Terms up to order ${\left(\frac{\Gamma }{k_{B}T}\right)}^5$ in the high-temperature expansion of the orientational specific heat of ortho-para alloys of solid ${\mathrm{H}}_2$ or ${\mathrm{D}}_2$ are evaluated. Good agreement is obtained between theory and experiment using a Padé approximant and effective values of the quadrupolar coupling constant, $\frac{{\Gamma }_{\mathrm{eff}}}{{\Gamma }_0}=0.83\mathrm{}$ for ${\mathrm{D}}_2$ and $\frac{{\Gamma }_{\mathrm{eff}}}{{\Gamma }_0}=0.80\mathrm{}$ for ${\mathrm{H}}_2$, where ${\Gamma }_0$ is the value for a rigid lattice. These values agree with other determinations of ${\Gamma }_{\mathrm{eff}}$, whereas the $T^{-2\mathrm{}}$ approximation for the specific heat yields anomalously small values of ${\Gamma }_{\mathrm{eff}}$.

The dependence of the recombination coefficient $\alpha$ of ${\mathrm{He}}_2^{+}$ ions and electrons on electron density and gas pressure is measured in helium afterglow plasmas where electron and gas temperatures are equal to 300° K, gas pressure ranges from 10 to 100 Torr, and electron density from ${10}^9$ to 5×${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Electron density decay measured by microwave interferometry is compared with computer solutions of a continuity equation for electrons which takes into account ambipolar diffusion and recombination effects. One finds $\alpha ={\alpha }_2+k_e{n}_e+k_{\mathrm{He}}{n}_{\mathrm{He}}$, where $n_e$ and $n_{\mathrm{He}}$ are the electron and neutral densities, respectively, and ${\alpha }_2\sim 5\times {10}^{-10\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$, $k_e=(2\mathrm{}\pm 0.7)\times {10}^{-20\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$, $k_{\mathrm{He}}=(2\mathrm{}\pm 0.5)\times {10}^{-27\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$. These values compare satisfactorily with results of theoretical computations for a collisional-radiative recombination mechanism including both collisions with electrons and neutrals. The effect of neutrals is particularly noteworthy as far as future experimental work on weakly ionized gases is concerned.

London's stress tensor of the phenomenological theory of superconductivity is identified with the pressure tensor of the gas of superelectrons.

The dispersion of the normal mode at the upper hybrid frequency (Propagating parallel to a uniform magnetic field) in a spatially inhomogeneous plasma has been calculated. The perturbed potential has been solved in $k$ space for the case of a Maxwellian plasma; in the limit $k_{\parallel }\ll k_{\perp }$, the potential function has been shown to satisfy the Whittaker equation. The results obtained are in good agreement with those obtained from a recent experiment.

In an earlier paper, the generalized Langevin approach of Mori and Zwanzig has been applied to generate an exact kinetic equation describing the time correlation of fluctuations in the microscopic phase density for a classical many-body system. In the present work, this kinetic equation is modeled using a single-relaxation-time form for the damping kernel, and the solution of this equation is then studied for large and small values of $k$ and $\omega$. The modeled kinetic equation is used to calculate transverse and lingitudinal current-current correlations in simple classical liquids, and the results are compared with the molecular-dynamics calculations of Rahman for argonlike systems.

The osmotic pressure of dilute solutions of ${\mathrm{He}}^3$ in liquid ${\mathrm{He}}^4$ has been measured at 0.32, 0.65, and 1.16°K for concentrations up to 2%. The technique does not require values of the ${\mathrm{He}}^4$ fountain pressure and is substantially more precise than previous methods. The results are analyzed in terms of the effective-interaction model of Bardeen, Baym, and Pines as extended to nonzero temperatures by Ebner. The results do not agree well with the theory.

The spectral width of "elastic" second-harmonic light scattering has been measured for a number of liquids. The broadening arises principally from rotational molecular motions and, ignoring possible nonadditive molecule-molecule interactions, the spectrum is shown to be related to the Fourier transform of the orientation-dependent pair distribution function $G(\overrightarrow{\mathrm{r}}, \overrightarrow{\Omega }, t)$. An irreducible spherical-tensor representation is employed. It is shown that a totally symmetric nonlinear-susceptibility tensor produces elastic harmonic light scattering characterized by just the first- and third-degree spherical elements of $G(\overrightarrow{\mathrm{r}}, \overrightarrow{\Omega }, t)$ and that the corresponding spectra may be independently determined. For Brownian orientational motion, these comprise Lorentzians whose widths are related to the tensorial orientational-diffusion coefficient. For isotropic orientational diffusion, the widths are in the ratio 1: 6. Dielectric-relaxation and depolarized Rayleigh scattering also relate to $G(\overrightarrow{\mathrm{r}}, \overrightarrow{\Omega }, t)$, and the results of all three experiments are compared. In no instance was isotropic orientational diffusion found to be a satisfactory model. A more extended study was made of carbon tetrachloride. Evidence was found both for nonadditive response and for coherent effects resulting from short-range orientational ordering. The extremely weak signals - often less than one photoevent per laser firing - necessitated development of a reliable high-repetition-rate $Q$-switched ruby laser and use of electronic data-accumulation techniques.

The binary-kernel technique, as used by Lee and Yang in quantum-statistical mechanics, involved computation of $l$-body contributions to the many-body equation of state retaining only the $\mathrm{}(l-1\mathrm{})\mathrm{}\mathrm{st}$ and $l\mathrm{th}$ powers of the binary kernel. In applying the method to the computation of classical hard-sphere fugacity coefficients, the known results can only be obtained if one sums over the action of an infinite number of kernels.

A generalized Saha equation for a partially ionized hydrogen plasma is derived and used for calculating the degree of ionization and the effective ionization potential for a wide range of electron densities and temperatures. The energy eigenvalues of the hydrogen atom in a plasma are obtained as solutions of the Schrödinger equation from a shielded potential $w\left(\mathcal{r}\right)$. $w\left(\mathcal{r}\right)$, determined by a self-consistent-field method, is found to be completely different from the Debye potential. A strong effect of the excited states at high temperatures is also established.

We describe a method for treating phase separation and interphase surfaces using an external force field. We believe the method to have some advantages over another recently proposed method.

Two contradictory conclusions on the question whether the local exchange potentials yield energy values consistent with the virial theorem are resolved here.

Classical rotational correlation functions for symmetric molecules are derived from quantum-mechanical results in neutron scattering. The analysis differs from an earlier calculation in that all values of the projection quantum number are considered. The results agree with those obtained by St. Pierre and Steele using classical statistical mechanics. Numerical results for ammonia and water are presented.

Profiles of nine lines of singly ionized nitrogen have been measured in a dense ($N_e=1.9\mathrm{}\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$) high-temperature (22 800 °K) plasma produced behind the reflected shock wave in a $T$ tube. Measurements were carried out that verified the fact that the plasma was homogeneous in both the radial and the axial directions and that the plasma was optically thin at the various wavelengths where the profiles were measured. The half-widths of the measured profiles show large discrepancies with earlier reported measurements of Day and Griem and, in most cases, show significant disagreement with the theoretical predictions.

Coherent interactions between short C${\mathrm{O}}_2$ laser pulses and a resonant absorbing S${\mathrm{F}}_6$ gas medium have been examined carefully, using a laser cavity-dumped rectangular pulse with a width of 20 nsec and input intensities from 10 to ${10}^4$ W/${\mathrm{cm}}^2$. Parameters studied included the width, shape, and delay time of the transmitted pulse after propagation through several absorption lengths of S${\mathrm{F}}_6$ cell. In addition to these measurements, we have observed two new phenomena, namely, optical free induction decay and edge echo, which are induced by this short rectangular C${\mathrm{O}}_2$ laser input pulse. Results indicate that although level degeneracy in S${\mathrm{F}}_6$ may be involved, the simple two-level model can account qualitatively for most observed phenomena, with the exception that the observed shape of a $2\pi$ pulse deviates significantly from the symmetric hyperbolic-secant function predicted by the simple two-level theory. In the $\pi$ pulse region, we have demonstrated that the free-induction-decay tail can be valuable for studies of atomic and molecular collision processes.

The shape and position of the Ni $L\alpha$ x-ray emission line ($\lambda =14.53\mathrm{}$ Å) has been studied as a function of bombarding electron energy ranging from 2 to 30 keV. The bombarding electrons struck the anode at normal incidence, and x rays were viewed by the spectrometer at a 90° takeoff angle. The Ni $L\alpha$ lines recorded under these conditions exhibit an increasing attenuation of intensity on the high-energy side and a shifting of the peak position towards lower energy with increasing bombarding electron energy. These changes are interpreted as due to differential self-absorption of the emission line in the anode. The emission measurements have been used to obtain the "$L$ self-absorption spectrum" of Ni, which agrees favorably with the absorption spectrum reported in the literature.

The cross section for ejecting electrons by 300-keV protons is calculated using the first term in the Neuman expansion of Faddeev's equation for the final state of the electron-proton-residual ion system. This approximation predicts a peak at 0° in the angular distribution of electrons ejected with a velocity approximately equal to the velocity of outgoing protons. Numerical results for He and ${\mathrm{H}}_2$ target gases are given and compared with the experiments of Rudd and co-workers. The qualitative behavior of the forward peak in the experimental angular distributions is well accounted for.

A cylindrical, retarding potential difference type of electron spectrometer with high-energy resoltuion has been used to study the structures in the $e^{-}$-He scattering cross section in a transmission experiment. Twenty-four structures have been observed between the transmission maximum due to ${\mathrm{He}}^{-}$ $1s2s^2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ at 19.30±0.01 eV and ${\mathrm{He}}^{+}$ $1s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ at 24.60±0.02 eV, 11 of which have been observed previously. The agreement with previous measurements for the positions of the structures is good where comparison is possible. Four of the new structures have been observed below the $n=2\mathrm{}$ states of He. The excitation onsets for $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$, $2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$, $2{}_{}{}^3{}_{}{}^{}P_{}^0{}_{}{}^{}$, and $2{}_{}{}^1{}_{}{}^{}P_{}^0{}_{}{}^{}$ have been observed, as well as some excitation onsets at higher energies. No structures have been observed below ${\mathrm{He}}^{-}$ ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$. Two structures have been observed above ${\mathrm{He}}^{+}$ ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ which were observed previously.