We study numerically the dynamical behavior of a chain of classical particles with competing and anharmonic interactions. Although this model possesses a complex potential-energy landscape with exponentially many metastable configurations, we find ergodic behavior at all temperatures we investigated. Ergodicity is tested with respect to several correlation functions of pseudospins, the spins describing the configurational degrees of freedom of the chain. The time dependence of the autocorrelation function is consistent with a stretched exponential for intermediate times with exponents between 0.6 and 0.75. The corresponding relaxation times fit very well with an Arrhenius law. Within a transition-state approach, it is shown that the relaxation dynamics can be described by a kinetic Ising model. The consequence of this result on the autocorrelation function and the central peak is discussed.

An alternative method, based on dielectric measurements on thick samples, for determining the Goldstone-mode rotational viscosity and the twist elastic constant of ferroelectric, chiral smectic-${\mathit{C}}^{\mathrm{*}}$ liquid crystals is presented. By the use of this method the temperature dependence of these quantities for p-decyloxibenzilidine-p’-amino-1-methylpropyl-cinamate (DOBA-1-MPC) and p-n-decyloxybenzylidine-p-amino-2-methyl-butyl-cinamate (DOBAMBC) has been determined. An Arrhenius-type behavior of the Goldstone-mode rotational viscosity was obtained with activation energies μ=0.33 eV for DOBA-1-MPC and μ=0.51 eV for DOBAMBC.

The phase separation of a mixture of equal-size particles interacting through a Lennard-Jones potential has been investigated making use of the hypernetted-chain integral equation (HNC). Fair agreement with simulation results, where available, shows that for the system considered the HNC equation can predict with a reasonable accuracy the onset of phase separation. The dependence of the critical point on the strength of the like and unlike interactions has then been studied in order to clarify the segregation mechanism.

A Schiff’s base ferroelectric liquid-crystal P-hexyloxybenzylidene-p’-amino-2-chloro-α-propyle-cinnamate was spread on water to create a monomolecular film. Lateral compression led to several collapses at areas in the relation 1:1/2: 1) / 3 , etc. suggesting that smectic layers had been formed. The transitions from layers n to n+1 are considered phase transitions, the entropies of which were determined on the basis of experimentally observed lateral pressures as a function of the number of layers and temperature. The effect of the aqueous subphase and its contribution to the entropy is evaluated. The initial layer is found to be of lower symmetry than the upper layers. The lateral film pressure contains a strong dynamic component that is used to determine the viscosity for the transition as a function of temperature and the speed at which the film is compressed.

A simple electro-optical technique was developed for measuring the rotational viscosity of nematic liquid crystals. These results were then used to confirm the newly developed Osipov-Terentjev (OT) theory [M. A. Osipov and E. M. Terentjev, Z. Naturforsch. A 44, 785 (1989)]. Another theory frequently used, the Diogo-Martins theory [A. C. Diogo and A. F. Martins, Mol. Cryst. Liq. Cryst. 66, 133 (1981)], is also used for comparison. By comparing the OT theory with experimental results of some Leslie coefficients, the activation energy, mean-field potential, fourth order of the nematic order parameter, and molecular length-to-width ratio of a liquid-crystal compound are obtained. The mean-field coupling constant is found to be about 1.8 ${\mathit{kT}}_{\mathit{c}}$, which is somewhat lower than the Maier-Saupe theoretical value of 4.5 ${\mathit{kT}}_{\mathit{c}}$.

This paper addresses two central issues in our understanding of self-trapping, or localization, of a light particle (e.g., electron, positron, or positronium atom) in a fluid. The first concerns the relationship between the mean-field calculations of localized states carried out from the mid-1950s until the present, and the direct calculation of light-particle (LP) properties from the adiabatic model. The second concerns the universality of the ring polymer representation of the LP, which has proved to be extremely useful for computing the properties of localized electrons. We first demonstrate that the mean-field theories of localization, which are derived from density functionals, can be obtained from the adiabatic model by employing a Gutzwiller type of approximation. We then study the application of the adiabatic model, via the polymer representation, to the annihilation of positrons and positronium atoms in a fluid. Expressions for the decay rate and its variance are derived in terms of the two- and three-point distribution functions for the polymer sites and positions of the atomic nuclei. The momentum distribution of the localized LP is shown to be proportional to the Fourier transform of the distribution function for the displacement between the ends of a broken polymer.

The connection between light scattering and nonlinear optical response is examined quantitatively for Kerr media and is generalized to include isotropic materials with nonlocal interactions near a critical point. A number of important quantities, including the nonlinear refractive index (${\mathit{n}}_2$) and the four-wave-mixing coefficient, are shown to acquire increasing grating wave-vector (q) dependence as the critical point is approached. This dependence is identical to the q dependence obtained for light scattering in the Ornstein-Zernike theory. Illustrative calculations are presented for near-critical xenon using the Redlich-Kwong equation of state and parameters obtained from independent measurements. These results indicate ${\mathit{n}}_2$ values of ${10}^{\mathrm{-}15}$–${10}^{\mathrm{-}14}$ ${\mathrm{m}}^2$/W and response times in the 1–10-μsec range. Calculations are presented for the phase-conjugate reflectivity in degenerate four-wave mixing that include both nonsaturable background loss and light-scattering noise. Reflectivities in excess of unity are predicted for 1.06-μm wavelength and cw pump intensities in the 10-kW/${\mathrm{cm}}^2$ range.

The development of a Lennard-Jones-type polymer-polymer interaction model is described, appropriate for monolayer polymer coatings on spherical solid particles suspended in a polymer fluid. This model is appropriate for polymer-coated particles interacting at very close proximity (angstroms). These molecular forces, derived from ab initio calculations for polyoxymethylene-polyoxymethylene interactions are added to the hydrodynamic forces of a Stokesian dynamics simulation in order to estimate the effect of a polymer coating on the behavior of suspended particles under sheared conditions. The relation between suspension microstructure, shear velocity, and their time evolution are studied in molecular-dynamics-like simulations.

The resonance method was applied to measure shear mechanical properties of liquids, and it has thus been established that such low-viscosity liquids as water, cyclohexane, hexadecane, and dibutyl phthalate possess a shear elasticity of about ${10}^6$ dyn/${\mathrm{cm}}^2$ at a frequency of shear oscillations of about 73.5 kHz. Now, according to the existing concepts on the nature of liquids, their shear elasticity should be observed only at a frequency of about ${10}^{11}$–${10}^{12}$ Hz. The present work maintains a comparison with the known works on measurements of the shear mechanical impedance of liquids. It has been shown that the results obtained here agree satisfactorily with measurements of the shear mechanical impedance of low-viscosity liquids at a frequency of 40 MHz.

We simulate the behavior of two weakly attractive atoms, either spin-polarized hydrogen atoms or ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms, on a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ cluster by examining the angular correlation of the atoms as they move on the surface of a sphere. We have calculated converged energy levels, wave functions, and reduced densities ${\mathrm{\rho }}^{\left(\mathit{J}\right)}$(${\mathrm{\theta }}_{12}$) for the low-lying states of these two systems. We also give the oscillator strengths for several transitions between low-lying states for the case of spin-polarized hydrogen. The energy levels can be arranged in patterns reminiscent of the rovibrational levels of a linear triatomic molecule, but the reduced densities differ significantly from those predicted by the molecular model. The lowest state for two ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms on a sphere of radius 13.0 bohr is bound, with E=-0.031 90 ${\mathrm{cm}}^{\mathrm{-}1}$.

The process of symmetric fragmentation of multiply charged alkali-metal clusters consisting of several tens of atoms is studied using the theory of shell corrections, which has been originally applied to the nuclear fission problem. The potential-energy surface of the fragmentation process is calculated for various sizes of clusters. The variation of the shell correction term due to shape deformation is found to be much larger than that of the energy of the liquid droplet if the mother cluster is metastable. Symmetric fragmentation is most likely to occur when the size of products is nearly equal to the magic number. The reaction paths of fragmentation into both magic and nonmagic clusters are also discussed.

A simple scaling argument for diffusion-limited reactions for both A+A→0 and A+B→0 is developed. Temporally asymptotic behavior of species densities in the absence of sources and reaction orders in the steady state in the presence of sources are discussed. We specifically address the effect of the spatial distributions of reactants on the reaction rates. For the A+A→0 reaction we recover existing results found by other methods. For A+B→0 in the absence of sources we find that the asymptotic behavior depends not only on the spectral dimension (as commonly believed) but also on the fractal dimension. In the presence of sources we agree with previous results when the steady-state density is not too low, but argue that a modification of these results may occur at very low densities.

We examine the validity of the empirical ‘‘linear mixing rule’’ for the determination of the thermodynamic properties of screened binary ionic mixtures interacting with effective density- and temperature-dependent screened Coulomb potentials. Comparison with hypernetted-chain calculations reveals generally good agreement and further verifies the reliability of the linear law, even when the response of the finite-temperature electron gas is taken into account.

A nonlinear Schrödinger equation that governs the nonlinear interaction of a quasistatic plasma slow response with ion acoustic waves for a two-ion collisional plasma is derived. The effects of collisions and concentration and mass of impurity ions on the instability of obliquely modulated ion acoustic waves are discussed. The following results are found: (i) In the case of a single-ion collisional plasma, for small values of k, the value of θ above which the wave is unstable increases in comparison to the collisionless case; (ii) the presence of heavy ion impurities has no effect on the stable region in k-θ plane either for collisionless or collisional plasmas; (iii) the presence of light ion impurities changes the modulationally unstable domain drastically. The variation of this effect with the concentration and mass of impurities and collisions for the collisional plasma is discussed in detail.

A self-consistent dynamic model for rf sheaths in the frequency range between the ion and electron plasma frequencies is developed and solved for arbitrary collision parameters and arbitrary rf sheath voltages. For floating dc sheaths with no rf voltage, and for collisionless and highly collisional rf sheaths at high rf voltages, the obtained solutions for the dc sheath voltage and the capacitive sheath width converge to the known limits. However, for rf voltages of tens and hundreds of volts, usually encountered in rf discharge applications, our results differ from those obtained using high voltage approximations and are in good agreement with the experiment. The found relations between the sheath characteristics show that the rf sheath capacitance and the equivalent sheath resistance, corresponding to ion acceleration losses, are practically independent of the rf voltage and the discharge current.

The energy loss in a plasma target was measured for different heavy-ion species, ranging from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ to ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ at an energy of 1.4 MeV/u. A discharge tube was used to generate a hydrogen plasma with a high degree of ionization and temperatures between 1 and 2 eV. An on-line diagnostic of the plasma was performed to measure the free-electron density and the electron temperature. Compared to neutral hydrogen of the same particle density, the plasma target shows an enhanced stopping power due to the increased energy transfer to the free plasma electrons, and a higher effective charge of the projectiles inside the plasma target. Theoretical predictions based on the Bethe-Bohr-Bloch stopping theory are in good agreement with the experimental results.

Stark profiles of B v and N v n=6 to 7 and 5 to 6 lines at 4945 and 2981 Å were measured in a laser-produced plasma. These lines are subject to the linear Stark effect and can be used as diagnostics for plasma density. For comparison with theoretical calculations of the Stark widths, we also measured the electron density independently using interferometry. The theoretical Stark widths of these lines were obtained scaling a previous calculation of the C vi n=6 to 7 transition with respect to the principal quantum number and core charge. The theoretical calculations compare well with our measurement where electron density can be determined accurately. Larger deviations were obtained for regions of high electron density, partly because of large uncertainty in the measurement of the electron density and partly because of large gradients in the electron density spatial distribution.

Amplification of soft x rays in Sr (Z=38) plasmas produced by second-harmonic laser irradiation of exploding foil targets is studied. Gains of 4.4 and 4.0 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, have been measured for the 164.1- and 166.5-Å J=2 to 1 transitions in Ne-like Sr at laser intensities of 1.3×${10}^{14}$ W/${\mathrm{cm}}^2$. Numerous x-ray and soft-x-ray transitions were identified in order to demonstrate typical spectra from x-ray laser plasmas and infer information regarding the plasma ionization balance. The J=0 to 1 transition at 159.8 Å, analogous to the well-known Se J=0 to 1 line at 182.4 Å, was not definitively observed due to a wavelength overlap with a Na-like Sr line. Two other J=0 to 1 transitions of interest at 84.9 and 133.0 Å were not observed. Reduced gain on the J=2 to 1 lasing lines was seen for several shots taken at a lower pump laser intensity of 7.0×${10}^{13}$ W/${\mathrm{cm}}^2$. Possible reasons for this are considered. The results of this work imply the feasibility of a high-output-power cavity geometry Ne-like Sr x-ray laser operating at 164.1 and 166.5 Å.

The energy and wave functions of two-particle states in a dense plasma are essentially influenced by the surrounding medium. However, the two-particle bound-state energies may remain unshifted over a large density interval. Two simple examples are given to demonstrate the compensation of many-particle effects such as self-energy and effective two-particle potentials in hydrogen plasmas. Neglecting self-energy and Pauli blocking, we obtain energy values that follow from numerical solutions of the Schrödinger equation with a Debye potential.

The semiclassical Bohr-Sommerfeld quantization condition is used to derive an approximate analytical expression for the state density of the hydrogen atom in a dense plasma. An ion-sphere model with an infinitely high potential wall is assumed. The expression leads to a universal curve that spans all values of the electron density. The curve is continuous and smooth over the entire energy range, starting from the hydrogenic state density for low-lying bound states and approaching the plane-wave state density in the high-energy limit of the continuum. The number of bound states is approximately proportional to the inverse of the square root of the electron density. Integration of the state density over the Boltzmann distribution of the electronic energy results in an ionization equilibrium relation, leading to modified Saha’s equation. The correction factor for this modified equation is a function of both the electron temperature and the electron density, and is expressed as a universal function of the ion coupling parameter.