We present an ab initio calculation of the double ionization of helium by protons and antiprotons, using the forced impulse method. Cross sections are larger for antiprotons than for protons, in accord with recently reported experimental results.

Without resort to any modeling, subgrid-scale eddy viscosity is computed from the results of high-resolution (${64}^3$ and ${128}^3$ grid points) direct numerical simulations of three-dimensional homogeneous, isotropic, decaying turbulence. In these simulations the eddy viscosity peaks sharply at the cutoff wave number, in rough agreement with the results of Kraichnan. In addition, in the low–wave-number range the eddy viscosity may be negative, contrary to the generally accepted concept of a subgrid-scale eddy viscosity. Some possible explanations of this behavior are discussed.

A new foilless diode has been developed that does not require an external magnetic field to control beam expansion of an intense relativistic electron beam (IREB). An ionized channel, formed with use of a laser, extends through the transport region and terminates at the cathode of an IREB accelerator. When a negative potential is applied to the cathode, channel electrons are expelled leaving a positive core that electrostatically attracts and focuses the emitted electrons. A 3.6-MeV, 25-kA electron beam has been generated and transported 1.3 m by use of this technique.

Excitation of nonlinear plasma oscillations by an intense ultrarelativistic electron beam is considered. It is shown, by analytical solutions of the one-dimensional relativistic fluid equations, that under certain conditions on the relative electron beam and plasma densities extremely large longitudinal electric fields can be generated in the beam’s wake. This scheme is a nonlinear version of the plasma wakefield accelerator, and is predicted to have advantages over the linear regime.

Analysis of ab initio cluster wave functions show that the shifts in the vibrational frequencies of CO/Cu and CN/Cu due to an applied electric field arise dominantly from a Stark effect. The CN bonding is largely ionic while CO has a dative covalent π bond. Consequently the Stark effect mechanisms are significantly different.

The path-integral method of quantum simulation is applied to an empirical model for hydrogen in niobium. Results for the density distribution of D, H, and the positive muon over the unit cell show the dramatic increase of quantum effects along this series. Calculations on the activation energy for diffusion confirm the importance of excited states at high temperature pointed out by Emin, Baskes, and Wilson and suggest that hydrogen diffusion is approximately classical in this regime.

Anomalous photoinduced transformations in amorphous Ge-based chalcogenide thin films are established as being due to photochemical modification of the surfaces, by photoemission studies. Mass measurements indicate that the giant thickness reduction on irradiation is predominantly due to the loss of material as a result of photogenerated volatile high vapor pressure oxide fractions on the surface. This extrinsic contribution contradicts the models of the phenomenon proposed so far, which are based purely on intrinsic structural transformations.

Neutron spin-echo measurements have been performed on the ionic system ${\mathrm{Ca}}_{0.4}$${\mathrm{K}}_{0.6}$(${\mathrm{NO}}_3$${)}_{1.4}$ around the glass transition temperature ${\mathrm{T}}_0$, in order to determine the time dependence of the density correlation function ${\mathrm{\phi }}_{\mathrm{q}}$(t), which plays a central role in recent theories. Above ${\mathrm{T}}_0$ the results reveal that ${\mathrm{\phi }}_{\mathrm{q}}$(t) contains two distinct ‘‘slow’’ components, the slower of which (a) has a stretched exponential form exp[-(t/τ${)}^{\mathrm{\beta }}$], (b) shows a slowing down as T approaches ${\mathrm{T}}_0$ which scales with the Stokes-Einstein diffusion constant, and (c) tends to become the nonergodic fraction of the structure factor at ${\mathrm{T}}_0$. .AE

Elemental iodine has been studied by ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ Mössbauer effect at 4 K to pressures of 30 GPa in a diamond anvil cell. Its structural transformations leading to metallization have been determined by the quadrupole interaction. At P≥16 GPa an ${\mathrm{I}}_2$-${\mathrm{I}}_2$ zigzag chain is formed leading to its metallic behavior and at P≥21 GPa the formation of a quasi two-dimensional system is observed. Up to 30 GPa the $1_2$ molecule remains the building block of the high-pressure structural modifications.

High-resolution core-level photoemission spectroscopy was used to study the initial growth and interaction of In on Si(100). The In—Si bonding coordination number, determined by quantification of the number of Si surface atoms selectively modified in the presence of an In adatom, is 3 for very low In coverages, and decreases to 2 for (1/2) -monolayer coverage. The results are consistent with a structural model deduced from electron diffraction, Auger, and scanning electron microscopy studies.

Neutron diffraction experiments on multilayer oxygen films on graphite have been carried out between 30 and 55 K. A compound solid-liquid film is observed in the range 40–48 K, exhibiting properties consistent with surface melting. The quantity of liquid in this range varies as ln(${\mathrm{T}}_{\mathrm{t}}$-T). This behavior ceases when the thickness of the liquid component reaches that of the film itself, giving rise to the ‘‘metastable liquid’’ phase of the triple point wetting process.

Wetting phenomena near the bulk critical point of an adsorbate are examined for the realistic case of long-ranged (e.g., van der Waals) forces, with use of mean-field theory for an Ising lattice-gas system. For moderately strong long-ranged forces new critical behavior appears at and near ${\mathrm{T}}_{\mathrm{c}}$, accompanied by both reentrant wetting and dewetting transitions and the appearance of two distinct wetting phases.

A simple interpretation is given of observed logarithmic dependence of the surface Fermi level on coverage of GaAS(110) in the range ${10}^{\mathrm{-}4}$–${10}^{\mathrm{-}1}$ ML in terms of the charging energy of microscopic metal clusters formed during the deposition process. New data taken on films formed at low temperature suggest that the clustering process is strongly temperature dependent.

We have formed an energetic (10–100 eV) beam of positronium (Ps) by scattering a well-collimated, small-diameter beam of 30–400 eV positrons off surfaces of Al(110) and clean and oxygen-coated Cu(100) at glancing angles of 6°–42° with respect to the surface. We determined the angular distribution (FWHM ≊20°) of the Ps beam and measured the relative Ps formation efficiency (3%–5%) as a function of incident positron beam energy and angle. Future uses of the Ps beam in atomic and surface physics will be discussed.

The elementary pinning force ${\mathrm{f}}_{\mathrm{p}}$ has been measured for a single vortex trapped in one of the superconducting layers of a cross-strip Josephson junction. At temperatures close to the transition temperature the vortex can be pushed across the junction by a transport current. The vortex is found to move in a small number of discrete steps before it exits the junction. The pinning force for each site is found to be asymmetric and to have a value of about ${10}^{\mathrm{-}6}$ N/m at the reduced temperature, t=T/${\mathrm{T}}_{\mathrm{c}}$=0.95. As a function of temperature, ${\mathrm{f}}_{\mathrm{p}}$ is found to vary approximately as (1-t${)}^{3/2}$. .AE

Two models which describe one-dimensional hopping motion of a heavy particle interacting with phonons are discussed. Model A corresponds to hopping in 1D metals or to the polaron problem. In model B the momentum dependence of the particle-phonon coupling is proportional to $k^{\mathrm{-}1/2}$. The scaling equations show that only in model B does localization occur for a coupling larger than a critical value. In the localization region this model shows close analogy to the Caldeira-Leggett model for macroscopic quantum tunneling.

A comparison of photoemission from shallow core levels with deep core-level spectroscopies allows a distinction between mixed valency and covalency in rare-earth systems. The determinative parameter distinguishing these spectroscopies is the Coulomb interaction between the core hole and the 4f shell. This approach confirms that the compound ${\mathrm{CeF}}_4$ is tetravalent, but does not deny the covalent admixture of 4f character into the valence band.

An intrinsic intergap surface state associated with the occupied dangling-bond state is found on high-quality cleaved, p-type GaSb(110) surfaces. This conclusion is derived from a high precision analysis of angle-resolved photoemission data taken with high energy and momentum resolution as well as high absolute accuracy with synchrotron and He?ize -2 roman I— radiation. Thus GaSb is besides GaP the second exception to the accepted rule that III-V’s do not have such states.

Carbon K soft-x-ray emission spectra of ${\mathrm{LiC}}_6$, ${\mathrm{KC}}_8$, and ${\mathrm{CsC}}_8$ are presented. An absolute value for the charge transfer from the alkali metal to the carbon π band is obtained. A model fit to the data reveals similar effects on both spectral shape and threshold energy, independent of the choice of the alkali-metal intercalant, and provides a measure of departures from the rigid-band approximation. An enhancement of the soft-x-ray emission spectrum near $E_{\mathrm{F}}$ is reported.

We present the two basic mechanisms leading to spatial complexity in one-dimensional patterns. They are shown to be the counterparts of the horseshoe formation mechanisms studied by Melnikov and Shilnikov in dynamical systems theory.