We have carried out studies of the interfacial roughness of a number of Nb/a-Si multilayers using cross-section transmission electron microscopy, wide-angle x-ray diffraction, and low-angle x-ray reflectivity and diffuse scattering. The multilayers studied were grown by sputtering in an Ar atmosphere at various pressures. The effect of the layer thickness and of the number of layers has also been studied. We observed a clear transition in the growth morphology when the sputtering pressure is raised above the thermalization pressure (≊9 mTorr) of the sputtered atoms. Both the mosaic of the Nb crystallites and the interface roughness increase dramatically when the Ar pressure exceeds 9 mTorr. The roughness of the various interfaces is strongly conformal and the samples with large roughness show a roughness that increases with deposited layer number. We discuss the quantitative extraction of these parameters from the x-ray data and the implications of these results for the physics of the deposition process.

Surface-extended x-ray-absorption fine-structure (SEXAFS) measurements have been obtained from p(2×2) overlayers of K on Ni(111) and Cu(111) surfaces. The data show that at 65–70 K, potassium occupies the atop site on both substrates, with chemisorption bond lengths of 2.92±0.02 Å on Ni and 3.05±0.02 Å on Cu. The identical adsorption site and small change in bond length, only slightly larger than that predicted from the difference in Cu and Ni lattice constants, are consistent with the expected dominance of adatom-adatom interactions in these near-saturation metallic adlayers. The inability to obtain SEXAFS data from K at lower coverages on these surfaces and at these temperatures, indicative of disorder/multisite occupation, is further evidence of a relatively weak alkali-metal substrate interaction even in the dilute-adatom, nonmetallic-overlayer regime.

Anisotropic surface diffusion of CO on Ni(110) and its coverage dependence have been measured using the method of linear optical diffraction from monolayer gratings. No coverage dependence below 0.7 ML was observed. Above 0.7 ML, the short-range CO-CO interaction due to orbital overlap seems responsible for the observed reduction of the diffusion activation energy with CO coverage. The corresponding interaction energy is estimated to be 0.077 eV from the result. Anisotropy in the surface diffusion is not significantly affected by the CO-CO interaction.

Results on laser-induced desorption of metal atoms from small metal particles are presented. Experiments have been performed on sodium, potassium, and silver particles supported on a LiF(100) single-crystal surface under ultrahigh vacuum conditions. Measurements include the determination of the desorption rate as a function of laser wavelength, laser intensity, average particle size, and substrate temperature, the determination of the kinetic energy of the desorbed atoms, the investigation of the optical spectra of the supported metal particles, and the study of the influence of adsorbate molecules on the desorption rate. Furthermore, theoretical extinction and absorption spectra of the metal particles have been calculated with the classical electrodynamical Mie theory as a function of average particle size and excitation wavelength. Also, the radial electric field at the particle surface was computed. The results of the experiments and theoretical calculations are combined to give a consistent picture of the mechanism of metal-atom desorption by electronic excitation with laser light. A realistic surface potential from which the atoms escape and nonlocal optical effects are taken into account. The latter introduce additional absorption channels by the formation of electron-hole pairs in the surface layer of the particle which relax into antibonding states before desorption occurs. Finally, the mechanism is discussed in the light of similar phenomena observed for thin metal films. Possibilities for future work are outlined.

A theory is developed for the specular reflection of thermal neutrons from an arbitrary smooth surface or interface using a Green-function technique and leads to a family of Born approximations that includes both the familiar plane-wave Born approximation (PWBA) and a generalized distorted-wave Born approximation (DWBA). The DWBA reduces to the PWBA if the wave-vector transfer, ${\mathit{q}}_{\mathit{z}}$, is much larger than the critical wave vector for total reflection, ${\mathit{q}}_{\mathit{c}}$, and to Fresnel’s formula for reflection from a sharp surface if ${\mathit{q}}_{\mathit{z}}$ is much less than the inverse surface thickness. Detailed numerical calculations of the reflectivity for a number of symmetric model surfaces show that it is only when ${\mathit{q}}_{\mathit{z}}$≫${\mathit{q}}_{\mathit{c}}$ that the reflectivity R is sensitive to the detailed shape and thickness of the surface potential. In particular, one requires experiments with ${\mathit{q}}_{\mathit{z}}$/${\mathit{q}}_{\mathit{c}}$ ranging from about 3 to 30, where R decreases from about ${10}^{\mathrm{-}3}$ to ${10}^{\mathrm{-}7}$, to obtain useful information on the shape and thickness of symmetric surfaces. From a comparison with the exact reflectivity for one particular surface, we find that the DWBA is typically three orders of magnitude more accurate than both the PWBA and an ad hoc model commonly used by experimenters for all values of ${\mathit{q}}_{\mathit{z}}$, and that the relative error in the DWBA is almost everywhere less than 0.01% which is, therefore, of the same order of magnitude as the intrinsic error in the Fermi pseudopotential itself. The high accuracy that we have found when the DWBA is applied to symmetric surfaces is in contrast to the relatively low accuracy that others have found when they applied this kind of approximation to thin films on substrates of low intrinsic reflectivity. The reason for this is discussed.

The interaction of ${\mathrm{SO}}_2$ with both stoichiometric and defective NiO(100) surfaces has been studied by x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, Auger-electron spectroscopy, and low-energy electron diffraction. Surface defects are found to play an important role in chemisorption. ${\mathrm{SO}}_2$ interacts only weakly with the stoichiometric NiO(100) surface at room temperature. The adsorption that does occur yields adsorbates having direct S-O bonds (i.e., the S 2p core-level binding energy is 167 eV); the species is presumably associatively adsorbed ${\mathrm{SO}}_2$. Subsequent heating of the ${\mathrm{SO}}_2$-exposed surface to 570 K results in desorption of the adsorbate without any changes in the S 2p binding energy. In contrast, the interaction of ${\mathrm{SO}}_2$ with the defective, reduced NiO(100) surface is much stronger. At 111 K only associatively adsorbed ${\mathrm{SO}}_2$ is present. Upon heating to about 260 K, in addition to the original adsorbates with direct S-O bonds, adsorbates having a S 2p binding energy of 162 eV are observed; these correspond to sulfide bonds to surface metal cations, which indicates that some of the ${\mathrm{SO}}_2$ has completely dissociated. By comparing the results from stoichiometric and defective NiO(100) surfaces, active adsorption sites that involve oxygen vacancies and steps are suggested as an explanation for the dissociation at room temperature, and a thermally activated dissociation mechanism is proposed that involves adsorption at oxygen-vacancy sites only.

A theory of electron tunneling through an adiabatically time-dependent potential barrier is applied to describe fluctuations of the scanning-tunneling-microscope current, induced by the motion of atoms in the tunneling gap. We show that atoms of a gas intervening between the tip and the sample introduce a noise small compared to the shot and thermal-electron noises. On the contrary, the noise induced by the movement of adsorbed atoms can be large enough to be observed. Simple formulas for the average current and temporal correlation functions of the current are offered to treat data on surface diffusion.

Photoabsorption cross sections for free, singly charged sodium clusters (14–48 atoms) have been measured by recording the light-induced evaporation. The measurements cover the region from 366 nm (3.4 eV) to 605 nm (2.0 eV) in 15 steps with a spectral resolution of ≃10 nm. The evaporation residues are separated in an electrostatic field and detected by a position-sensitive microchannel plate. The spectra are dominated by surface-plasma oscillations of the valence electrons with a cross section that exhausts ≊70–100 % of the dipole sum rule. Over the mass range considered, the mean energies of the resonances are constant (≃2.64 eV), and redshifted compared to the corresponding experimental bulk-plasmon energy but slightly blueshifted compared to the resonance energy of neutral clusters. A double structure of the resonance peak is observed for nonmagic clusters. It is discussed and interpreted in terms of spheroidal deformations.

Spontaneous desorption (SD) denotes a process in which atoms and molecules are transferred as singly charged ions from the solid phase into the gas phase. For this purpose the solid sample does not need to be bombarded by particles (keV or MeV ions, electrons) from an external source. An electric field between the sample and an acceleration grid with a field strength at the sample surface below the threshold value for field desorption is the only prerequisite to initiate the SD process. The SD mechanism is the subject of the present paper. A number of experimental results obtained recently are summarized. It follows that SD is a two-step process. Primary ions are field desorbed in a first step from the edges of the grid where an enhanced electric field exists. These ions are mainly molecular ions and stem from adsorbates at the grid. They are accelerated in the electric field between the grid and sample to keV energies and sputter, in a second step, secondary ions from the sample with a sputter yield that exceeds the yield of atomic ions considerably.

The interference of fluorescent x rays in a layered material has been observed in grazing exit experiments. The exit angular dependence of the x-ray fluorescence intensity of a Cr/Au layered thin film clearly shows an oscillation structure that represents the interference of emitted x rays. As predicted by the reciprocity theorem, the results obtained under the grazing exit condition agreed with the results obtained under the grazing incidence condition quite well.

We investigate the influence of quadratic adatom-phonon couplings on the phonon-assisted adatom tunneling on solids based upon the early work of Tonks and Dick. We find that the incoherent tunneling at high temperatures (T⩾${\mathrm{T}}_{\mathrm{D}}$/3) is not simply thermally activated but described by a rate ${\mathrm{\omega }}_{\mathrm{ij}}$≈(Δ/ħ${)}^2$exp[-${\mathrm{E}}_{\mathrm{a}}$/${\mathrm{k}}_{\mathrm{B}}$T+${\mathrm{E}}_{\mathrm{a}\prime }^2$/(${\mathrm{k}}_{\mathrm{B}}$T${)}^2$] where Δ is the renormalized tunneling matrix element, ${\mathrm{E}}_{\mathrm{a}}$ is the conventional activation energy caused by linear adatom-phonon couplings, and ${\mathrm{E}}_{\mathrm{a}\prime }$ is an effective energy arising from the quadratic couplings. We determine the conditions when the activated tunneling behavior is qualitatively altered by the quadratic couplings. As a result of reduced dimensionality, the latter play much more important roles in the tunneling diffusion of hydrogen on metal surfaces than they do in the interstitial tunneling in solids.

A spin-resolved x-ray-photoelectron-spectroscopy study of the deep core levels of ferromagnetic Fe are reported. The 2${\mathit{p}}_{3/2}$, 2${\mathit{p}}_{1/2}$, and 2s core levels, as well as the more shallow 3p level, all display interesting spin-dependent structures. These findings indicate the importance of intra-atomic exchange in deep-core-level photoemission from Fe. The electronic response of the metal to core-hole creation is found to be dependent on the spin of the core hole.

It is found that thick (1000 Å) films of fcc Co can be stabilized via epitaxial growth onto Cu(110) substrates at room temperature. These metastable films transform into polycrystalline films of the stable hcp phase if annealed to temperatures above 200 °C. The unusual stability of the fcc phase in the (110) growth direction is linked with the fact that the phase transformation to hcp involves a major disruption of the crystal structure, compared with, e.g., growth of fcc Co on Cu(111). The stabilization of single-crystal fcc Co at room temperature has enabled measurements of the room temperature magnetic properties in these films, for comparison with hcp Co.

We find a paramagnetic to diamagnetic ordering transition of a two-dimensional electron gas on a mesoscopic tube in a magnetic field as the tube radius is varied. Spin effects (Pauli paramagnetism) are explicitly considered. The dependence of magnetization on temperature is calculated. A relationship between the periodicity of the magnetization and the de Haas–van Alphen oscillations is presented. These results are directly pertinent to the recently synthesized helical microtubules of graphite.

We consider infrared absorption in charged and photoexcited ${\mathrm{C}}_{60}$ using a Su-Schrieffer-Heeger model supplemented by a force field model to treat molecular vibrations. Intensities are calculated assuming the principal cause of absorption is the charged phonon effect. Our calculated frequencies for ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}^{\mathrm{-}6}$ are in good agreement with experiment but the calculated intensities are at best in qualitative agreement. For photoexcited ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}^{\mathrm{-}}$ the calculations indicate a number of additional infrared peaks which are in principle accessible to experiment.

We have systematically studied the interaction between the ${\mathrm{C}}_{28}$ fullerene molecule and the group-IVA endohedral atoms C, Si, Ge, and Sn, using state-of-the-art electronic structure and total-energy calculations based on density-functional theory. We find no covalent bonding between these atoms and the fullerene cage. The nature of the cage-atom interaction involves charge transfer from the endohedral atom to the cage which produces weak, mostly ionic binding. The degree of charge transfer increases with the atomic number of the endohedral atom. By comparing the electronic structure of the group-IVA endohedral cage compounds to ${\mathrm{ZrC}}_{28}$ (a group-IVB endohedral), we find that the latter bonds strongly to the cage due to a covalent interaction between the Zr 4d states and the ${\mathrm{C}}_{28}$ states. We extrapolate the trends found for the group-IV endohedrals to suggest other possibly stable systems and to provide insight to the stability of the experimentally observed ${\mathrm{UC}}_{28}$ complex.

By applying the numerically computed energy levels and wave functions of the π electrons of the ${\mathrm{C}}_{60}$ molecule following the Su-Schrieffer-Heeger model, we calculated the second-order quadrupole nonlinear susceptibility ${\mathrm{\chi }}_{\mathit{z}\mathit{z}\mathit{z}\mathit{z}}^{\mathit{Q}}$ for the ${\mathrm{C}}_{60}$ medium and the calculated value is 1.7×${10}^{\mathrm{-}20}$ ${\mathrm{m}}^2$/V for the fundamental wave λ=1.064 μm. For the fundamental wave λ=1.064 μm, the second-order susceptibility ${\mathrm{\chi }}_{\mathit{z}\mathit{z}\mathit{z}}^{\mathrm{}\left(2\right)\mathrm{}}$ caused by the second-order nonlinear quadrupole polarization has the magnitude 1.1×${10}^{\mathrm{-}13}$ m/V (i.e., 2.5×${10}^{\mathrm{-}9}$ esu). This result is in excellent agreement with the recent experimental measurements. In addition, we also calculated the quadrupole-induced reflection and transmission second-order harmonic generation by further considering the effect of finite film thickness.

Linear-response theory for open (infinite) systems leads to an expression for the current response which contains surface terms in addition to the usual bulk Kubo term. We show that this surface term vanishes identically if the correct order of limits is maintained in the derivation: the system size L must be taken to infinity before the adiabatic turn-on rate δ of the perturbation is taken to zero. In contrast to recent claims this shows that linear-response theory for open systems is gauge invariant without modification of the continuity equation. We show that a simpler derivation of the Landauer-Büttiker equations may be obtained consistently from the bulk Kubo term, noting that surface terms arising here are nonvanishing because they involve the opposite order of limits, δ→0, then L→∞.

We show that the Fermi distribution can be represented by an infinite sum of contour integrals in the complex energy plane. The expansion converges very rapidly and a few terms are sufficient to obtain high accuracy.

The photonic band structures of a square lattice array of metal or semiconductor cylinders, and of an fcc array of metal or semiconductor spheres, are computed numerically. The frequency-dependent dielectric function of the metal or semiconductor is assumed to have the free-electron form ε(ω)=1-(${\mathrm{\omega }}_{\mathit{p}}^2$/${\mathrm{\omega }}^2$), where ${\mathrm{\omega }}_{\mathit{p}}$ is the plasma frequency of the charge carriers. A plane-wave expansion is used to transform the vector electromagnetic wave equation into a matrix equation. The frequencies of the electromagnetic modes are found as the zeros of the determinant of the matrix. In general, the plane-wave method is found to be most effective for structures with filling fractions f≤0.1% but provides a semiquantitative description of the band structures of these systems for f>0.1%.