Enhanced damping of an electron plasma wave due to induced nonlinear decay into a second electron wave plus an ion wave is demonstrated experimentally. The particular ion distribution allows decay into both positive and negative energy ion waves. The threshold for decay into the positive energy waves is shown to be consistent with theory.

We have measured the temperature dependence of the flow birefringence in the isotropic phase of $p$-methoxybenzylidene $p-n$-butylaniline. We have observed a divergence as the phase transition is approached. We have determined the steady shear viscosity ${\eta }_0$ and the coupling coefficient $\mu$ between velocity shear and the order parameter as a function of temperature. We have also measured the real part of the shear impedance and compared the results with light-scattering data. Our results are well described by de Gennes's theory.

The Kawasaki formula $D=\frac{\mathrm{kT}}{6\pi \eta \xi }$ is experimentally verified using the measured values in a cyclohexane-aniline critical mixture near $T_c$. Its application in a wide range of temperatures above $T_c$, where the viscosity is known, is completely verified by the measurements of the diffusion coefficient $D$ and allows a direct determination of the correlation length $\xi$ in a domain where no other optical method is suitable. We show that the power law $\xi =\xi {\left(\epsilon \right)}^{\nu }$ is followed up to 40° from $T_c$.

We have observed anomalous ultrasonic attenuation in pure superconducting Nb[100] for $q{l}_e\approx 1$. The anomalous behavior is believed to be associated with the high purity of the sample, which has a measured resistivity ratio of 6500. The attenuation data have been analyzed by assuming that the material has two energy gaps, one with a standard BCS value of $2\frac{{\Delta }_1\mathrm{}\left(0\right)\mathrm{}}{k{T}_c}=3.4\mathrm{}$ and the other with $2\frac{{\Delta }_2\mathrm{}\left(0\right)\mathrm{}}{k{T}_c}=10\mathrm{}$.

Two years ago Piller, Seraphin, Markel, and Fischer reported electroreflectance measurements on disordered germanium; their results allowed the interpretation that the $E_0$, $E_0+{\Delta }_0$ doublet is still present in the completely amorphous state. I demonstrate here that the electroreflectance peak positions are determined entirely by interference effects in the thin-film sample, and hence have no bearing on the electronic structure of amorphous Ge.

Results of the isothermal pressure dependence and isobaric temperature dependence of the $q\approx 0$ transverse optic mode for TlBr are given. These results indicate that the anomalous temperature dependence of this mode arises from strong quartic contributions to the anharmonic self-energy.

The cluster-variation method is applied to calculate the orientational ordering of $o-{\mathrm{H}}_2$ molecules on fcc and hcp lattices. The $\mathrm{Pa}3\mathrm{}$, $\mathrm{Pc}{a2\mathrm{}}_1$, and $\frac{{P2\mathrm{}}_1}{c}$ structures are examined. A fcc-hcp structural transition very near to an orientational ordering transition is predicted. These transitions result because of the anisotropic intermolecular interactions. The isotropic interactions are expected to play only a very minor role.

Mössbauer-effect measurements on extremely small (∼60 Å) crystallites of $\gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3$ show that the spin configuration differs from the Néel type found in large crystallites. It is proposed that the ions in the surface layer are inclined at various angles to the direction of the net moment.

Precision measurements of the magnetic thermal expansivity of RbMn${\mathrm{F}}_3$ near its critcal (Néel) temperature are reported. The critical exponents are $\alpha =+0.007\mathrm{}\pm 0.02$ and ${\alpha }^{\prime }=-0.100\mathrm{}\pm 0.03$, in disagreement with scaling predictions. The rounding about $T_c$ due to sample inhomogeneity is observed to be asymmetric.

The channeling technique has been employed to study the lattice location of Tl, Pb, and Bi implanted into iron single crystals. The results, when compared with measured magnetic hyperfine fields acting on Pb in ferromagnetic iron, provide evidence for the association of high-field sites with Pb in nonsubstitutional positions.

We have measured the variance of the number of neutrons emitted by individual fragments in fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ as a function of the fragment mass and the total kinetic energy released. The variance does not show pronounced "sawtooth" structure as a function of the fragment mass. We deduce from this that the correlation coefficient between the excitation energies of complementary fission fragments is substantially less than unity for at least some portions of the fragment mass-ratio distribution.

The $E4\mathrm{}$ transition moments between the ground state and the $4^{+}$ rotational state in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Sm}$ and ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$ have been determined from Coulomb excitation experiments with 10-12-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ projectiles. Quantum mechanical corrections have been applied to the calculations used in the analysis of the data. The resulting $E4\mathrm{}$ moments are about twice those expected from previously measured ${\beta }_4$ deformations.

A rocket-borne radiometer measurement of background radiation in the spectral range from 6 to 0.08 mm has yielded a flux which corresponds to an equivalent backbody temperature of ${3.1}_{-2.0\mathrm{}}^{+0.5\mathrm{}}$ K.

The real part of the $\rho$-nucleon forward scattering amplitude has been obtained by a measurement of the photoproduction of asymmetric electron-positron pairs from a carbon target. A determination of the relative $\rho -\omega$ production phase is also made. At a mean incident photon energy of 4 GeV the ratio of the real to the imaginary part of the $\rho$-nucleon amplitude is found to be - 0.28 ± 0.12.

A new $K^{*}$ resonance which we denote $K_N\mathrm{}\left(1760\right)\mathrm{}$ is observed. Its mass is 1759 ± 10 MeV and its width 60 ± 20 MeV. We assign it to the natural—spin-parity series on the basis of 5—standard—deviation evidence for a $K\pi$ decay mode. Decays into $K^{*}\mathrm{}\left(890\right)\mathrm{}\pi$ and $K\rho$ with the same mass and width are observed with 4—standard—deviation significance. It is suggested that this state is the analog of the $g$ meson. In addition, we see evidence for further high-mass structure.

We study the inclusive reactions ${\pi }^{-}+p\to {\pi }^{-}$ +anything at 8, 18.5, and 24.8 $\frac{\mathrm{GeV}}{c}$ and ${\pi }^{+}+p\to {\pi }^{-}$ +anything at 18.5 $\frac{\mathrm{GeV}}{c}$. Distributions of the particle density $\frac{\omega \left(\frac{d\sigma }{d{p}_{\parallel }}\right)}{{\left({\sigma }_T\right)}_{\infty }}$ at $p_{\parallel }\lesssim 1 \frac{\mathrm{GeV}}{c}$ for the "inclusive" ${\pi }^{-}$ indicate an approach to limiting behavior for ${\pi }^{+}p$ at current accelerator energies. Invariant cross sections for ${\pi }^{-}p$ decrease significantly in magnitude as the incident energy increases. The decrease is consistent with an energy dependence similar to the form $A+{Bs}^{\frac{-1}{2}}$ suggested by Regge phenomenology, approaching the ${\pi }^{+}p$ limiting distribution as a limit.

An alternative to Dirac's factorization of the Klein-Gordon equation is developed; the resulting two-component, $m\ne 0$ equations are proved Poincaré invariant. The particles interact chirally and minimally with the electromagnetic field ($g=2\mathrm{}$). Our equations yield factorizations of Kramers's equations and a conserved chiral current (without ${\gamma }_5$ projection) to implement in a natural way the Feynman—Gell—Mann approach to weak interactions. Besides possessing sharp chirality, the particles possess a new dichotomic quantum number.

The direct sum-over-states method of calculation of dynamic polarizabilities is used to compute the most accurate values for the frequency-dependent polarizability of the ground state yet obtained by ab initio calculations. Accurate values of Rayleigh depolarization ratios are also obtained. A Rayleigh depolarization ratio for Lyman-$\alpha$ radiation (unpolarized light) of 0.0562 is obtained. The feasibility of direct computation of the electric dipole components of the resolvent operator for molecules is established.

Cascade transitions have been used to produce polarized atoms in optically inaccessible excited states. The polarized fluorescent light which is emitted when the atoms decay can be used to measure hyperfine structures, lifetimes, and other atomic parameters of these inaccessible states. Preliminary measurements of the hyperfine interval for the second excited $S$ states of ${\mathrm{K}}^{39}$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Rb}}^{87}$, and ${\mathrm{Cs}}^{133}$ are (in MHz) 46(2), 271(15), 565(40), and 684(50), respectively. To our knowledge, these are the first measurements of excited $S$-state hyperfine intervals in alkali atoms.

A rigorous proof is given for the existence of a phase transition in the Widom-Rowlinson model in two dimensions.