YbP${\mathrm{O}}_4$ was found to show weak green (497 nm) luminescence when excited by infrared (1 μm) light. It is concluded, on the basis of measurements of excitation intensity dependence and decay time, that this luminescence is due to the emission of a photon by a pair of excited ${\mathrm{Yb}}^{3+}$ ions in cooperation.

The compound ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$ was prepared by two different methods. Neutron diffraction did not reveal any difference in the crystallographic structure and the Néel temperature ($T_{\mathrm{N}}\sim 57$°K) of the samples of the two preparations. The samples did, however, exhibit at $T<\mathrm{}{T}_{\mathrm{N}}$ two distinctly different magnetic structures. It is argued that the appearence of these magnetic polytypes is related to the two-dimensional magnetic behavior of this compound and could in general be expected to occur in compounds exhibiting this behavior.

The electric susceptibility of a two-center bound polaron is discussed as a function of applied magnetic field. Periodic maxima observed in the dielectric constant in rutile are shown to result from zero-field ($H=0\mathrm{}$) splitting induced by impurity ions.

The spectrum shape of the $\beta$ particles emitted in the $0^{-}\to 0^{+}$ decay of ${}_{}{}^{206}{}_{}{}^{}\mathrm{Tl}$ was determined using a $4\pi$ solid-state $\beta$ spectrometer. The sources used were isotopically separated and extremely thin. The shape factor shows only small deviations from a statistical form. The half-life was determined as ${\tau }_{\frac{1}{2}}=4.27\mathrm{}\pm 0.05$ min.

A simple model of the nucleon-nucleus optical potential is used to explore the effects of exchange. The resulting potential is close to those found from analysis of actual scattering. Exchange effects are important.

Neutron-proton and neutron-deuteron total cross sections have been measured directly at the Princeton-Pennsylvania Accelerator using time of flight to determine the incident neutron momentum. The results cover the region from 700 to $2900 \frac{\mathrm{MeV}}{c}$ with a typical accuracy of 0.8% for each of 26 momentum bins. The data are not consistent with the most precise previous measurements in the same momentum range.

On the basis that the Pomeranchukon is a branch point with vanishing strength at the tip, we relate the small-$t$ dependence of the Pomeranchukon to the properties of the secondary trajectory $P^{\prime }$. We then use the exchange degeneracy between $P^{\prime }$ and $\rho$ to deduce the ${\mathrm{}\left(\mathrm{form}\mathrm{factor}\right)\mathrm{}}^4$ behavior as conjectured by Wu and Yang. Multiple-scattering correction at large $t$ then modifies the $t$ dependence in accordance with the Chou-Yang description. High-energy behavior is discussed.

We have analyzed the superallowed $\beta$ transitions of mirror nuclei and calculated the Gamow-Teller matrix elements in order to determine $g_A$ and $g_V$. Agreement with the more recent measurements is obtained.

The total cross section for meson-meson scattering at high energy is calculated using a simple, but plausible, $\mathrm{SU}\mathrm{}\left(n\right)\mathrm{}$-symmetric multiperipheral model. The resulting ${\sigma }_T$ is of the order $\frac{16{\pi }^3}{NM_V^{}{}_{}{}^2}$, where $M_V$ is the central mass of the dominant low-energy resonance multiplet in elastic meson-meson scattering and $N$ is the dimensionality of the multiplet of the incident mesons. The nongeometric character of this result is discussed.

An optical double-resonance effect, requiring two optical frequencies, is demonstrated in the infrared for the molecule C${\mathrm{H}}_3$F, and provides high-precision dipole-moment values for both the ground and an excited vibrational state. The method is a particularly convenient and precise means for probing optically excited molecular levels.

We present some measurements on the instability threshold of $p-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y}-n$, $p-\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{e}$ butylaniline (MBBA) under ac fields of frequencies 10-1000 Hz. The threshold curves, in both the low- and high-frequency regimes, are interpreted by a simple extension of the Helfrich theory to time-dependent phenomena.

Radiation exceeding thermal bremsstrahlung by more than three orders of magnitude has been observed from a theta-pinch plasma. The spectrum consists of two broad and partially overlapping bands centered approximately at the electron plasma frequency and its first harmonic.

The dependence on temperature and magnetic field of the diamagnetism due to fluctuations above $T_c$, while showing marked deviations from the behavior expected on the basis of the Ginzburg-Landau theory, appears to follow a universal behavior if one introduces a new characteristic field $H_s$, which may be expressed in terms of other known material-dependent parameters.

The attenuation of 10-GHz longitudinal sound waves in superconducting molybdenum ($T_c=0.914\mathrm{}$ K) and cadmium ($T_c=0.500\mathrm{}$ K) shows the high-frequency behavior predicted by the BCS theory. In particular, the onset of the rapid drop in attenuation with decreasing temperature that is characteristic of superconductors is shifted downward to the temperature $T_{\nu }$ (0.905 K in molybdenum and 0.490 K in cadmium) at which the superconducting energy gap equals the phonon energy. The analysis of the measurements indicates a large anisotropy in the energy gaps of both metals.

The two-photon absorption constant is measured experimentally with high resolution in the exciton region of KI. Fine structure is resolved in which the $2P$ exciton is seen at 6.263 eV. The peak at 6.285 eV is identified as a LO-phonon—assisted transition. The latter assertion is verified theoretically by invoking third-order time-dependent perturbation theory.

The imaginary part of the dielectric response function ${\epsilon }_2$ is calculated in the second-order perturbation theory for the Fröhlich electron—optical-phonon and hole—optical-phonon interactions. Coulombic attraction between the electron and the hole is included exactly. The interaction produces a maximum near the energy $E_{1s}$ (energy of $1s$ exciton) $+\hslash {\omega }_0$ (energy of optical phonon) accompanied by minima on both high-energy and low-energy sides. Positions of the maximum and the minima agree well with the experiment.

It is shown that the random-phase approximation static dielectric constant of a semiconductor with a ${\Gamma }_8$ conduction—valence-band degeneracy is nondivergent at zero momentum transfer for any concentration of ionized impurities (including the intrinsic material) so long as $T\ne 0$. The temperature dependence of the dielectric constant is calculated. It is shown that in the nondegenerate limit the dependence of the interband polarization is pure $T^{-\frac{1}{2}}$. Illustrative calculations are presented for $\alpha -\mathrm{S}\mathrm{n}$.

We show that localized states in solids can be conveniently investigated using techniques which are widely applied in the theory of atomic and nuclear spectra. The effective-mass theory for shallow acceptor states is formulated in a simple way and a meaningful classification of these states is obtained. The eigenvalue problem is reduced to simple radial Hamiltonians which are solved for the most important acceptor states.

A simple mechanical model has been found which gives the same angular velocity dependence of energy and angular momentum as postulated in the variable moment of inertia description of rotational states. The possibility for one of the two coefficients of this description to adopt negative values follows naturally from this model. Also the nonzero ground-state energies, which in some cases result from the variable moment of inertia description, are easily understood.

The dynamic model of the giant dipole resonance has been extended to describe nuclei of quite different collective character, e.g., rotators, vibrators, and transition cases on an equal footing.