We present results of nonequilibrium molecular-dynamics calculations for the triple-point Lennard-Jones fluid undergoing shear flow. The calculations show that this simple fluid exhibits a wide variety of non-Newtonian behavior ranging from viscoelasticity, to shear dilatancy and flow birefringence. It is shown that the constitutive relations describing these phenomena are consistent with nonanalytic functional forms. For every property so far studied these functional forms agree with those predicted by long-time tail theories. However, the size of each effect is found to be several orders of magnitude greater than theoretical predictions.

Molecular-dynamics calculations for a three-dimensional model liquid of 108 particles were performed to probe pair and triplet interactions. The triplet potential was assumed to have the Axilrod-Teller form, while the pair interaction was represented by the Lennard-Jones (12-6) potential. The computations were done to simulate liquid Ar at densities between 1.3 and 1.5 g/${\mathrm{cm}}^3$ and at a temperature of 100 K. The findings of Schommers for a two-dimensional model liquid are partly supported. The details of our calculations indicate, however, that the three-dimensional system exhibits a different behavior compared to that of the two-dimensional model. In the three-dimensional system both the static and dynamic correlation functions are affected by the triplet interactions. The influence of the triplet interaction on the time-dependent correlations is not as pronounced as in the two-dimensional system. For higher densities the effect of the Axilrod-Teller potential on the correlation functions diminishes, whereas the effect on the energy does not.

Secondary Langmuir-wave decay into another Langmuir wave and an ion-acoustic wave is shown to provide a very efficient saturation mechanism for two-plasmon decay and stimulated Raman scattering. Near the quarter critical density, where two-plasmon decay dominates Raman scattering, large ion fluctuations are expected at the saturated stage of the two-plasmon instability.

Experimental studies of the wavelength dependence of the following laser-plasma coupling processes were performed: (1) laser-light absorption, (2) energy transport, and (3) ablation and target-acceleration behavior. At a fixed intensity around ${10}^{14}$ W/${\mathrm{cm}}^2$, the ablation process was dominated by heat transport by cold electrons for 0.53-and 1.06-μ m lasers and by hot electrons for a 10.6-μ m laser.

This article presents a general analysis of some aspects of the interaction of classical particles with the classical electromagnetic zero-point field (cemzpf). The analysis provides a possible observational test for stochastic electrodynamics (SED). A convergence form factor derived semiclassically supports the narrow linewidth and related approximations of SED by introducing a typically sharp frequency cutoff. An extended classical charge monopole can then be shown to perform a simple jiggling motion under the influence of the cemzpf. Besides this motion (same as polarizable particles), monopolar particles also display a random walk in velocity space which leads them to ever-increasing translational kinetic energies. Hence, classical particles under the influence of the cemzpf display a conspicuous behavior because of the following well-known interrelated results: First, no velocity-dependent forces exist for classical particles moving exclusively through the cemzpf. Second, both monopolar and polarizable particles in SED are predicted to perform a random walk in velocity space due to the action of this field. Only collisions may provide a stopping mechanism. An analysis of the work of Boyer and others concerning particle collisions with walls, suggests the idea that collisions transfer energy from an unconfined gas of mutually colliding particles to the random field. Using this, a Fokker-Planck model for an unconfined gas of mutually colliding classical particles is constructed. It displays a universal equilibrium energy spectrum $E^{-\mathrm{const}}$ for the gas particles under the cemzpf as seen from any point fixed to co-moving coordinates. Primary cosmic rays have such an energy distribution. This motivated the proposal of a zero-point field (zpf) cosmic-ray acceleration mechanism in a previous work. Such a proposal requires a careful examination. However, methodologically speaking, one should first examine the alternative possibility that the behavior predicted in SED for classical particles does not occur in nature. If that would happen to be the case, then SED and the cemzpf concept should be critically revised. That the cemzpf concept may apparently lead to difficulties, is seen by presenting a paradoxical example where a monopolar particle moving through the cemzpf is predicted to suffer an enormous frictional force due to the surrounding zpf. The prediction obviously violates the Lorentz invariance of the cemzpf energy density spectrum. But the paradox is easily resolved by realizing the improper ultrarelativistic behavior of the Lorentz-Dirac equation which is used in the example. Extreme care must then be exerted in the use of the equations of motion of classical charged particles when moving under the influence of a zpf. The search for internal contradictions in SED, not related with the well-known renormalization and other difficulties of classical electrodynamics, has so far been unsuccessful. This and several points of rigor here and elsewhere included, are enough to indicate that the conspicuous behavior of classical particles discussed here is correctly predicted from the assumptions of SED. It is therefore proposed that this predicted behavior may serve as an observational test for the validity of SED.

Results of "molecular dynamics" simulations are reported for a model of a fully ionized strongly coupled hydrogen plasma. Quantum effects are taken into account through the use of effective pair potentials; at short distances, these differ significantly from the bare Coulomb potential. Static properties of the plasma are shown to be well described by hypernetted chain theory. The ion- and electron-velocity autocorrelation functions have been computed and the electrical conductivity turns out to be roughly twice that expected on the basis of the electron self-diffusion coefficient. The predictions of Vlasov theory for the damping and dispersion of the plasmon mode are found to be in generally poor agreement with the results of the computer "experiments", but the collective dynamical properties are successfully described by a memory-function scheme in which explicit account is taken of ion-electron correlations. Prospects for future work are briefly reviewed.

New calculations on the core-excited $1s2s3s{}_{}{}^4{}_{}{}^{}S$ and $1s2p3d{}_{}{}^4{}_{}{}^{}D_{}^o{}_{}{}^{}$ states of neutral lithium combined with very accurate experimental and theoretical data confirm an earlier estimate of the position of the lowest quartet state, $1s2s2p{}_{}{}^4{}_{}{}^{}P_{\frac{3}{2}}^o{}_{}{}^{}$ at 57.4128(2) eV above the Li ground state. The uncertainty of the present result, ±0.2 meV, is five times smaller than our previous one and it arises, in part, from the uncertainty in the conversion factor from ${\mathrm{cm}}^{-1\mathrm{}}$ to eV. Good agreement with recent experimental work of Rassi et al. (57.41±0.015 eV) is obtained. The positions of all identified quartets have been recalculated from the latest experimental data and new theoretical results.

Blackbody radiation is expected to produce a temperature-dependent Stark shift in atomic Rydberg states. Unfortunately, in "one-electron" atoms, all the Rydberg levels experience the same shift, and the effect is only observable on an optical transition from the ground state. However, in a many-electron atom the presence of valence-state perturbers leads to unequal shifts of the Rydberg levels. This should make it possible to observe the frequency shift of a transition between two neighboring Rydberg states at a frequency in the radio-frequency range. This eliminates the need for a very stable visible laser, which so far has been the major technical obstacle to the observation and utilization of the effect. We present as an example a specific case in Ba.

Recent work on the perturbation theory of the Dirac equation in a central field is extended to include scalar-type interactions. All corrections to the energies and wave functions, including corrections to the positions of the nodes in excited states, are expressed in quadratures in a hierarchical scheme without the use of either the Green's function or the sum over the intermediate states.

Recent data reported by Zander and Andrews for $K$-shell ionization of ${}_{22}{}^{}{}_{}{}^{}\mathrm{Ti}$, ${}_{26}{}^{}{}_{}{}^{}\mathrm{Fe}$, ${}_{28}{}^{}{}_{}{}^{}\mathrm{Ni}$, and ${}_{30}{}^{}{}_{}{}^{}\mathrm{Zn}$ by 60-150-keV protons are reanalyzed in terms of the ECPSSR theory of Brandt and Lapicki. The ECPSSR approach takes into account the energy-loss effect (E) as well as the Coulomb deflection (C), perturbed-stationary state (PSS), and relativistic (R) effects. Agreement between theory and experiment is improved, and the remaining discrepancies are partially attributed to inaccuracies of the wave functions utilized in the calculation of inner-shell ionization cross sections.

It is demonstrated that the theory presented in a recent letter by Lin which goes "beyond mean field" in the treatment of the isotropic-nematic transition is in less satisfactory agreement with experiment than is the prevailing mean-field theory.

We report some quantitative studies of four-wave mixing in aligned nematic-liquid-crystal films. Using an unfocused cw laser of 200 mW power, at an estimated intensity of 5 W/${\mathrm{cm}}^2$, we have observed a conversion efficiency of ${10}^{-2\mathrm{}}$. The third-order nonlinear polarization that is responsible for this wave-mixing process is attributed to the optically induced molecular reorientation. A quantitative calculation of the wave-mixing efficiency and the intensity dependence is presented. It is shown that the experimental results are in good agreement with the theoretical predictions.

The integral equation for the probability density of the number of ions produced in matter as a result of complete degradation of electrons is solved numerically using a Fourier transform. Data are presented with reference to argon, which show that if inner-shell contributions are neglected, the probability distribution becomes Gaussian around a source energy of 5 keV. However, results obtained including the contributions from $L$-shell and Auger electrons indicate that a normal distribution of ions can be expected only at higher energies.

It is shown that the transverse photon-electron interaction significantly modifies the conclusions of Gross and Rafelski and changes the mutual cancellation of the exchange and relativistic corrections to the inner-shell electrons of atoms with $Z>\mathrm{}40\mathrm{}$ instead of $Z>\mathrm{}58\mathrm{}$.

The cancellation of the exchange and relativistic corrections to the energies of inner shells in heavy atoms is discussed.

Analytic $Z$-dependent electron-ion potentials have been obtained for all isoelectronic sequences from He to Cs. These potentials have a parametrized Yukawa component for each electron shell. Parameter values were determined by iteratively solving the Dirac equation until a match with experimental ionization energies was obtained. The screening parameters for each isoelectronic sequence were fit to simple $Z$-dependent functions. Working systematically through the periodic table and using preceding fits to obtain the screening parameters for closed inner shells, I found it necessary to evaluate only the screening parameter for the valence shell of the parent ion. Excited states having the same parentage as the ground state are also given to good accuracy by these potentials. The energy offset of excited states having a different parentage than the ground state is obtained from simple $Z$-dependent functions.

Stimulated two-photon induced Raman scattering (hyper-Raman scattering) in an amplifier consisting of four-level atoms is analyzed semiclassically, via the coupled density matrix and Maxwell equations. We consider conditions typically encountered in the optical regime and in vapors. We show that even in the linear regime, it is characterized by a significant depletion of the atomic ground-state population, thus invalidating perturbative analyses based, for instance, on the nonlinear susceptibility tensor. Two major regimes of operation are found, one in which the population is completely consumed by the interaction ("linear regime"), and one in which the pump laser is depleted ("nonlinear regime"). Both regimes are, however, characterized by small photon conversion efficiencies. The tuning range is also briefly discussed.

Using a method based on the many-body perturbation theory, the dipole and quadrupole polarizabilities and Sternheimer ionic antishielding factor have been calculated for ${\mathrm{Cr}}^{+}$, ${\mathrm{Mn}}^{2+}$, ${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{4+}$, and ${\mathrm{Ni}}^{5+}$ in Ar $3d^5$ isoelectronic configuration. The results include all the electron-electron interactions to first order and are similar in accuracy to the coupled Hartree-Fock method.