Electron angular distributions from ionization in collisions of $\mathrm{He}\left(2\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}S\right)$ and $\mathrm{Ar}\left({}_{}{}^1{}_{}{}^{}S\right)$ have been studied in terms of molecular-orbital correlation diagrams and a semiclassical and probabilistic model for the collision process. Consideration of the united-atom limit for this system indicates that the electronic angular momenta $\lambda =0, 1, \mathrm{and} 2$ play an important role. The interaction potential responsible for electron emission was assumed to decrease exponentially with increasing distance, and was parametrized by fitting experimental results at a fixed collision energy. The values of the parameters show that $\lambda =0\mathrm{}$ is the dominant partial wave and lead to backward-peaked distributions. Calculations performed at other energies show an increasing anisotropy with increasing collision energies.

Adiabatic potential-energy curves have been calculated for the interaction of several $n=3\mathrm{}$ triplet states of helium with ground-state helium. Mechanisms for excitation-transfer in collisions of $3{}_{}{}^3{}_{}{}^{}S$ and $3{}_{}{}^3{}_{}{}^{}P$ with ground-state atoms are suggested by the results of these calculations. A deexcitation cross section corresponding to $3{}_{}{}^3{}_{}{}^{}S\to 2{}_{}{}^3{}_{}{}^{}P$ excitation-transfer has been calculated using the Stueckelberg-Landau-Zener approximation and is consistent with existing measurements. The existence of a small hump in the potential curve causes the cross section to decrease rapidly at low energies. Thus, a strong temperature dependence of effective cross sections is expected. A possible mechanism for associative ionization is also discussed.

The cross sections for $L$-shell and subshell ionization by direct Coulomb excitation of argon by incident heavy charged particles are evaluated. Incident particles are described in the plane-wave Born approximation, and nonrelativistic Hartree-Slater (HS) wave functions are used for the atomic electrons. Form factors, energy distributions, and ionization cross sections are compared with those obtained from screened hydrogenic wave functions. At most incident energies the HS results for the total ionization cross section are only slightly smaller than those obtained with screened hydrogenic wave functions, but considerable discrepancies are found for form factors and energy distributions near the ionization threshold.

An approximate integral form of the close-coupling formalism has been applied to the charge-transfer process in the proton-helium collision problem. Results for the ground-state capture cross section have been obtained with the inclusion of the effect of proton-nucleus interactions and compared with the available experimental and theoretical results. The present values of the capture cross section are in very good agreement with the experimental observations at high as well as intermediate incident energies. At low energies the nature of the experimental curve has been correctly reproduced. Results for the differential capture cross sections in the forward direction and the total elastic cross sections have also been reported.

Improved calculations of the first several autoionization states of the lower symmetries (${}_{}{}^{1,3}{}_{}{}^{}S, {}_{}{}^{1,3}{}_{}{}^{}P_{}^o{}_{}{}^{}, {}_{}{}^{1,3}{}_{}{}^{}D_{}^e{}_{}{}^{}$) are reported. Unshifted energies are obtained by diagonalizing $\mathrm{QHQ}$ using a Hylleraas basis with more terms than used previously. Shifts, widths, and photoabsorption shape parameters $q$ (where appropriate) are obtained with the additional use of exchange approximate nonresonant continuum functions. Our previous calculations of ${\mathrm{H}}^{-}$ resonances are recapitulated and slightly augmented by using various improved nonresonant continua and correcting small previous errors. Comparison with other calculations and experiments is made. The latter is very satisfactory except for the lowest ${}_{}{}^1{}_{}{}^{}P$ autoionization state of He for which a more accurate experimental result is required.

Intensity fluctuations for a single-mode laser near threshold are calculated from the Scully-Lamb equation of motion for the reduced density matrix of the radiation field. The magnitude and correlation time of the intensity fluctuations are contrasted with the results obtained from the usual Fokker-Planck analysis of the rotating-wave, Van-der-Pol (RWVP) model. When the photon number at threshold is not large, these calculations predict measurable differences from the RWVP Fokker-Planck results. Two methods are used to calculate the dynamics. One is a direct eigenfunction expansion of the Green's function of the equation of motion. The other is based on the calculation of the initial derivatives of the correlation function.

The evolution of the radiation field in a model for interacting radiation and matter in cooperative-atom states is studied via the density matrix. The exact treatment possible for small numbers of cooperating atoms reveals effects not apparent from approximation treatments for long interaction times and/or strong interactions. The superradiance concept needs modification under these conditions, and the photon statistics can be greatly modified by the interaction. The representation for the density matrix uses eigensolutions of the model obtained earlier.

For certain types of classical processes a fluctuation-dissipation theorem (FDT) holds. The validity of such a theorem is preserved in each order of mass renormalization in the perturbation scheme recently developed by Martin, Siggia, and Rose (MSR). As a consequence, whenever the response function can be expressed in terms of the two-point correlation function via a FDT, the perturbation scheme of MSR simplifies considerably. Especially, it reduces to the scheme constructed by Kawasaki for random processes obeying detailed balance which are (i) linearly damped and (ii) linearly driven by Gaussian white noise.

The propagation of slow electrostatic waves across a magnetic field ${\overrightarrow{\mathrm{B}}}_0$ ($\frac{k_{\parallel }^2}{k_{\perp }^2}\lesssim \frac{m_e}{m_i}$) near the lower hybrid frequency ($\omega \gtrsim {\omega }_{\mathrm{LH}}$) has been investigated experimentally in a large uniform magnetoplasma. The dispersion is in good agreement with the fluid description; warm-plasma effects appear near $k_{\perp }{r}_{\mathrm{ci}}\simeq 2\pi$. Propagation of wave packets reveals the dispersive and backward wave nature of the modes ($v_g>0\mathrm{}$, $v_{\mathrm{ph}}<0\mathrm{}$, $v_g\ll \left|v_{\mathrm{ph}}\right|$), the different directions of phase and group velocity ($\frac{\omega }{\overrightarrow{\mathrm{k}}}\sim \perp \frac{\partial \omega }{\partial \overrightarrow{\mathrm{k}}}\sim \parallel {\overrightarrow{\mathrm{B}}}_0$) and the cone structure of the antenna radiation pattern.

The breakdown of the validity of the Ornstein-Zernike form for the behavior of the order-parameter fluctuations is investigated for sound absorption and dispersion near the critical point. Since sound propagation involves the temperature derivative of the correlation function, it is a very sensitive way to test departures from the Ornstein-Zernike behavior. Several functional forms have been used and the results compared with recent experimental data for xenon. A good qualitative agreement for high-frequency data close to the critical point, not possible when using the Ornstein-Zernike form, is obtained with the expression proposed by Fisher and Langer for large values of $q\xi$. The latter is also satisfactory from a physical point of view since it reproduces the specific-heat anomaly.

Unstable, time-independent transitions that cause changes in the spatial symmetry of a nonequilibrium system are analyzed. The physiochemical processes include the nucleation, growth, and dissociation of particle aggregates and molecular diffusion. A bifurcation problem is treated in which there occurs a time-independent transition to a spatially inhomogeneous stationary solution of the nonlinear diffusion equation. Repeated instability and branching can occur through the excitation and interaction of the harmonics that the perturbation comprises. New stationary regimes, each with a different spatial symmetry, can be attained by the superposition of a stable stationary value of the perturbation upon the unstable stationary solution. Harmonic interactions cause stationary-state branching and shifting. Asymptotically, the nonequilibrium system approaches a stable, time-independent spatial configuration that is an inherent property of the system itself. Instabilities occur in the direction of stationary regimes with decreasing characteristic dimensions and are shown to increase the number of degrees of freedom of the system. The transitions are (i) considered to be the zero-frequency analog of the transition between laminar and turbulent flow proposed by Landau and (ii) resemble thermodynamically a second-order phase transition. The second variation of the entropy contained in the first harmonic of the perturbation vanishes at the critical point. Energy-transfer mechanisms of unstable transitions are discussed.

A spin-1 lattice-gas model, similar to the Blume-Emery-Griffiths model for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures, is shown to describe condensation and solidification of a simple fluid. The Ising-like Hamiltonian of the system involves quadrupolar and dipolar interactions, which are responsible for condensation and solidification, respectively. The molecular-field approximation is used, and the ordinary phase diagram of a simple fluid is reproduced. However, for some range of the parameters, the liquid-gas equilibrium curve disappears. Also, the melting curve may exhibit a tricritical point: For pressures larger than the tricritical pressure, critical melting is found. Other physical applications of the model are briefly discussed.

A spin-1 lattice-gas model is shown to describe condensation and phase separation in a binary fluid. The Ising-like Hamiltonian involves quadrupolar, dipolar, and crossed quadrupolar-dipolar interactions and is treated in the molecular-field approximation. Various experimental phase diagrams are reproduced qualitatively, exhibiting liquid-gas and liquid-liquid equilibrium lines, critical, azeotropic, and triple lines.

The spin-1 lattice-gas model used in paper II to represent the behavior of a binary fluid is shown to represent as well the behavior of a ternary fluid kept at a fixed pressure. Tricritical points and quadruple points in binary and ternary fluids are described within the same model in the case of a zero crossed dipolar-quadrupolar interaction $L$. They are compared to those predicted or observed in simple fluids and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures.

The results of a molecular-dynamics study of a simple model of a molten salt are reported. The interionic pair potential which is used consists of the Coulombic term an an inverse-power repulsion which is assumed to be the same for all ions. The structure of the liquid is found to be dominated by charge-ordering effects and the calculated equilibrium properties are in good agreement with the predictions of the hypernetted-chain approximation. The relation between the self-diffusion coefficient and the electrical conductivity is discussed, and the observed deviations from the Nernst-Einstein relation in real molten salts are shown to have a natural explanation in terms of short-lived cross correlations. Data on the spectra of charge and particle density fluctuations are presented. At small wave numbers there is a propagating optic-type mode which shows a strong negative dispersion, but no Brillouin peak is seen even at the lowest wave number which is accessible. The data are analyzed in terms of a single-relaxation-time model incorporating the low-order spectral moments, for which we give explicit formulas. The fit achieved is fair, but the low-frequency behavior of the charge fluctuations at small wave numbers is incorrectly reproduced, and there is evidence for the necessity of introducing a second relaxation time. Comparison is made with results previously obtained for the classical one-component plasma.

The density fluctuations in molten salts are being investigated with a combination of neutron-scattering and computer-simulation techniques. In this paper results are given for measurements of the coherent inelastic neutron scattering from molten RbBr at 986 K, in the wave-vector range $1\lesssim Q\lesssim 7$ ${\mathrm{\AA }}^{-1\mathrm{}}$. In view of the similarity between the ${\mathrm{Rb}}^{+}$ and ${\mathrm{Br}}^{-}$ ions with regard to size, mass, and neutron scattering length, the scattering function measured in these experiments closely reflects the behavior of the number density fluctuations in the melt, regardless of ion type.

Electron temperatures different from neutral and ion temperatures can persist for appreciable periods in a low-density plasma decaying by dissociative recombination. Such temperature differences can be initially due to the process generating the plasma. The energy dependence of the recombination cross section can also result in an elevation of the electron temperature during the recombination process. The time dependence of the electron temperature and recombination rate is calculated for a variety of experimental conditions. Corrections to the observed recombination rate for these effects are important only if the dimensionless parameter $C=\left(\frac{16}{3}\right){\left(\frac{2m_{e}k{T}_A}{\pi }\right)}^{\frac{1}{2}}\left(n_A\frac{{\sigma }_{\mathrm{MT}}}{m_A}{n}_e^0{\alpha }_R^A\right)$ is less than unity. ($T_A$, $n_A$, and $m_A$ are the neutral particle temperature, number density, and mass, respectively; $n_e^0$ and $m_e$ the electron initial number density and mass; ${\sigma }_{\mathrm{MT}}$ the electron-neutral momentum-transfer cross section; and ${\alpha }_R^A$ the electron-ion dissociative recombination rate constant at $T_e=T_A$.)

A spectrum broadening due to the self-phase modulation of laser light was observed in a laserproduced deuterium and hydrogen plasma. Qualitative treatments of the density modulation due to the self-focusing process and the modulational instability are discussed. The theoretical estimation of spectrum broadening agrees fairly well with the experimental results.

The magnetic field associated with the dense pinch in a plasma focus device has been measured from the Zeeman splitting of the Cv, $2s-2p$ emission. Values of the order of 1 MG are derived, in agreement with pressure balance between the self-field due to currents in the pinch and the kinetic pressure of the plasma.

The asymptotic high-frequency ($\omega \to \infty$) and static ($\omega =0\mathrm{}$) behavior of the quadratic longitudinal polarizability, combined with the Kramers-Kronig relations, leads to sum rules analogous to the linear "conductivity" and "compressibility" rules. Further, Kubo-type sum rules are derived by taking frequency moments of the nonlinear fluctuation-dissipation theorem; these lead to the external quadratic polarizability summing to static linear response functions.