Dynamic diffraction coupling is examined in superfluorescence with use of a semiclassical model in which diffraction and transverse density variations are rigorously included. The Cs data are correctly simulated for the first time.

Using the 002 reflection of a LiF single crystal, we have observed phase-matched parametric down conversion of 8-keV ($\mathrm{Cu}-K\alpha$) x rays into single photons at 7.7 keV; the corresponding idler photons are in the extreme vacuum ultraviolet at 335 eV. Agreement well within the estimated experimental uncertainty is found with theory, which predicts that this process can provide a means for the direct determination of valence-electron charge distributions.

We have studied the effect of a background of many modes on the growth of a tearing mode. A third-order calculation with random-phase approximation is presented for a static background. In that case, the tearing mode is further destabilized (stabilized) if the ${\Delta }^{\prime }$ values of the nonlinearly driven modes are positive (negative). This explains the destabilization of the ($m=3\mathrm{}$; $n=2\mathrm{}$) tearing mode observed in nonlinear three-dimensional calculations.

X-ray studies have been performed on a new liquid-crystal material which exhibits a noncrystalline $B$ phase. Using free-standing liquid-crystal film techniques, we find that this $B$ phase has short-range in-plane positional correlations but long-range, three-dimensional, sixfold bond-orientational order. We interpret our results in terms of a system of interacting two-dimensional hexatic layers.

Energy-transfer events have been observed between a He atom and a Cu(001) surface at 16°K. Only phonon creation events were observed which were confined to the subspecular region. Parallel momentum was conserved and bulk and surface modes detected. For an ordered Xe layer, both phonon creation and annihilation events were detected and parallel momentum was not conserved.

Polarized reflectance measurements from the far infrared to the visible are reported for ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$. At low temperatures, both parallel and perpendicular polarizations show plasma edges in the infrared with metallic reflection below these edges, indicating two- or three-dimensional behavior. The transverse plasma frequency is $\sim \frac{1}{5}$ of the parallel plasma frequency. At higher temperatures, the plasmon in the perpendicular polarization becomes highly overdamped, indicating a crossover to one-dimensional behavior.

Valence-electron plasmons in an amorphous silicon-carbon alloy system $a-{\mathrm{Si}}_x{\mathrm{C}}_{1-x} :\mathrm{H}$ ($0<~x<~1.0$) are observed as the satellites of core-electron levels in the x-ray photoemission spectra. The atomic density of $a-{\mathrm{Si}}_x{\mathrm{C}}_{1-x} :\mathrm{H}$ as a function of $x$ is determined from the valence-electron density $n_v$ obtained from observed plasmon energy $\hslash {\omega }_p$ with use of $\hslash {\omega }_p=\hslash {\left[\right(\frac{4\pi e^2}{m}\left)n_v\right]}^{\frac{1}{2}}$ and the results of thermal effusion experiments. The results are discussed in relation to the coordination of carbon atoms in $a-{\mathrm{Si}}_x{\mathrm{C}}_{1-x} :\mathrm{H}$.

Extremely thin (monolayer to 100 Å) continuous metallic films of Pt can be formed on Si substrates. In the thinnest films the conductance is found to be $S=S_0+{\alpha }^1\left(\frac{e^2}{{\pi }^2\hslash }\right)$ $\ln T+\delta T^2$, with ${\alpha }^1=0.75\mathrm{}\pm 0.15$ from liquid-He temperatures up to $T>~390$ K. Thicker films show a transition as a function of temperature from two- to three-dimensional behavior, with a corresponding resistivity minimum. The negative magnetoresistance is in good agreement with predictions of localization theory.

The photoelectrochemical cell [$p$-type InP(Ru)]/HCl-KCl/Pt(Rh) converts 12% of the incident solar energy into two useful chemicals, hydrogen and chlorine, by photoassisted electrolysis of aqueous hydrochloric acid. At the threshold for electrolysis, the voltage required is reduced from 1.3 to 0.65 V. Hydrogen evolution takes place at microscopic islands of catalysts such as Rh, Ru, and Pt. The high efficiency of the cell derives from a thin surface oxide on InP, preventing carrier recombination, and from efficient transport of electrons to the catalyst.

Above a region of excitonic transitions the polydiacetylene DCHD [poly 1, 6-di(N-carbazolyl)-2, 4-hexadiyne] shows at the photoconduction edge a large electroreflectance response for fields parallel to the polymer. The signal broadens with field strength indicating a high-field limit for electroreflectance. Evaluation yields $E_g=2.335\mathrm{}$ eV, $m^{*}=0.05\mathrm{}{m}_0$, and $\mu =2800\mathrm{}$ ${\mathrm{cm}}^2$/V sec. The influence of a field perpendicular to the polymer chain is 3 orders of magnitude smaller. This unique anisotropy indicates that delocalization of the electrons occurs only along the polymer.

A large anisotropy in the electrical conductivity of CdS appearing below 30°K is reported. This anisotropy casts serious doubt on the customary view that impurity hopping supplies the low-temperature conductivity. We present a new model for this conductivity based upon the electronic screening of the piezoelectric polarization around dislocations. Numerical solutions of the highly nonlinear Poisson equation quantitatively justify this model.

Chemical reactions near critical points of multicomponent systems exhibit slowing down. If all the components participate in the reaction, the rate of reaction vanishes as ${\left[\frac{(T-T_c)}{T_c}\right]}^{\gamma }$, independently of the nature of the constituents. All known experiments are explained.

Incommensurate perturbations of classical orbits lead to an almost periodic Hill's operator whose spectrum, we argue, is a Cantor set, but one with large Lebesgue measure. Applied to the rings of Saturn, this implies that the complex groove structure in the rings approximates a Cantor set. We also emphasize the possible relevance of the sun in producing "side gaps" which magnify the apparent gap size.

It is shown that vacuum strings produced in the early universe at the grand unification phase transition can generate density fluctuations sufficient to explain the galaxy formation.

We have made, for the first time, a direct reconstruction of the $\mathrm{pp}$ elastic-scattering matrix at 579 MeV from a series of experiments performed at the Schweizerisches Institut für Nuklearforschung polarized-beam line. Fifteen observables consisting of the polarization, two-spin correlation and transfer parameters, and three-spin parameters were measured at seven angles between 66° and 90° (c. m.). The experimental results and reconstructed amplitudes are presented and compared to phase shift analysis.

Predictions based on our proposed SU(7) unification of color, electroweak, and generation symmetries are obtained concerning neutrino masses and mixing, and concerning $B$-meson decay modes.

A semimicroscopic model of the nucleus is presented in which spin-isospin order is realized as zero-point one-dimensional oscillations of spin-up protons and spin-down neutrons against spin-down protons and spin-up neutrons. This phase is favored by the one-pion-exchange potential in light deformed nuclei. The model is characterized by the lowering of the energy of the first excited state and enhancement of the $B\left(M2\mathrm{}\right)\mathrm{}$.

We investigate the physical origin of the energy dependence of the isovector potentials $V_{\tau }$ and $V_{\tau \sigma }$ within a model which includes the one-pion and one-$\rho$—exchange potential as well as the second-order effects of the tensor force.

The ${\overline{\nu }}_e$ spectrum has been computed for fission-product decay following a 30-d irradiation of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ by thermal neutrons. Estimated uncertainties lie in the range (7-15)% for $E_{\overline{\nu }}<6\mathrm{}$ MeV. Analysis relied on comparisons of calculated $\beta$-ray data with recently obtained experimental $\beta$-ray spectra. The ${\overline{\nu }}_e$ spectrum is softer than all other calculated ${\overline{\nu }}_e$ spectra. Cross sections (${10}^{-44\mathrm{}}$ ${\mathrm{cm}}^2$/fission) were calculated as $\sigma ({\overline{\nu }}_e+p\to n+e^{+})=58\mathrm{}\pm 3$, $\sigma ({\overline{\nu }}_e+d\to n+p+{\overline{\nu }}_e)=2.7\mathrm{}\pm 0.2$, and $\sigma ({\overline{\nu }}_e+d\to n+n+e^{+})=1.04\mathrm{}\pm 0.13$.

The energy dependence of the reaction cross section and the corresponding tensor analyzing power has been measured for the system ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$-${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ by two different methods. The tensor analyzing powers observed can be interpreted in a sharp-cutoff model as caused entirely by the static deformation of the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ projectiles.