Electroreflectance spectra in PbTe lead to a clear determination of the band separations at $L$. Our results suggest that the accepted parity labeling of some of these states may be incorrect.

The energy distribution of slow positrons diffusing from chemically cleaned and partially outgassed polycrystalline tungsten, molybdenum, and chromium is made up of two principal parts, a broad asymmetric peak at 0.2 ± 0.1 eV with a width of ~ 2 eV, and a second sharp, usually symmetric, peak with an energy which depends upon the material under study. The second peak is believed to be associated with the bulk properties of the material, in particular, with the electronic density of states.

An anisotropic direct-transition model for single-crystal semiconductors is shown to predict the direct-transition features seen in experimental photoemission spectra for Ge(111) for $h\nu \lesssim 20$ eV. By comparing theory with experiment, all the conduction and valence bands at $L$ and $X$ within 1 Ry of the gap are determined. Comparison of experiment with current band models suggests that an ∼ 10% self-energy correction may be needed to describe high-energy optical transitions.

It is shown that a simple random-walk formalism is not sufficient to derive ac conductivities when the influence of the first-waiting-time distribution is considered.

We have observed an anomalous broadening of two fluorescence lines in Pr${\mathrm{Cl}}_3$. The new effect is observed only when the terminal state is a magnetic doublet and only at low temperatures ($T<\mathrm{}100\mathrm{}$ K). However, the effect is seen far above any transition or ordering temperature. The broadening can be, at least partially, suppressed by application of a large magnetic field. The effect is almost certainly a cooperative phenomenon arising from an interaction between ground- and excited-state ions.

The compositional dependence of the dc conductivity of ${\mathrm{As}}_x{\mathrm{Te}}_{1-x}$ films with $0.3<x<\mathrm{}0.78\mathrm{}$ was measured. The resistivity over the entire compositional range investigated could be described with an effective-medium percolation theory. No adjustable parameters enter into this first analytical description of the compositional variation of a transport property in an amorphous multicomponent semiconductor.

The correlation between the magnetic hyperfine field $H_{\mathrm{hf}}$ and the electronic moment $\mu$ at the neptunium site in a number of neptunium conducting compounds is presented. A simple linear relation $H_{\mathrm{hf}}=\left[\frac{\left(1915\mathrm{} \mathrm{kOe}\right)}{{\mu }_{\mathrm{B}}}\right]\mu$ is found. This behavior is similar to that observed in lanthanide systems and suggests a strong localization of the $5f$ electrons in these materials. The implications of this result for our understanding of magnetism in the actinides are considered.

We have studied the properties of two rotational bands in ${}_{}{}^{167}{}_{}{}^{}\mathrm{Lu}$ for high spin values. Backbending is observed in the ${\frac{7}{2}}^{+}\mathrm{}\left[404\right]\mathrm{}$ band, but not in the ${\frac{1}{2}}^{+}\mathrm{}\left[541\right]\mathrm{}$ band. These results are in agreement with the rotation-alignment model.

From Glauber calculations of the absorption cross section of nucleons on air (${\sigma }_{\mathrm{abs}}$) we find that present extensive-air-shower data cannot resolve the question of the asymptotic energy behavior of the nucleon-nucleon total cross section [${\sigma }_t\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$]. Relative differences between various asymptotic extrapolations for ${\sigma }_t\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$ are reduced by about a factor of 3 in ${\sigma }_{\mathrm{abs}}$ through the Glauber conversion. Novel techniques or conventional experiments with a better determination of electron and muon numbers in extensive air showers will be required in order to obtain useful information on proton-proton cross sections in the asymptotic region.

Polarization up to 15% has been detected in the decay of electronic levels in many-electron monatomic systems which were excited by the beam-foil method, the foil being tilted with its plane at 65° to the particle velocity. Quantum beats were found even with an external magnetic field oriented parallel to the particle velocity, indicating that coherence is produced in the beam-foil interaction itself. Results are reported for various levels in He i, O ii, and Ar ii.

The molecular-beam magnetic-resonance method has been used to measure the Stark effect of the metastable $c{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$ electronic state of ${\mathrm{H}}_2$. The magnitude of the effect is found to be about ${10}^4$ times greater than for the ground electronic state and to depend sensitively upon vibrational quantum number. A theoretical treatment of the effect has allowed us to identify at least three vibrational states of ${\mathrm{H}}_2$ which are metastable, including $v=0\mathrm{} \mathrm{and} 1$.

The high-resolution $L_{2,3}{M}_{4,5}{M}_{4,5}$ Auger-electron spectrum from zinc vapor has been measured using electron-impact excitation. The spectrum has been decomposed into components and interpreted by comparison with calculated line energies and intensities. The calculations have been carried out for $\mathrm{jj}$ coupling in the initial states and for intermediate coupling with configuration interaction in the final states. Good agreement between the calculated and the experimental results has been found.

We have measured, for the first time, the spin-lattice relaxation rate between hyperfine levels of Kramers-doublet ions dilutely dispersed in crystals in zero external magnetic field. The expected amount of reduction in the rate, at temperatures well below 2°K, relative to the rates at higher fields, is not realized. The measurements have been made for the ${}_{}{}^{143}{}_{}{}^{}\mathrm{Nd}_{}^{+3\mathrm{}}{}_{}{}^{}$ ion substituted dilutely for ${\mathrm{La}}^{+3\mathrm{}}$ in La${\mathrm{Cl}}_3$ crystals.

A nuclear spin echo ("pseudosolid echo") analogous to a "solid echo" is predicted to be observed even within the motional-narrowing approximation, whenever the molecular motion symmetry gives rise to a nonzero average dipolar spin coupling. It is considered as a new characterization of molecular motion anisotropy. New features are discussed. They are well illustrated by "pseudosolid echoes" observed, for the first time, on protons bound to long chains in a molten polymer system. Echoes are shown to reflect chain constraints.

The renormalization-group transformations for a square Ising spin lattice are evaluated in a four-cell-cluster approximation to obtain the critical surface in a three-dimensional space of spin-interaction parameters. Agreement is found with known critical curves, particularly Baxter's analytic solution.

It is shown that spin waves exist in the hydrodynamic regime in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. Hydrodynamic equations ruling these spin waves are derived. Spin waves are studied in the axial (Anderson-Brinkman-Morel) and in the isotropic (Balian-Werthamer) states.

It is shown that the use of the concept of effective photons offers a simple explanation of some recent experimental results of air breakdown by a 20-psec laser pulse, in which a quadratic dependence of breakdown laser power on lens focal length has been found. It is also demonstrated that the use of the same concept yields a different focal-length dependence in the case of gas breakdown with rectangular and Gaussian spatial beam profiles.

Intense radiation impinging on a plasma near the critical density may be absorbed by exciting electrostatic plasma waves. In inhomogeneous plasmas these oscillations convect out of the interaction region to lower densities where the waves have lower phase velocities. Electrons whose velocity is not much greater than the thermal velocity then absorb the wave. In this process, a very few highly energetic electrons are produced. We present theory and simulation for the absorption of these waves in an inhomogeneous plasma.

The theory of the cylindrical-geometry analog to the toroidal trapped-electron-scattering mode has been developed, and the resulting equations have been solved numerically. Observations of oscillations occurring near the electron bounce frequency have been made in a plasma confined by a spatially periodic magnetic field. These oscillations were found to be low-amplitude, high-mode-number, drift-type modes having properties consistent with theoretical predictions.

Self-consistent sub-band splittings and inter-sub-band optical matrix elements are calculated for $n$-type accumulation layers at temperatures low enough that the bulk carriers are frozen out. The energy splittings are sensitive to the concentration of acceptor impurities in the surface space-charge layer.