as the stability of images obtained by the atomic force microscope (AFM) operating in a fluid. As an example, the AFM images of highly oriented pyrolytic graphite surfaces in liquid and in vacuum are calculated for comparison.

Orientational ordering transitions in ${\mathrm{C}}_{70}$ are studied by constructing a Landau free energy in terms of order parameters describing long-range orientational order. This theory predicts that the transition from the orientationally disordered state into a partially ordered state, where the long axes of the molecules are parallel to one another, is discontinuous with an elastic distortion. Order parameters describing the lower temperature transition, where spinning about the long axis becomes hindered, are also discussed.

Electron energy loss spectroscopy and high-resolution images of carbon nanotubes, obtained using a 2 nm probe and energy resolution of 0.5 eV, revealed a 15 eV surface plasmon for tubes containing ≤12 cylindrical graphitic sheets. As the number of sheets increases further the 24 eV bulk plasmon quickly dominates the low-loss spectra. It is concluded that carbon nanotubes should be semimetallic, as is graphite. The first few graphitic sheets of nanotube structures act as if they are effectively decoupled electronically. As the radius of curvature increases then the interlayer structures approach the structure of graphite, as do the electronic properties. The present results do not exclude the possibility that the electronic structure of carbon nanotubes is the same as that of graphite, due to the operation of a conformal invariance principle.

We develop a model in which the ultraviolet dielectric tensor of planar graphite is transported to the spherical geometry of a nanoscale multishell fullerene with a central cavity. This is accomplished by assigning to every point of the multishell fullerene a local dielectric tensor identical to that of graphite with its c axis aligned along the local radial direction. The dynamic, multipolar polarizabilities of the model fullerene are obtained from the exact solutions of the nonretarded Maxwell equations. The ultraviolet absorption spectrum of the hollow fullerene is calculated as a function of the ratio of the inner and outer radii. Comparisons of the theoretical absorption spectra with the 2175-Å interstellar extinction hump and with recent absorption measurements for synthetic multishell fullerenes indicate that the dielectric properties of graphite are qualitatively adequate for understanding the optical data. However, difficulties persist with both the astrophysical and laboratory absorption peaks which lead us to consider the possible role of multishell fullerene aggregation into small or large clusters. It is found that the effect of clustering is important and reduces but does not remove completely the quantitative difficulties of the graphitic multishell model. Finally theoretical electron-energy-loss spectra (EELS) of these structures with an empty or filled cavity are calculated from the multipolar polarizabilities of the model. The results indicate that spatially resolved EELS measurements should be ideally suited to study the dielectric properties of individual multishell fullerenes and to ascertain to what extent they differ from those of planar graphite.