An analytical expression is obtained for the average energy loss of atomic projectiles scattered by homonuclear diatomic molecules in the absence of electronic excitations.

The excitation of reconnecting modes is commonly considered to rely solely on global properties of the current-density distribution. Here a class of simple resistive magnetohydrodynamic models is studied analytically to show that local properties of the current profile at the reconnecting surface become important in some realistic physical regimes.

The ion-clustering mechanism which is the explanation usually invoked for the long collisional lifetime of metastable muonic helium is examined. Based on current knowledge of similar clusters, this mechanism does not work.

We show that the observed ordering of Rydberg states of one-electron atoms can be understood by assuming that these states are basically hydrogenic in nature. Much of the confusion concerning this point is shown to arise from the failure to differentiate between hydrogenic ordering as the nuclear charge approaches infinity, and hydrogenic ordering for an effective charge of one. The origin of $\kappa$ ordering of Rydberg levels suggested by Sternheimer is considered within this picture, and the predictions of $\kappa$ ordering are compared with those obtained by assuming hydrogenic ordering.

Radiation from highly excited Rydberg states $N_2^R\mathrm{}(B,v=5\mathrm{})\mathrm{}$ and $N_2^R\mathrm{}(B,v=4\mathrm{})\mathrm{}$ have been observed at 3532 and 3538 Å, respectively. These states are formed by a reaction of ground-state ${\mathrm{N}}_2$ with He $2^{3}S$. The Rydberg states have been observed at low pressure in a flowing afterglow and at high pressure in an electron beam discharge.

The distortion of the structure of a simple fluid of spherical particles when subjected to a shear is analyzed. The orientational distribution of particles around a central particle is examined via the radial distribution function. General symmetry arguments are advanced. Computer-simulation results using the technique of homogeneous shear nonequilibrium molecular dynamics are reported for the inverse-12 soft-sphere system. It is shown that the fluid displays non-Newtonian behavior.

The cross sections for single and double electron transfer in ${\mathrm{He}}^{2+}$ +Li collisions have been measured absolutely for center-of-mass kinetic energies in the range 4.8-98.6 keV for single and 32.0-98.6 keV for double electron transfer. Both cross sections are sufficiently large to support suggested applications to the development of soft-x-ray lasers and diagnostic techniques for fusion plasmas. The data are compared with recent theoretical results and with concurrent experimental data.

The $Q$ of an overmoded (wavelength much less than cavity size) irregular cavity is calculated by making use of the ergodic theorem.

Relative Rayleigh-scattering cross sections from nitrogen have been measured for various pulse durations and wavelengths of incident laser radiation. No pulse-duration dependence has been observed for laser pulses as short as 5 ns, and classical theory is found to be still valid over the pulse-width range ($5 \mathrm{ns}\le \Delta t\le 110 \mathrm{ns}$) of our observations.

Studies of the electronic properties of condensed rare-gas liquid systems have been conducted with the use of nonlinear excitation in the ultraviolet range. Analysis is furnished for several media, including binary (Ar-Kr, Ar-Xe, and Kr-Xe) and tertiary (Ar-Kr-Xe) systems. Evidence is reported for the selective excitation of impurity species, molecule-molecule electronic energy transfer in the liquid phase, and the identification of the 162.5-nm emission in Xe-Kr with the KrXe excimer.

The repeated-ring kinetic equations are accurately solved for the velocity-correlation function of a tagged particle in a three-dimensional Lorentz gas. Analytical expressions for the long-time behavior and for both the low- and high-density limits of the diffusion constant, calculated within the repeated-ring approximation, are also presented.

The interaction of singly periodic nonlinear waves of relativistic amplitude and electron-positron plasmas is investigated by means of fully self-consistent, relativistic, electromagnetic-particle computer simulation. When the frequency of a superluminous wave is slightly above the relativistic cutoff frequency, the initial waveform, be it sinusoidal or sawtooth (a self-consistent cold-fluid-theory solution), breaks up and creates a very hot plasma with a large particle flux $\Gamma \lesssim 2n_{0}c$ in the longitudinal direction. We find an important deviation from the cold-fluid-theory solution in the form of localized density spikes at the extrema of the wave magnetic field. For a wave frequency well above the cutoff, an initial sinusoidal waveform changes little but still creates a large longitudinal particle flow. Our observations of a large longitudinal flux of high-energy particles confirms a suggested mechanism for cosmic-ray acceleration.

We present here a detailed experimental study of nonlinear optical scattering in nematic liquid crystals using low-power cw lasers. The scattering is effected by the phase grating created by two intersecting same-frequency lasers. These lasers interfere spatially to create an index modulation via their reorientation effects on the molecules. When aided by a dc magnetic field, the reorientation and nonlinear responses of the medium are enhanced. We have quantitatively measured the dependence of the diffraction efficiency as a function of the optical intensity and the magnetic field ranging from well below to far above the critical orientation threshold values. The scattering is also analyzed in terms of its dependence on the temperature, sample thickness, and the scattering geometry. The experimental results are in good agreement with theoretical expectations. We also point out some high-intensity effects and applications of this phase grating, e.g., in wave-front conjugation and holographic imaging.

This paper employs a hybrid virial — Debye-chain cluster expansion to reinterpret a collective-coordinate calculation of electric microfield distributions in a plasma. A numerical evaluation of corrections previously neglected is presented. Comparisons with Monte Carlo and molecular-dynamics results are included.

Recent analyses of the Yvon-Born-Green equation strongly suggest that if it has a critical point, the critical correlations are, for spatial dimension less than four, negative at long range. We present here new numerical evidence that the YBG equation, in spatial dimension three, does not have a critical point, in contrast to the conclusions reached in previous numerical work. Our evidence comes from new numerical solutions which are closer to the supposed critical point and of greater precision than was achieved in the previous work. The extrapolations inferred from the previous work are not followed by the new solutions. The conclusion that there is no true critical point is based on three observations. (1) None of our numerical solutions are negative at long range. (2) The inverse compressibility does not extrapolate to zero along any isochore in the vicinity of the supposed critical point. (3) The inverse correlation length does not extrapolate to zero in the vicinity of the critical point, but rather goes through a nonzero minimum at a point of maximum, but not infinite, correlation length.

The patterns of convection to be expected in laterally large Rayleigh-Bénard cells close to onset are analyzed using the amplitude equation approach of Newell and Whitehead and Segel. This equation allows the introduction of a Lyapunov functional, which may be used to order the stability of various patterns. Two competing effects are identified: a surface effect that favors rolls approaching the sidewalls normally, and so tends to produce $O\left(1\right)\mathrm{}$ rotations of the roll orientation over the cell, and bulk effects that favor straight parallel rolls. The competition between these effects, and the role of defects in the structure, are studied in some simple examples.

Deuterium NMR spectral patterns from cholesteric liquid crystals (twisted nematics) for the case where the pitch axis aligns perpendicular to the direction of the magnetic field are evaluated in terms of three quantities or effects: (i) an asymmetry parameter in the quadrupole interaction which has been motionally induced by biaxial ordering of the molecules, (ii) a self-diffusion constant for migration of the molecules along the pitch axis, and (iii) distortion of the cholesteric helix in the presence of the applied magnetic field. Calculated spectral patterns which include the above effects are fitted to experimentally recorded spectral patterns from a selectively deuterated nematic that has been twisted by the addition of a chiral compound. It is demonstrated how values of the asymmetry parameter and the self-diffusion constant can be separately measured and helix-distortion-effects studied.