The spin-singlet and spin-triplet optical transitions of negative donor centers ${\mathit{D}}^{\mathrm{-}}$ in GaAs/(Al,Ga)As quantum wells in high magnetic fields are considered both theoretically and experimentally. The measured and calculated transition energies are shown to be in very good agreement for the ${\mathit{D}}^{\mathrm{-}}$ singlet transition (1%). The two strong triplet transitions that are predicted by theory are not observed in the experiment. This is thought to be due to accidental energy coincidences with one-particle excitations such as the electron cyclotron resonance and the 1s→2${\mathit{p}}^{+}$ transition of neutral donors.

We have used time-resolved photoluminescence spectroscopy to measure the temperature of a gas of quasi-two-dimensional excitons. The exciton gas consists of both spatially direct and spatially indirect excitons in a coupled GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As double-quantum-well system. We show by experiment and modeling that the ratio of the intensities of the recombination radiation constitutes a sensitive, calibrated thermometer for the electron system. The technique is insensitive to inhomogeneous effects such as well-width fluctuations and is feasible at temperatures as low as several K.

We present a magnetoreflectivity study of a ZnSe/${\mathrm{Zn}}_{0.85}$${\mathrm{Mn}}_{0.15}$Se multiple-quantum-well structure. The spin splittings of the two confined excitons ${\mathit{e}}_1$${\mathit{h}}_1$ and ${\mathit{e}}_2$${\mathit{h}}_2$ are studied as a function of applied magnetic field. Differences in the energies and intensities between the two excitonic transitions are discussed and interpreted using a variational calculation. We show that these diluted-magnetic-semiconductor-based structures may be interesting hosts for the study of heavy-hole/light-hole band mixing, as the energies of the confinement hole states change with the application of an external magnetic field.

A mechanism of metastable ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ ${\mathrm{He}}^{\mathrm{*}}$ deexcitation is proposed for a NO-covered Pd(111) surface in competition with the resonance ionization of the excited noble-gas atom, based on potential-energy curves (transition matrix elements are not calculated). The quantum-mechanical and electrostatic interactions between He and NO and their interaction with the semi-infinite Pd(111) surface are fully accounted for within a spin-unrestricted screened Hartree-Fock theory. For the system ${\mathrm{He}}^{\mathrm{*}}$-NO/Pd(111) resonance ionization of metastable ${\mathrm{He}}^{\mathrm{*}}$ dominates at large distances. Close to the classical turning point of the projectile the strong interactions in the system will lead to an electronic configuration of lowest energy which corresponds to the formation of a ${\mathrm{NO}}^{\mathrm{-}}$ negative ion ${\mathrm{He}}^{+}$-${\mathrm{NO}}^{\mathrm{-}}$/Pd(111). Penninglike transitions can occur in the complex ${\mathrm{He}}^{+}$-${\mathrm{NO}}^{\mathrm{-}}$/Pd(111) even though the metastable helium atom has been resonantly ionized. In this complex the negative ${\mathrm{NO}}^{\mathrm{-}}$ ion is in a triplet configuration. An interesting aspect of the experimental metastable deexcitation spectrum is that a structure due to the NO 2${\mathrm{\pi }}^{\mathrm{*}}$ electron appears only for ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ ${\mathrm{He}}^{\mathrm{*}}$ and not for ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ ${\mathrm{He}}^{\mathrm{*}}$.

In the present theory this is explained by a spin-selection mechanism, which relies on the reflection of the metastable thermal ${\mathrm{He}}^{\mathrm{*}}$ atom, if its 2s electron has antiparallel spin orientation relative to the unpaired 2${\mathrm{\pi }}^{\mathrm{*}}$ electron of adsorbed NO. For parallel spin orientation of the ${\mathrm{He}}^{\mathrm{*}}$ 2s and NO 2${\mathrm{\pi }}^{\mathrm{*}}$ electrons the approach of the ${\mathrm{He}}^{\mathrm{*}}$ projectile towards the target is nonactivated. In this way the 1s electron of ${\mathrm{He}}^{\mathrm{*}}$ retains the information of whether it originates from singlet ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ ${\mathrm{He}}^{\mathrm{*}}$ or triplet ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ ${\mathrm{He}}^{\mathrm{*}}$ atomic beams despite the resonance ionization of the metastable helium atom at closer distances.

Photoemission from different alkali core levels (Na 2p, K 3p, Rb 4p, and Cs 4d) has been studied for thin alkali films deposited on four different 4d metal surfaces: Mo(110), Rh(111), Rh(110), and Pd(100). Alkali atoms at the interface, the bulk, and at the surface of the adsorbed alkali film are found to have different core-level binding energies. It is found that the core-level binding-energy shifts of the alkali metals induced by the 4d-metal substrate increase with increasing atomic number of the alkali metal and with increasing atomic number of the 4d metal. Thermodynamical quantities such as interface segregation energies and adhesion energies are deduced from the layer-resolved shifts. Estimates of the experimental binding-energy shifts are given using semiempirical calculations, and certain complications in doing that are discussed.

The atomic geometry of the Al(111)-(√3 × √3 )R30° structure formed by adsorption of Na on Al(111) at room temperature has been determined by analysis of extensive low-energy electron-diffraction (LEED) measurements. It is shown conclusively that Na atoms occupy a sixfold-coordinate site formed by displacing every third Al atom in the first layer of the substrate. This confirms the results of a recent study by surface extended x-ray-absorption fine structure.

We report high-resolution electron-energy-loss (HREEL) spectra at low energy (0–20 eV) on solid multilayer films of Ar. The present results are compared with optical data of similarly deposited films as well as with electron-energy-loss spectroscopy of Ar in the gas phase. Relatively wide and intense bands located at the energy of the n=1,1’ and 2,2’ bulk excitons, which have been measured optically, are ascribed to the longitudinal branch of the bulk excitons. In the energy range of the n=1,1’ surface excitons, up to four narrow peaks are resolved with HREEL spectroscopy. At higher energy loss between the first and second bulk-exciton manifold, we have features ascribed to the n=2 surface excitons along with several extra peaks, which have not been reported previously. The observed trend is discussed in terms of Wannier-type exciton series split by the spin-orbit interaction and originating from the surface of the solid.

Low-energy ${\mathrm{D}}^{+}$ scattering is employed to explore the nature of the bonding of polycrystalline alkaline-earth oxides MgO, CaO, SrO, and BaO, with particular emphasis on the investigation of the ionicity of the topmost-layer atoms. Increasing ionicity as one goes to the heavier cations is concluded from the probability of the resonance neutraliztion of the ${\mathrm{D}}^{+}$ ions, which is consistent with the conventional chemical arguments based on electronegativity scales but is in apparent contradiction to the results of recent ab initio cluster-model calculations. It is also concluded that the metallic Ba layer is formed rather patchily on the BaO surface after the heat treatment up to 1000 °C. This is probably because free Ba atoms, being supplied by the reaction of BaO with the Ta substrate, are precipitated at the BaO surface. Another example is concerned with the interactions of the Ba adatoms with Si(001) and Pt(111) surfaces; Ba is found to have marked covalency with the substrate atoms.

Electron trajectories in x-ray photoemission from solids are partially randomized by elastic collisions, and thus the angular distribution of photoelectrons leaving the surface is different from that for isolated atoms. This problem is approached in the present work by extensive Monte Carlo simulations of electron trajectories resulting from photoionization of the gold 4s, 4${\mathit{p}}_{3/2}$, 4${\mathit{d}}_{5/2}$, and 4${\mathit{f}}_{7/2}$ subshells by Mg characteristic x rays. Calculations were made for the full range of angles of x-ray incidence and for all possible positions of the electron energy analyzer. In comparisons with intensities predicted from the common formalism in which elastic scattering is neglected, it was found that the elastic-scattering effects can be accounted for with two correction factors. These factors are, to a large extent, independent of experimental geometry for certain ranges of angles. The correction factors depend only slightly, for example, on the photoelectron exit angle in the range 0°–30° with respect to the surface normal. The present results indicate that the magic angle (the angle between the direction of x rays and the direction of signal electrons at which the effects of angular anisotropy can be avoided) is not a single constant value of 54.7° (as found for isolated atoms) but a much larger value that depends on the electron exit angle and the photoelectron subshell. Furthermore, it has been found that elastic-scattering effects can be neglected for certain experimental configurations. The current for a given photoelectron line is then equal to the current calculated from the common formalism, but this equality occurs at different angles between the incident x rays and the detected electrons depending on the photoelectron line and the electron exit angle.

Moiré patterns observed in scanning tunneling microscopy (STM) of monolayer graphite on transition-metal surfaces are investigated theoretically. STM images of the monolayer graphite on the TiC(111) surface are calculated using the tight-binding method. The calculated STM images show the moiré patterns which reproduce well the experimental STM images. The moiré patterns cannot be explained in terms of a simple superposition of the atomic structures of the substrate and the overlayer. The calculated moiré patterns strongly depend on the bias voltage. At a bias voltage, peaks of the STM image are located at the atomic sites of the substrate, which can be regarded as an example of the trans- parency effect observed in STM. It is found that three factors are important for the appearance of the moiré patterns: (1) the strength of the interaction between overlayers and substrates, (2) the height of the tip in scanning over sample surfaces, and (3) the bias voltage that reflects the energy region where bands of the overlayer and the substrate are strongly mixed.

Electronic states of graphite overlayers formed on the TaC(111) surface have been investigated with the use of scanning tunneling microscopy and photoelectron spectroscopy. The graphite film grows on the substrate layer by layer. The thickness of the overlayer has been adjusted precisely to be either one or two monolayers. The physical properties of the monolayer graphite film are modified by chemical bonding at the interface. This interfacial bonding becomes weak upon the formation of the second layer of graphite, which makes the properties of the double-layer graphite film similar to those of bulk graphite.

A study aimed at understanding the geometrical and electronic properties of barium and oxygen coadsorbed on the tungsten (001) surface has been carried out by means of work-function measurements (Δφ), Auger-electron spectroscopy, low-energy electron diffraction, inverse photoelectron spectroscopy, and relativistic-electronic-structure calculations. A report of the experimental measurements and a comparison with theoretical results from embedded-cluster-model calculations are presented. Our experimental studies show that the work function of the W(001) surface (φ=4.63 eV) is lowered to approximately 2.3–2.4 eV by coadsorption of 1 ML of Ba and O regardless of the order of deposition of these two species. The technique of IPS in the isochromat mode was used to determine the unoccupied electronic-energy band structure for ordered c(2×2) Ba and O layers on W(001). Several spectral features are observed above the Fermi level (${\mathit{E}}_{\mathit{F}}$), which we assign to transitions into Ba and W d states. The measured two-dimensional electronic band structure is independent of the order of Ba and O deposition. Using embedded-cluster-model calculations, we investigated two possible adsorption configurations of an ordered c(2×2) adlayer of Ba and O on W(001): ‘‘tilted,’’ where Ba and O are placed on alternate fourfold-hollow sites, and ‘‘upright,’’ where the adsorbed atoms lay above the same site with Ba outermost. The calculated densities of states for the tilted geometry show distinct peaks above ${\mathit{E}}_{\mathit{F}}$ originating from Ba and W d orbitals and are in good agreement with the experimental results.

I analyze the sliding motion of an elastic block on a substrate with a layer of lubrication molecules. This model of boundary lubrication illustrates how the surface stress generated by the lubrication layer at the block-substrate interface is transmitted to the upper surface of the block. It shows that it is essential to correctly incorporate the elastic properties of the block and of the substrate; otherwise, incorrect results will result. In light of the theoretical results I discuss the sliding friction measurements of Yoshizawa and Israelachvili.

The BH moiety is isoelectronic with C. We have studied the stability of the (BH${)}_{60}$ analogue of the ${\mathrm{C}}_{60}$ fullerene as well as the dual-structure (BH${)}_{32}$ icosahedron, both of them being putative structures, by performing local-density-functional electronic calculations. To aid in our analysis, we have also studied other homologues of these systems. We find that the latter, i.e., the dual structure, is the more stable although the former is as stable as one of the latter’s lower homologues. Boron hydrides, it seems, naturally form the dual structures used in algorithmic optimization of complex fullerene systems. Fully relaxed geometries are reported as well as electron affinities and effective Hubbard U parameters. These systems form very stable anions and we conclude that a search for BH analogues of the ${\mathrm{C}}_{60}$ alkali-metal supeconductors might prove very fruitful.

A resonance phenomenon in the zeroth diffraction order of a gold-wire grating is explained by the excitation of surface polaritons. This effect is connected with a strong enhancement of the electromagnetic fields on the wire surface and consequently with a peak of power losses in the grating material. Measurements of the zeroth-order transmittance have been performed on gold gratings with periods of 1 and 2 micrometers in the near-infrared region which are in agreement with theoretical results. Furthermore, dispersion relations of the first-order coupling mode are presented having large energy gaps in the center of the Brillouin zone. It is shown that this energy gap strongly depends on the wire profile. In this coupling branch, however, practically no dispersion could be observed for optical wavelengths less than the grating period.

We report on structural and electrical properties of thin granular ${\mathrm{Pd}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{-}\mathit{x}}$ films with palladium (Pd) metal volume fractions 0.3<x<0.34, approaching the percolation threshold (${\mathit{x}}_{\mathit{p}}$=0.3) from the metallic side. As revealed from transmission-electron microscopy, granular ${\mathrm{Pd}}_{\mathit{x}}$${\mathrm{C}}_{1\mathrm{-}\mathit{x}}$ films consist of small spherical Pd clusters with mean diameters Φ ranging between 3 nm<Φ<4 nm, embedded in an amorphous carbon (C) matrix. The Pd clusters are only weakly coupled, forming an infinite percolative network within the amorphous-C matrix. The whole network is progressively disrupted with decreasing x. The overall conductivity behavior of the films is metallic, strongly influenced by electron localization and electron-electron interaction effects. The temperature dependence of the dc conductivity follows σ(T)∝${\mathit{T}}^{1/2}$ over a large range in temperature at elevated temperatures, similar to what is expected for three-dimensional (3D) homogeneous systems. However, below distinct low temperatures ${\mathit{T}}_{\mathrm{\delta }}$ we observe characteristic deviations from the σ(T)∝${\mathit{T}}^{1/2}$ law towards a stronger than logarithmic temperature dependence of σ(T), and a saturation of σ(T) for T→0. This does not result from a dimensional (3D→2D) crossover with respect to localization and electron-electron interaction, but is discussed as resulting from the influence of the granular film structure on electronic transport, since ${\mathit{T}}_{\mathrm{\delta }}$ is for all films related to the mean cluster diameter Φ via ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathrm{\delta }}$=[N(${\mathit{E}}_{\mathit{F}}$)${\mathrm{\Phi }}^3$${]}^{\mathrm{-}1}$. Here, ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathrm{\delta }}$ is the average energy level separation within the small metallic clusters due to quantum size effects (QSE). These are known to be of significance in the transport properties of insulating granular films. With this paper we propose that likewise QSE are of importance for granular metallic films.

Auger-photoelectron-coincidence spectroscopy has been used to study the effect of alloying on the Ni ${\mathit{L}}_{23}$VV Auger spectrum in the Ni/Fe system. Data have been collected for the Ni ${\mathit{L}}_{23}$VV Auger line in coincidence with the 2${\mathit{p}}_{3/2}$ and 2${\mathit{p}}_{1/2}$ photoelectron line from single-crystal Ni and a ${\mathrm{Ni}}_{50\mathrm{\%}}$${\mathrm{Fe}}_{50\mathrm{\%}}$ alloy. Alloying is shown to affect the valence band and this in turn alters the components of the Auger line shape in coincidence while the overall singles lines may appear similar. A narrowing of the valence band and its shifting away from the Fermi level is seen to affect the bandlike contribution to the spectrum, the Coster-Kronig rate, and the intrinsic initial- and final-state shake-up and/or shake-off processes.

We have found that the intensity and polarization of secondary electrons emitted from Au/Fe(110) oscillate periodically as a function of the Au film thickness. The periods of both oscillations are 15 Å, but they have opposite polarities. This result can be attributed to confined energy states in the Au film.

The elastic backscattering of medium-energy (200–2000 eV) electrons from noncrystalline materials has been investigated experimentally, theoretically, and with a Monte Carlo (MC) technique. A theory for elastic electron backscattering has been developed. In this theory, single scattering events are treated exactly while the double and higher order scattering processes are dealt with in the transport approximation. An analytical expression for the angular distribution of backscattered electrons has been derived. It has been shown that the contributions of singly and multiply scattered electrons are of the same order of magnitude. Therefore, the exact shape of the differential elastic cross section is of crucial importance for the interpretation of experimental data. The theoretical results are in good agreement with MC simulations in a broad range of electron energies and elements. Experimental data on the differential reflection coefficient in the energy range 300–1000 eV for polycrystalline Al, Ni, Pt, and Au for scattering angles ranging from 45° to 135° are presented. The measured angular distributions agree well with theory.

The nonlinear optical response of regularly stopped (vicinal) jellium surfaces is calculated with the aim of obtaining a microscopic understanding of the isotropic and anisotropic contributions to the second-harmonic surface polarization. The ground-state electronic properties are determined self-consistently using the local-density-functional scheme and the nonlocal dynamical response to the applied laser field is treated within the time-dependent local-density-approximation approach. Although the screening effects are not yet completely taken into consideration in the present work, these calculations provide detailed information on various important aspects of the induced harmonic surface currents: their perpendicular and parallel spatial distribution, their relative magnitudes and phases, and their variation with frequency and with step density. The overall size of the anisotropic polarization terms is found to be much smaller than the leading isotropic contribution due to the normal response of the terraces. These results differ from recent experiments of second-harmonic generation from stepped Al surfaces which suggest, at 1.5 eV fundamental frequency, isotropic and anisotropic surface polarization sources of comparable magnitude.