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.

By measuring the pressure dependence of the velocity of sound, we have determined both the pressure dependence of the density and the Grüneisen constant $u$ of liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Measurements were made below 0.1 K and in the vicinity of 0.5 K. Our determinations of the pressure dependence of the density agree quite well with that determined by Boghosian and Meyer, who used a capacitance bridge. Since the latter results rely on the validity of the Clausius-Mossotti relation and a pressure-independent electric polarizability, the present work can be interpreted as supporting both of these assumptions. We found that $u\left({\rho }_0\right)\equiv \left(\frac{\rho }{c}\right)\frac{\mathrm{dc}}{d\rho }=2.84\mathrm{}$ under the vapor pressure at 0.1 K. Using this value of $u$ to calculate the attenuation of sound according to a three-phonon mechanism, we obtain an attenuation of less than half the measured value. Thus, the present theory of sound attenuation must be incomplete.

Electronic multipolar polarizabilities of closed-shell isolated atomic systems are calculated by a variational method using, as trial functions, the perturbed wave functions given by the uncoupled Hartree-Fock method. The external field is replaced by an effective parametric field taking into account the coupling between the deformations of the various orbitals. Numerical results obtained for ${\mathrm{F}}^{-}$ and ${\mathrm{Cl}}^{-}$ show a much better agreement with the experimental data.

The absolute electron-impact ionization cross section of the metastable $\mathrm{He}\left(2s{}_{}{}^3{}_{}{}^{}S\right)$ state of helium has been measured by the application of a double-electron-beam technique. The cross section at the plateau is about 6.5×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ ± 30%. The ionization of the metastable $\mathrm{He}\left(2s{}_{}{}^1{}_{}{}^{}S\right)$ state of helium has also been observed. This absolute ionization cross-section measurement is in good agreement with the experimental results for lithium and with the theories for $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ when compensation for the differing ionization potentials is made.

Measurements of the volume recombination coefficient of a highly ionized potassium plasma demonstrate the strong inverse temperature dependence predicted theoretically, and provide further confirmation of the three-body nature of the recombination process.

The beam-foil light source was used for a study of the Stark effect on He II ($\lambda 4686$Å) and ${\mathrm{H}}_{\beta }$. For the former, the $p$ and $s$ components were detected for transverse and longitudinal fields. The transverse-field data suggest that levels of high $j$ are underpopulated relative to their equilibrium numbers. The longitudinal-field data are anomalous. For $H_{\beta }$, splitting was observed, but the arrangement precluded the detection of polarization. The data also imply underpopulation of levels of high $j$.