We report a new and more precise measurement of the magnetic moment of the ${\Sigma }^{-}$ hyperon. Our result of (-1.48± ${0.37)\mathrm{\mu }}_{\mathit{N}}$ is in agreement with the sign given by simple SU(3) theory, but differs somewhat from the value of - ${0.9\mathrm{\mu }}_{\mathit{N}}$ predicted by the theory with no mass breaking.

The photon spectrum from radiative and charge-exchange capture of pions in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ was measured in a high-resolution pair spectrometer yielding the new value of the Panofsky ratio $P_3=\frac{({\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to {}_{}{}^3{}_{}{}^{}\mathrm{H}+{\pi }^0)}{({\pi }^{-}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to {}_{}{}^3{}_{}{}^{}\mathrm{H}+\gamma )}=2.68\mathrm{}\pm 0.13$. An impulse-approximation analysis is presented which gives a value $P_3=2.5\mathrm{}$. In addition, the branching ratios for the ${}_{}{}^3{}_{}{}^{}\mathrm{He}+{\pi }^0$, ${}_{}{}^3{}_{}{}^{}\mathrm{H}+\gamma$, and the combined ${}_{}{}^2{}_{}{}^{}\mathrm{H}+n+\gamma$ and $p+n+n+\gamma$ channels are measured to be 17.8± 2.3, 6.6± 0.8, and 7.4± 1.0%, respectively.

$\mathrm{Ne} K$ x-ray and Auger-electron production cross sections have been measured in 50-MeV Cl + Ne collisions for incident charge states of 5+ to 15+. The ionization cross section and mean fluorescence yield were derived from the data. A factor 20 rise in the x-ray cross section over the charge-state range studied is primarily due to a factor 7.2 increase in the mean fluorescence yield. The factor 2.8 rise in the ionization cross section can be explained by a decreased screening of the projectile nuclear charge.

Relative $K$-shell ionization cross sections were measured for 7.1-MeV/amu projectiles of ${\mathrm{He}}^{2+}$, ${\mathrm{B}}^{5+}$, ${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{7+}$, ${\mathrm{O}}^{8+}$, ${\mathrm{F}}^{9+}$, and ${\mathrm{Ne}}^{10+}$ on targets of Ca, Fe, Y, and Cd. The scaled cross section ratios $R=\frac{\left[4\mathrm{}\sigma \right(Z_1\left)\right]}{\left[Z_1^{}{}_{}{}^2\sigma \mathrm{}\right(2\left)\right]}$ exhibit linear $Z_1$ dependence characteristic of the effects of increased binding of target electrons and polarization. The absence of higher-order $Z_1$ dependence suggests that the contributions to the ionization $\sigma$ by charge exchange to the empty orbitals of the projectile are relatively small.

The two-quantum annihilation of positrons in flight with the $K$-shell electrons of silver has been investigated using 300-keV positrons. The experimental result (7.7 ± 6.4) × ${10}^{-27\mathrm{}}$ ${\mathrm{cm}}^2$/${\mathrm{sr}}^2$ has been obtained for the double-differential angular cross section of this process, at 30 and - 100° for each annihilation photon with respect to the incident positron direction.

A microscopic theory of the flow alignment in nematic liquid crystals is presented. The calculation is based on a thermodynamic sum rule derived earlier. The temperature dependence of the flow-alignment angle is obtained in terms of the nematic order parameter $S\left(T\right)\mathrm{}$. The theory favors stable flow alignment, but the orientational instability found by Gähwiller cannot be ruled out.

I analyze Graham's theory of the threshold region of the convection instability in a fluid layer heated from below. I consider the situation where vertical boundary walls insure that a single spatial mode dominates in the threshold region. Equilibrium properties and the temporal correlation times for both the amplitude and intensity of that mode are presented.

Neutron inelastic scattering at a momentum transfer of about 15 ${\mathrm{\AA }}^{-1\mathrm{}}$ has been used to measure the scattering law $S(Q,\omega )$ with high accuracy for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at 1.2 and 4.2°K. At this momentum transfer the impulse approximation is nearly valid and momentum distributions for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ can be determined directly from the data. The momentum distributions for the two temperatures show features not previously observed and provide new information to compare with theoretical results.

The excluded-volume exponent $\nu$ is determined from a neutron scattering experiment by polystyrene chains, in the "intermediate" momentum-transfer range. The chains were successively dispersed in a good solvent, in a $\Theta$ solvent, and in the bulk material. The new result is the value of $\nu$ for the bulk material, which is discussed below.

The threshold is derived for parametric excitation of Alfvén waves in a uniform plasma in which the background magnetic field is modulated sinusoidally in time. Including the plasma pressure we show that ion acoustic waves can also be excited directly. Both cases are absolutely unstable for any interaction length. Finally, we analyze the subsequent decay of the excited Alfvén wave into an ion acoustic and another Alfvén wave.

The plasma energy confinement time required to achieve a practically useful power amplification factor $Q_E$ can be reduced substantially if the plasma is heated by an energetic ion beam that reacts with the bulk ions as it thermalizes. The contribution to $Q_E$ arising from the beam-plasma fusion reactions can be maximized by maintaining the post-injection energy of the beam ions near an optimum value, for example, by means of slow magnetic compression.

Magnetic fields in excess of ${10}^7$ G are expected to arise in laser-produced plasmas in which a discontinuity in atomic weight of the target occurs in the focal spot. The upper limit to $B$ is controlled by fluid disassembly of the field layer due to magnetic pressure. This may provide a means for direct measurement of these fields in laser-produced plasmas.

The degree to which the surface atomic arrangement influences the bonding geometry of sulfur atoms on (001), (110), and (111) Ni surfaces is investigated by an analysis of experimental low-energy electron diffraction data. For all surfaces, the sulfur atoms reside in high-coordination sites—the atomic hollows of the surface; all nearest-neighbor Ni-S bond lengths are less than those of stable bulk compounds.

Observations on lattice instability are demonstrated by transmission and diffraction electron microscopy together with an electron multiplier. The latter displays the coupled atomic fluctuations on an oscilloscope screen. The WKB method has been used to calculate a single-atom transition time. The results indicate that fluctuations should be present and that quantum mechanical considerations are important in the nucleation of solid-state phase transformation. A standing-wave model is suggested for the pretransition lattice instabilities and is experimentally supported.

The electronic structure of a model "surface complex" consisting of a Ni diatom and an ethylene molecule has been calculated using the self-consistent-field, $X\alpha$, scattered-wave method. Comparison of calculated $\pi$-orbital bonding shifts with recent photoemission spectra for chemisorption of ethylene on nickel favors a $\pi$-bonded complex over a di-$\sigma$-adsorbed complex. Charge distributions of various molecular orbitals indicate that the C-C double bond is weakened more for the latter molecular arrangement, suggesting such a complex as a likely intermediate in heterogeneous reactions.

Using the results of self-consistent augmented-plane-wave band-structure calculations, electron mass enhancement factors and superconducting transition temperatures have been calculated for V, Nb, NbC, TaC, and HfC. Good agreement with experiment is obtained.

The first accurate measurements of the algebraic values of the photoelastic tensor of silicon are reported. The values, determined acousto-optically, are $p_{1111}=-0.094\mathrm{}$, $p_{1122}=+0.017\mathrm{}$, and $p_{1212}=-0.051\mathrm{}$. The results are in strong disagreement with calculations based on the Phillips-Van Vechten theory of ionicity. It is shown that the widely used assumption of a universal power law describing both the intermaterial and intramaterial variations of the homopolar average energy gap with volume must be reinterpreted or revised.

Experimental evidence for a transition from localized to extended states in a two-dimensional band tail is obtained by measuring the conductivity in $n$- and $p$-type inversion layers in Si as a function of electron density and temperature (4.2 to 0.4°K). A transition from metallic to thermally activated conductivity is observed as a function of electron density, while the temperature dependence at electron densities below the transition shows both thermally activated nearest-neighbor hopping and variable-range hopping as proposed by Mott.

A theory of the angular resolved photoemission from localized adsorbate orbitals is presented in which the effects of the final state are discussed in detail. Numerical results show a strong dependence of the energy-resolved angular distribution on the symmetry of the bonding orbitals and on the geometry of the adsorption site.

Two nuclides with a proton excess of 4 have been produced via the ($\alpha ,{}_{}{}^8{}_{}{}^{}\mathrm{He}$) reaction, and their masses measured. The mass excess of ${}_6{}^8{}_{}{}^{}\mathrm{C}$ is 35.30 ± 0.20 MeV, and ${}_{}{}^8{}_{}{}^{}\mathrm{C}$ is therefore unbound with a measured width of ${220}_{-140\mathrm{}}^{+80\mathrm{}}$keV. The ${}_{}{}^8{}_{}{}^{}\mathrm{C}$ mass excess is 0.57 MeV less than the Kelson-Garvey prediction. For ${}_{12}{}^{20}{}_{}{}^{}\mathrm{Mg}$, a mass excess of 17.74 ± 0.21 MeV is found, indicating that ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$ is nucleon stable. Knowledge of the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$ mass permits for the first time a test of the isobaric multiplet mass equation in an isobaric quintet, and good agreement is found.