Multifragment events from 35, 40, 45, and 55 MeV/nucleon ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$${+}^{12}$C, ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$, ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Cu}$, and ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ reactions can be assigned to sources characterized by their velocity. At each bombarding energy, the probabilities of threefold, fourfold and fivefold events increase substantially with decreasing source velocity, but are independent of the target mass. To remove the bombarding-energy dependence, a simple transformation has been applied which gives the excitation energy of the fused system in the simple incomplete-fusion picture. These ‘‘excitation functions’’ appear to independent of both the system and bombarding energy.

${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ NMR spectroscopy has been used to probe directly the distribution of xenon atoms confined in atomic-size Na-A zeolite cavities. For mean xenon occupancies less than about three Xe atoms per α-cage, the guest populations are well described by binomial statistics. At higher guest loadings the finite volumes of the xenon atoms become significant, as reflected by a fit of the experimental populations with a hypergeometric distribution. The data and hypergeometric analysis indicate a maximum occupancy of seven Xe atoms/cage. At the highest xenon loadings the experimental distribution is narrower than hypergeometric.

Optical pumping has been used to enhance the pulsed NMR signal of ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$, allowing the detecting of low-pressure xenon gas and of xenon adsorbed on powdered solids. We observe an increase in sensitivity of more than 2 orders of magnitude over conventional NMR, the current limitation being the laser power. Adsorbed xenon is observed at 298 K on graphitized carbon (about 10 ${\mathrm{m}}^2$g and on powdered benzanthracene (∼0.5 ${\mathrm{m}}^2$g) below 170 K. The increased sensitivity of this technique allows the study of a large class of amorphous materials with surface areas below 10 ${\mathrm{m}}^2$/g including semiconductors, polymers, metal oxides, and catalysts.

Coincidence detection of photon pairs produced in parametric fluorescence, in conjunction with a Michelson interferometer in which one member of each pair acquired a geometrical phase due to a cycle in polarization states, has allowed the first observation of Berry’s phase at the single-photon level. We have verified that each interfering photon was essentially in an n=1 Fock state, by means of a beam splitter following the interferometer combined with a triple-coincidence technique. The results can be interpreted in terms of a nonlocal collapse of the wave function.

The isotope shift for the $2^3$S-$2^3$P transition between ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ has been measured with a laser-microwave spectroscopy technique to a precision of 1 ppm, or 30 kHz. Our result is 100 times more precise than previous measurements and is by far the most precise isotope-shift measurement ever made for a multielectron system. It provides, for the first time, not only a check for the specific mass shift, but also for the calculations of the nuclear volume shift, the relativistic corrections involving the coupling between electrons, and the QED contributions.

We show that dc-electric-field coupling of the 2s and 2p states in atomic hydrogen leads to resonantly enhanced second-order susceptibility with reduced absorption at the second-harmonic wavelength, and exact phase matching at the center of the Stark-split components.

We present a physical model constructed from the Navier-Stokes equation to describe the evolution of the probability distribution function of transverse velocity gradients in 3D isotropic turbulence. Quanitative agreement with data from direct numerical simulations of isotropic turbulence for a wide range of Reynolds number is obtained. The model is based on a concrete physical picture of self-distortion of structures and interaction between random eddies and structures; the dynamical balance explains the non-Gaussian equilibrium probability distributions.

A new analytical model and numerical simulations are used to show that an ion-density cavity can support large potential drops, extended over a distance related to the cavity width, for several electron transit times. When the applied potential drop exceeds a critical value, which depends on the cavity depth, the drop is instead concentrated in a cathode sheath. The existence regions for the two different states in the parameter plane are found from the simulations and shown to agree with the regions predicted theoretically. The results are consistent with available experimental data.

The safety factor within a tokamak plasma has been measured during a major disruption. During the disruption, the central safety factor jumps from less than unity to above unity, while the total current is unchanged. This implies that total reconnection has occurred. This observation is in contrast to the absence of total reconnection observed during a sawtooth oscillation in the same device.

A new technique for producing dense (${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$), low-temperature (2–6 eV), optically thin, and well characterized laser-produced plasmas is used to measure, for the first time, the inverse bremsstrahlung absorption and the Coulomb logarithm at 0.5 μm in a strongly coupled plasma. Extrapolations of the standard high-frequency calculations of inverse bremsstrahlung absorption underestimate the absorption in this regime by a factor of 2 or more, whereas calculations based on strongly coupled plasma theory and static classical theory agree with the measurements.

We study the evolution of patterns formed by injecting a reactive fluid with viscosity μ into a two-dimensional porous medium filled with a nonreactive fluid of unit viscosity. We treat the ‘‘mass-transfer limit,’’ in which the time scale of the chemical reaction between the injected fluid and the porous media is much faster than the time scale of reactant transport. We formulate a three-parameter position-space renormalization group and find two crossovers: (1) from the first diffusion-limited-aggregation (DLA) to the Eden point—due to finite viscosity, and (2) from the Eden to the second DLA point—due to chemical dissolution. We also calculate the crossover exponent and the crossover radius.

We develop a self-consistent-field model for copolymer adsorption onto laterally heterogeneous surfaces. Using this model, we can predict how varying the adsorbate-surface interaction energies and the chain architecture affects the properties of polymer films on nonhomogeneous, solid interfaces. These predictions are useful in designing interfacial materials that contain ordered domains of distinct polymers and, thus, can exhibit unique electrical and optical properties.

Molecular-dynamics calculations have been used to investigate the transition to an orientational-glass state in (NaCN${)}_{1\mathrm{-}\mathit{x}}$(KCN${)}_{\mathit{x}}$ mixtures. A previously unsuspected feature of the anion ordering was found. Whereas ${\mathrm{Na}}^{+}$ ions act as nucleation centers which cause neighboring ${\mathrm{CN}}^{\mathrm{-}}$ to lock into (100) orientations, other ${\mathrm{CN}}^{\mathrm{-}}$, which do not have a ${\mathrm{Na}}^{+}$ nearest neighbor, strive to freeze into (111) pockets, but at a lower temperature. The competition between these two tendencies leads to an anomalous temperature dependence of the orientational order parameters at low ${\mathrm{Na}}^{+}$ concentration.

A study of the scattering of x rays from the liquid-vapor interface of ethanol at room temperature is reported and compared in detail to predictions based on standard continuum capillary-wave theory. Quantitative agreement is obtained and, in particular, the asymptotic power laws in the diffuse scattering characteristic of the algebraic decay of the density-fluctuation correlations at the surface are verified. The root-mean-square fluctuation of the liquid surface is obtained as 6.9±0.2 Å.

We present the first experimental observation of an asymmetric diffusion behavior of dopant atoms in silicon. Point defects were injected into the silicon by formation of the silicide compound ${\mathrm{Pd}}_2$Si at 200 °C. This flux of point defects led to an asymmetric broadening of a narrow buried layer of dopant atoms, with diffusion occurring preferentially towards the silicide/silicon interface. The asymmetric diffusion behavior of the dopant atoms is a general phenomenon since we observed it for n- as well as p-type dopant. We discuss several mechanisms that could be responsible for this new diffusion phenomenon.

We examine the vibrational spectrum of a glass of soft spheres produced by quenching an equilibrated liquid (produced via constant-energy molecular-dynamics simulation) to zero temperature. Normal-mode analysis shows clearly the existence of (quasi)localized modes at the low-frequency end of the vibrational spectrum. The modes are found to be localized around atoms whose neighborhood structure differs significantly from the average glass environment. The effective masses of these modes range upwards from 10 atomic masses.

Vibrations of Sb on (110) surfacesof III-V semiconductors (InP,GaAs,GaP) were investigated by Raman scattering from the submonolayer range up to a few monolayers. Interface phonons, involving the epitaxial first monolayer of Sb and the upper substrate layer, are observed. They are clearly identified by their coverage dependence. The peaks are most pronounced for Sb on InP. They fulfill the polarization selection rules for Sb chains along (11¯0). Moreover, they reveal that for thicker Sb coverages the epitaxial structure of the first Sb monolayer remains intact.

We have measured the momentum dependence of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathrm{g}}^{}{}_{}{}^{}$←X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ transition in cubic solid nitrogen. We have evaluated the oscillator strength of the transition and compared it to the markedly different strength found in analogous measurements on the gas, and conclude that local-field effects present in the solid are responsible for the differences found. This is the first momentum-dependent measurement of local fields in a solid. The local-field enhancement factor increases by about a factor of 3 over the momentum range covered, and is consistent with a simple model calculation for small momenta.

Wide-band-gap semiconductors typically can be doped either n-type or p-type, but not both. Compensation by native defects has often been invoked as the source of this difficulty. Using first-principles total-energy calculations we show that, for ZnSe and diamond, native-defect concentrations are too low to cause compensation. For nonstoichiometric ZnSe, native defects compensate both n-type and p-type material; thus deviations from stoichiometry cannot explain why ZnSe can be doped only one way. In the absence of a generic mechanism, specific dopants should be examined case by case.

We argue that for a pinned 2D electron lattice in a magnetic field thermally activated bound dislocation pairs contribute to the conductivity from which an activation energy ${\mathit{E}}_{\mathit{d}\mathit{p}}$ can be deduced. Near the melting point, this activation energy ${\mathit{E}}_{\mathit{d}\mathit{p}}$ approaches zero as ${\mathit{E}}_{\mathit{d}\mathit{p}}$=${\mathit{E}}_0$(1-ν/${\nu }_0$), where ν is the filling factor. We calculate ${\mathit{E}}_0$ from the change in the zero-point energy of the phonons as a dislocation pair is created and find agreement to within 30% with the experimental result of Willet et al.