We have developed a technique to produce stationary nonequilibrium states in a molecular-dynamics system; this method is based on the introduction of stochastic boundary conditions to simulate the contact with a thermal wall. The relaxation times involved in such contact are short enough (∼${10}^{-11\mathrm{}}$ sec) to make the technique suitable for computer experiments. The method allows the simulation of bulk properties in a system coupled with a heat reservoir and the study of the local thermodynamical equilibrium. Furthermore, it gives a physical description of the heat transfer near a thermal wall. The method has been applied to simulate high thermal gradients in a region of dense fluids ranging from the gas-liquid coexistence line to the freezing line, to check the validity of the linear thermal response (Fourier's law). We have found that the linear region extends at least up to gradients of the order of 1.8×${10}^9$ K/cm for argon. In the bulk region where boundary effects are negligible we have verified the validity of the local equilibrium hypothesis for all simulated gradients.

Computer-simulation studies of soft-sphere crystals reveal that under isothermal homogeneous shear the crystal melts through a series of intermediate phases. At the highest strain rates the thermodynamic and transport properties indicate a normal-shearing fluid phase. The results suggest that the behavior of Bingham plastics and fluids exhibiting a yield stress are manifestations of shear-induced phase transitions.

A simple extension of McMillan's mean-field model of the nematic- and smectic-$A$ phases is reported. Specifically, terms proportional to fourth-order Legendre polynomials are included in the mean field. For a reasonable range of parameter choices this mean field can restrict the most probable directions of the long axis of a molecule in a smectic layer to a cone with the axis of the cone parallel to the optic axis. This result is related to recent quadrupole-magnetic-resonance observations of molecular order, and also to x-ray experiments which find that the repeat distance of the smectic layers is less than the all-trans length of the molecule. Finally, a possible connection to recent theories of the smectic-$A$—smectic-$C$ transition is pointed out.

The static-coherent-scattering function $S(\overrightarrow{\mathrm{k}},N)$ for a single polymer chain consisting of $N$ units with full excluded-volume effect is calculated by the renormalized perturbation method in chain conformation space. $S(\overrightarrow{\mathrm{k}},N)$ is given in a closed form to order $\epsilon$($=4-d,d$ being the spatial dimension) as a function of $x=k^2〈S^2〉$ with $〈S^2〉$ the radius of gyration. It is found that $I\left(x\right)=\frac{S(\overrightarrow{\mathrm{k}},N)}{S(\overrightarrow{0},N)}$ is very close to the $I_0$ without any excluded-volume effect; the difference between $I\mathrm{}\left(x\right)\mathrm{}$ and $I_0\mathrm{}\left(x\right)\mathrm{}$ is sufficiently small to probably preclude experimental measurement. However, there still remains the possibility of observing the excluded-volume effect on the dissymmetry coefficients which magnifies the difference between $I\mathrm{}\left(x\right)\mathrm{}$ and $I_0\mathrm{}\left(x\right)\mathrm{}$.

Cross sections for the excitation of the hydrogenic ion ${\mathrm{Ar}}^{17+}$ from the $1s$ to the $2s$ and $2p$ states are computed on the basis of a pseudostate expansion. Collision strengths for several hydrogenlike systems are compared.

This paper considers nonlinear decay of a beam-driven plasma mode into an Alfvén wave plus a circularly polarized electromagnetic wave near the electron-plasma frequency. Explicit expressions for the growth rate and threshold are obtained analytically. Relevance of our work to space as well as laboratory plasmas is discussed.

The characteristics of recombination luminescence in an argon-nitrogen gas mixture is reported. The authors' interest lies in the moderation of subexcitation electrons by collisions with gas atoms. It is concluded that the measured time delay for recombination luminescence decay is well explained by nearly the same thermalization time for subexcitation electrons of energy > 0.4 eV.

An exact solution is found for the functional renormalization-group equations describing intermittency in area-preserving mappings of the plane. Analysis of the solution yields two scaling laws for the duration of laminar flow in between chaotic bursts.

The behavior of the Liapunov number for the quadratic map in the intermittently chaotic region near period three is studied as a function of $\epsilon$, the distance of the map parameter from bifurcation. The contributions which arise from the regular and chaotic portions of the time series are separately analyzed and their ${\epsilon }^{\frac{1}{2}}$ dependence is discussed.

The emission, transition probability of the $\mathrm{Ca} (4s6s{}_{}{}^1{}_{}{}^{}S\to 4s4p{}_{}{}^1{}_{}{}^{}P)$ transition at 586.8 nm has been measured relative to the known strength of the $\mathrm{Ca} (4\mathrm{}{p}^2{}_{}{}^1{}_{}{}^{}D\to 4s4p{}_{}{}^1{}_{}{}^{}P)$ transition at 585.7 nm by two-step laser excitation. The ratio of transition probabilities was found to be $\frac{A_{586.8}}{A_{585.7}}=(3.5\mathrm{}\pm 0.5)\times {10}^{-1\mathrm{}}$ resulting in an $A$ value for the 586.8-nm transition of 2.35×${10}^7$ ${\sec }^{-1\mathrm{}}$.

The elastic differential cross section of electrons from molecular hydrogen has been calculated in the independent-atom model using the Glauber approximation for both direct and exchange scattering amplitudes. Polarization effects have been considered through the modified Glauber amplitude for the direct scattering. For the exchange amplitude the result obtained by Franco and Halpern has been used. Numerical results for various incident electron energies have been carried out. The polarization contribution has been found to have negative effects in bringing the calculated results close to the experimental values.

An alternative explanation for the origin of the $K{\beta }^{\prime }$ satellite of the $K{\beta }_{1,3}$ x-ray emission line of the first transition elements ($21\le Z\le 29$) has been given using the Bohm and Pines theory of plasma oscillations in solids. The present calculated values for the energy separation ($\Delta E$), and the relative intensity of the $K{\beta }^{\prime }$ satellite with respect to the $K{\beta }_{1,3}$ x-ray emission line, agree better with the experimentally observed values than the values calculated by the exchange-interaction theory of Tsutsumi et al., Salem et al., and Ekstig et al.

The different rates of self-consistent-field (SCF) convergence in the Hartree-Fock (HF) and $X\alpha$ methods are examined. The difference is due to the absence (HF) or presence ($X\alpha$) of the self-Coulomb interaction of each orbital. As a result the occupied orbitals are stabilized (destabilized) in the HF ($X\alpha$) method. A perturbative method is introduced which improves the rate of SCF convergence in $X\alpha$ at minimal increase in computational effort.

Cross sections are calculated for the charge transfer of ${\mathrm{C}}^{3+}$ ions in atomic hydrogen at impact energies up to 5 keV. Agreement is satisfactory with both the low-energy data below 250 eV and the high-energy data above 2 keV. The calculations show that the cross-section behavior is largely due to capture into the $1s^{2}2s3s{}_{}{}^3{}_{}{}^{}S$ state of ${\mathrm{C}}^{2+}$ for which the cross sections pass through a minimum at an energy near 250 eV and increase rapidly beyond the minimum out to 5 keV.

We report experimental results on the photoionization spectra of excited sodium atoms in a well-defined $m_l=1\mathrm{}$ substate in the presence of a dc electric field. The various spectra corresponding to a definite polarization of the ionizing light exhibit characteristic features. It appears in addition that they are strikingly perturbed by the Earth's magnetic field.

Strong ion turbulence generated in a C${\mathrm{O}}_2$-laser-heated gas-target plasma has been studied using ruby-laser Thomson scattering. Spectral and temporal features along with fluctuation levels which have been determined appear to account for the anomalous absorption previously observed.

The transition from Couette to Taylor vortex flow is described by a time-dependent Ginsburg-Landau model. The effects of the fluctuations and of the boundaries are investigated. It is shown that the nature and the localization of the mean-field transition are modified by these effects in a way depending on the size of the system.