We have measured the ${\mathrm{He}}^4$ electrostatic fine-structure intervals $7^1{D}_2-7^1{F}_3$ and $7^1{D}_2-7^3{F}_3$ neat 31.5 GHz. The latter transition appears because of spin-orbit mixing of singlet and triplet $7F$ states of the same $J$. The 3-ppm precision represents a ${10}^4$-fold improvement over the mean $7^{1}D-7F$ spectroscopic value. We have also measured the mean $7F-7G$ and $7G-7H$ intervals. The microwave-optical method used appears widely applicable to atomic and molecular Rydberg states.

It is shown that the results of a recent laser-produced—plasma interaction experiment by Dean et al. can be explained by collisional processes.

A high-frequency (${\omega }_{\mathrm{ci}}\ll \omega \simeq {\omega }_{\mathrm{pi}}$) electrostatic instability observed in a toroidal turbulent-heating device has been interpreted in terms of ion-acoustic waves excited by a radial, electron temperature gradient across a toroidal magnetic field. The waves have been found to propagate across the field, in the direction opposite to the electron diamagnetic velocity. Observed anomalous electron thermal transport has been explained by the instability.

Observation of the time development of parametric instability reveals new oscillatory saturation states, whose frequency is proportional to the pump intensity. The self-consistent pump field simultaneously exhibits similar but 180° out-of-phase oscillations. Application of a small auxiliary driver wave ($E_t\sim 0.1E_0$) at the electron sideband lowers the threshold and dramatically enhances the intensity of excited ion wave. Growth rate and functional dependence of threshold on ion damping are verified.

The attenuation and velocity of sound in liquid ${\mathrm{He}}^4$ have been calculated using the Landau-Khalatnikov kinetic equations and the phonon Boltzmann equation. A detailed comparison between theory and experiment is made at 0.35°K and good agreement is obtained over a wide range of requencies.

Coefficients are derived for a one-dimensional Fokker-Planck equation describing the evolution of an electron energy distribution. These coefficients include acceleration of electrons between collisions in addition to the collision terms. The coefficient for the average rate of change of the electron energy, $\frac{〈\Delta \epsilon 〉}{\Delta t}$, is the same as obtained with the "average-electron" theory; but the expression for the dispersion, $\frac{〈{\left(\Delta \epsilon \right)}^2〉}{\Delta t}$, has not appeared previously.

We have observed light scattering from the density variations in the first- and second-sound modes generated simultaneously by a thermal transducer in superfluid ${\mathrm{He}}^4$. Measurements of the temperature dependence of the density variations in both sound waves agree with the theoretical treatment by Lifshitz of the density-temperature coupling.

The intensity of a large-amplitude, unstable plasma mode has been reduced by applying a pump signal to parametrically couple it to another damped, natural mode of the plasma column. A suppression of the mode is observed when the frequency of the pump ${\omega }_0$ is equal to the difference between the damped-mode frequency ${\omega }_a$ and the unstable-mode frequency ${\omega }_b$ (${\omega }_0={\omega }_a-{\omega }_b$). The observed behavior of the mode energies is compared with a weak parametric-mode-coupling theory for lumped circuits, which includes nonlinear saturation terms for the unstable mode.

We give a coherent quantitative interpretation of both the frequency and temperature behavior of the proton spin-lattice relaxation time $T_1$ in the nematic phase of $p$-azoxyanisole. The frequency dependence of $T_1$ is dominated by the long-wavelength thermal fluctuations in the orientational order. The temperature dependence of $T_1$ has an important contribution from short-range phenomena involved in the motion of methyl protons. The activation energy for this motion is $W\approx 9$ kcal/mole.

A simple lattice gas with a phase diagram similar to that of argon is described.

Trapping of beam electrons has been observed in the potential wells of large-amplitude electrostatic waves ($E=7\mathrm{}\times {10}^3$ V ${\mathrm{m}}^{-1\mathrm{}}$) resulting from beam-plasma interaction. Trapping is attended with the growth of a red-shifted sideband near the main peak in the frequency spectrum and with spatial oscillations in the amplitude of the main peak.

The superconducting resistive transition of the layer compound Nb${\mathrm{Se}}_2$ has been observed for crystals a few layers thick. The $T_c$ decreases as the crystal thickness is reduced below six Nb${\mathrm{Se}}_2$ layers, and a $T_c$ of 3.8°K is predicted for a single layer. No significant increase in the transition width is observed as the number of Nb${\mathrm{Se}}_2$ layers is reduced. The results are compared to recent work on intercalated layer compounds.

The energy-loss spectra of 25-keV electrons transmitted through thin amorphous Ge and Si films have been measured in the range of very small energy losses between 25 and 400 meV. The resolution was 4 and 6 meV. The main intensity in the spectra is located at low energy and is caused by "defect-induced" two phonon excitation.

Left-right asymmetry is expected in the scattering of electrons by magnetic impurities in metals; we present a simple calculation of the asymmetry in a virtual-bound-state model. When a magnetic field lines up the impurity moments, this left-right asymmetry should generate an extraordinary Hall effect proportional to the impurity concentration. This effect appears to occur in many magnetic alloys (e.g., ${\mathrm{Ni}}^{-}$, ${\mathrm{Cu}}^{-}$, and ${\mathrm{Au}}^{-}$ based alloys).

The existence of an impurity-activated $E_{\mathrm{g}}$ resonance mode in NaCl:${\mathrm{Cu}}^{+}$ has been suggested by several previous experiments. Raman data presented here reveal this resonance directly and also reveal the three components of the first overtone of the 23.5-${\mathrm{cm}}^{-1\mathrm{}}$ infrared resonance mode. The frequencies of the $E_{\mathrm{g}}$ resonance and the $E_{\mathrm{g}}$ component of the overtone are shifted as a result of a strong anharmonic coupling. Their line shapes and strengths are considerably altered by an interference between the Raman amplitudes. A reasonable fit to the data has been obtained using a simple theory.

We study the effect of the exchange interaction on the dielectric response of an electron gas in the ferromagnetic state. For a parabolic band, the dielectric constant of the minority-spin electrons becomes negative, and the magnetization associated with the screening charge around a unit point charge can be larger than $1{\mu }_B$.

The reaction ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}({\pi }^{+},{\pi }^0){}_{}{}^{91}{}_{}{}^{}\mathrm{Nb}_{}^{*}{}_{}{}^{}$ was observed by detecting the decay proton $\tilde{p}$ from the isobaric analog state ${}_{}{}^{91}{}_{}{}^{}\mathrm{Nb}_{}^{*}{}_{}{}^{}$ in the sequential process ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}({\pi }^{+},{\pi }^0\tilde{p}){}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$. The observed cross section for an incident pion energy of 30 MeV is 1.1 ± 0.8 mb, and is in reasonable agreement with theoretical calculations.

It is pointed out that neutrinos with a finite mass could be unstable. We discuss the consequences of this possibility for solar-neutrino experiments.

We study $\pi \pi$ scattering in the reactions ${\pi }^{-}p\to n{\pi }^{+}{\pi }^{-}$ and ${\pi }^{+}n\to p{\pi }^{+}{\pi }^{-}$ at 7 $\frac{\mathrm{GeV}}{c}$ using the absorption-modified one-pion-exchange formalism. Fitting the $\pi \pi$ angular distributions we obtain $S$-, $P$-, and $D$-wave $\pi \pi$ phase shifts and inelasticities for $0.6<M\left(\pi \pi \right)<\mathrm{}1.5\mathrm{} \frac{\mathrm{GeV}}{c^2}$. We find an $I=0\mathrm{}$ $S$-wave resonance with $M=1.25\mathrm{}\pm 0.04$ and $\Gamma =0.3\mathrm{}\pm 0.1 \frac{\mathrm{GeV}}{c^2}$. Support for our phase shifts in the $f^0$ mass region is obtained from data on ${\pi }^{+}n\to p{K}^{+}{K}^{-}$ and ${\pi }^{+}n\to p{\pi }^0{\pi }^0$ at 7 $\frac{\mathrm{GeV}}{c}$.

New data are presented on longitudinal and transverse momentum spectra of $\Lambda$ and ${K_1}^0$ produced in $\mathrm{pp}\to \Lambda +$ anything and $\mathrm{pp}\to {K_1}^0+$ anything at 13, 18, 21, 24, and 28 $\frac{\mathrm{GeV}}{c}$. The $\Lambda$ spectra show little variation with energy. The ${K_1}^0$ inclusive cross section increases by ∼ 50% over our energy range, indicating that for this process the threshold for scaling and limiting fragmentation may be above 30 $\frac{\mathrm{GeV}}{c}$. Tests of Mueller-Regge phenomenology are discussed.