Measurements have been made of the polarization properties of the two photons emitted in the decay of metastable atomic deuterium in an experiment in which three linear polarizers, instead of the normal two, are used. The quantum-mechanical predictions for these imperfect polarizers have been calculated and shown to be in good agreement with the experimental results. The results have also been used to test local realistic models which are capable of explaining all existing two-polarizer-type experiments, and have provided conclusive evidence against these theories.

A general exact procedure for separation of the energy of the overall rotation from the "internal" (vibrational) energy in any (arbitrarily floppy) $N$-body system is formulated. It is based on following the detailed dynamics of the system and it retains all the dynamical couplings allowing for the flow of energy between the rotation and vibrations. Still, it leads to a complete instantaneous separation of the energies in that the Coriolis term in the Hamiltonian is zero at all times. The utility of the new scheme is illustrated.

We suggest an explanation for the structure, observed by Giese and Horsdal, in the projectile angular dependence of the ratio of double to single ionization produced by fast protons in collision with helium. We postulate that two incoherent mechanisms, a first that produces slow electrons and a second that produces fast electrons, combine to give the effect.

Saturated oscillation of a free-electron laser operating in the near infrared has been achieved with 200 Torr of ${\mathrm{H}}_2$ gas introduced into the optical cavity. Plasma effects which limited output power in previous gas-loaded, free-electron laser experiments have been eliminated through the use of a small doping fraction of an electron-attachment gas. It is necessary to flow the gas mixture through the oscillator cavity to avoid depletion of the dopant through dissociation. Measured gain is consistent with theory, as is the observed wavelength tuning of 0.7 μm.

In our recent experiment, the hyperfine splittings of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ were observed in the infrared spectrum. Now we present an ab initio quantum-mechanical calculation of hyperfine structure in the (1-0) vibrational band of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ground electronic state. The hyperfine interaction coefficients $b$, $c$, and $\gamma$ are calculated by using the numerical multiconfiguration self-consistent-field method at a set of closely spaced internuclear distances and then averaged over nuclear coordinates for individual vibration-rotation states. The theoretical spectra agree well with the experimental observations.

Hysteretic spiral turbulence is a remarkable phenomenon of coexistence of turbulent and laminar domains in Taylor-Couette flow. We observe and measure for the first time a nonuniform pitch in long geometries and its dependence on boundary conditions at the cylinder ends, and we explain these results within the framework of phase dynamics. We also discuss the influence of secondary flow on the azimuthal width of the spiral.

We report measurements of synchrotron radiation emission in the wavelength region from 30 to 400 μm where coherent enhancement was predicted. The power ratio relative to a blackbody source exhibits no such enhancement and can be accounted for by the incoherent emission theory. This discrepancy is explained by showing that the coherent enhancement is reduced when proper account is taken of the relative phases of all emitting particles.

We have performed one-dimensional magnetohydrodynamic computer calculations of the formation and evolution of the solid-deuterium-fiber $Z$ pinch. With use of a tabulated atomic data base and "cold-start" initial conditions, our computations show that current is carried by hot plasma which has been ablated from the solid fiber. The computations suggest that the experimentally observed instability growth may coincide with the complete ablation of the central fiber.

It is shown that anisotropy in gradient has a substantial effect on the ion-temperature-gradient-driven mode. A gradient in the parallel temperature is needed for an instability to occur, and a gradient in the perpendicular temperature can either enhance or diminish the instability. The latter may be the basis for a stabilization scheme for this mode in tokamaks via strong and appropriate ion-cyclotron-resonance heating. The physical reason for this important role difference is also presented.

When an electrode is immersed in a laboratory plasma with magnetized electrons and its bias is pulsed with respect to the plasma potential so as to collect either ions or electrons, the initial current in the plasma is always carried by electrons. The dc current front is preceded by oscillatory dispersive currents which, for our experimental conditions, are associated with whistler waves. Thus, the current front advances along ${\mathbf{B}}_0$ not at particle speeds but at the whistler wave speed.

Dynamic light scattering measurements on oriented samples of lyotropic smectics have been performed. The layer compressibility modulus $B$ is extracted from the anisotropic dispersion relation of the so-called slip mode, which arises from the coupling between concentration and layer displacement fluctuations. The samples are diluted with three different solvents (repeat distances 50 to 800 Å): The behavior of $B$ suggests either undulation or electrostatic repulsions between membranes, depending on the swelling solvent.

Forward-recoil spectrometry reveals that the surface of a blend of deuterated polystyrene ($d$-PS) and protonated polystyrene (PS) is enriched with $d$-PS relative to the bulk; the surface excess of $d$-PS increases almost linearly with the bulk volume fraction $\phi$ of $d$-PS at 184°C. The results may be interpreted quantitatively with mean-field theory, and yield a form of the bare surface energy of the blend consistent with the surface enrichment being driven by a surface-energy difference between $d$-PS and hydrogenated polystyrene ($h$-PS) of isotopic origin.

With use of scaled variables, the equation of viscous fingering in radial geometry for zero surface tension is written in a form which admits singular solutions. It is then shown in an example that there exist singular solutions which are tree graphs. The dynamics of these solutions is not uniquely determined by the differential equation. The nonuniqueness corresponds to kinking, branching, and tip splitting at the growing ends, which need not happen, but which may happen. The qualitative resemblance of these ramified, underdetermined solutions to the experimental viscous fingering patterns in radial geometry is pointed out.

In situ studies of the isothermal crystallization of an amorphous alloy (Ni${\mathrm{Zr}}_2$) have been performed using time-resolved x-ray diffraction, with a time resolution as short as 3 ms. In the temperature range studied (628-680 K), a transient structural precursor is present during the crystallization. The maximum volume fraction reached by this precursor during crystallization increases with the rate of crystallization.

We show that when the angular dependence of three-body forces is adjusted to describe covalent-metallic phase transitions rather than small-amplitude atomic vibrations, a simple and accurate force field is obtained which is easily extended to describe energies and structures of ${\mathrm{Si}}_n$ vapor-phase clusters ($3\le n\le 10$). Our model may have widespread applications to amorphous phases, melting, and extended defects.

We have observed the capture of polarized electrons into excited terms of atoms after the interaction of fast ions with a magnetized Fe(110) surface at grazing incidence. The spin polarization of captured electrons results in a modified circular polarization fraction of fluorescence light. This experiment has considerable potential as a new analytical tool for investigating surface magnetism with extreme surface sensitivity, as a method for detailed studies of ion-surface interaction, and as a means to produce nuclear spin-polarized beams.

A theory of ballistic conduction through a constriction is developed which predicts a steplike structure with amplitude $\frac{e^2}{h}$ in the conductance as a function of Fermi energy or width. For a sharp (abrupt) constriction the steps are only quantized to 1 part in ${10}^3$ at $T=0\mathrm{}$. Disorder destroys the steps and they are modulated by novel resonant structure in certain regimes of shape and temperature.

Band-edge absorption spectra for a probe beam under the action of a nonresonant pump beam are calculated and analyzed within the Hartree-Fock theory. As a result of many-body interactions, one obtains an exciton optical Stark shift with a constant (2D) or slightly increasing (3D) exciton oscillator strength in agreement with several recent experiments and in striking contrast to corresponding atomic observations. Apparently, the deviating results of earlier semiconductor experiments were partly caused by dynamical effects and partly by the presence of real excitations.

The illuminated $n$-Si/aqueous-N${\mathrm{H}}_4$F interface has been studied by attenuated-reflectance infrared spectroscopy under conditions in which photoetching occurs. An anomalously strong Si-H stretch band was observed to develop in the region of 2100 ${\mathrm{cm}}^{-1\mathrm{}}$ when the silicon was illuminated. The intensity of the band was found to increase linearly with time under steady illumination, rapidly exceeding values corresponding to monolayer hydrogen coverage. The absorbance decayed slowly in the dark. The results indicate that the etching process involves a ${\mathrm{Si}}^{2+}$ intermediate which can disproportionate to form a hydrogenated amorphous silicon overlayer which builds up progressively as photoetching proceeds.

We report on experimental evidence of the important role of phonon broadening in very small CdSSe crystallites. At low temperature, spectral hole burning is observed. These results are in agreement with the special conditions of quantum confinement and with the polar character of this semiconductor.