Three binary fluids, aniline-cyclohexane, nitrobenzene-$n$-hexane, and isobutyric acid—water, have been studied by light scattering and turbidity techniques near their critical points, both at the thermodynamic equilibrium and under a shear flow, as a function of the variables temperature $T$, relative concentration $M$, shear rate $S$, and wave vector $\overrightarrow{\mathrm{q}}$. The following results have been obtained: (i) Out of equilibrium. The region where a shear affects the critical behavior has been determined in the plane ($M$,$T$); the crossover temperature varies as $M^2$, and the coexistence curve exhibits the classical exponent ($\beta =\frac{1}{2}$). A small temperature change due to the shear was detected; its value is about four times lower than that calculated by the Onuki-Kawasaki theory. The susceptibility versus $T$, $M$, $S$, and $\overrightarrow{\mathrm{q}}$ is well represented by the Onuki-Kawasaki formulation, in particular the exponent $\gamma$ shows the classical value ($\gamma =1\mathrm{}$). (ii) At equilibrium. The susceptibility and the correlation length have been measured on the critical isochore above $T_c$, on the coexistance curve, and on the critical isotherm. The universal amplitude ratios $\frac{{\xi }_0^{+}}{{\xi }_0^{-}}$, $\frac{C^{+}}{C^{-}}$, $R_{\chi }^{+}$, and $Q_2$ have been obtained. The typical time taken by the system to return to equilibrium after having been perturbed by shear has been analyzed in terms of mass diffusion.

The diffusion and localization of a classical particle of diameter ${\sigma }_0$ in an environment of fixed spherical scatterers of diameter ${\sigma }_1$ in arbitrary dimensionality is considered by extending the self-consistent current relaxation theory for the Lorentz gas of a point particle and overlapping spheres. The variation of the diffusion coefficient with scatterer density $n$ and diameter ratio $\delta =\frac{{\sigma }_0}{{\sigma }_1}$ is calculated analytically. Different factorization schemes are examined and compared. The overlapping $\delta =\infty$ and nonoverlapping $\delta =0\mathrm{}$ Lorentz gas are discussed briefly as special cases. The critical density $n_c\left(\delta \right)$ in the ($n$,$\delta$) phase diagram separating diffusive from localized behavior is established and good agreement with Monte Carlo results for the percolation density of hard-core disks is obtained.

It is shown that in the strong-potential approximation using the full-peaking approximation the cross section for electron capture is invariant when any term which depends on the internuclear coordinate $R$ is added to the expansion potential. That is, Wick's theorem holds in the strong-potential approximation with full peaking.

Nuclear-spin-polarized samples of up to 200 Torr ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$ have been obtained through spin-exchange collisions with optically pumped Rb vapor. The exponential time constant associated with buildup and decay of this polarization along the quantization axis is in excess of 15 h in 3-cm-diam Pyrex cells. From the data and estimates given, samples of ${}_{}{}^{21}{}_{}{}^{}\mathrm{Ne}$ with densities greater than 1 STP and polarization in excess of 1% appear feasible.

The positronium negative ion (${\mathrm{Ps}}^{-}$), the system composed of two electrons and a positron, has been reinvestigated theoretically. Using a Hylleraas wave function with two nonlinear parameters and more than 200 linear terms, we have obtained excellent values of binding energy and annihilation lifetime of the particle-stable ${}_{}{}^1{}_{}{}^{}S_{}^e{}_{}{}^{}$ ground state. In addition, we have examined the question of stability of the ${}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$ state discussed by Mills, and we agree with his conclusion that the state is probably not stable against breakup into $\mathrm{Ps}\mathrm{}(n=2\mathrm{})+e^{-}$. Improved limits on the critical "positron" mass for binding the ${}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$ state have also been obtained.

Classical-trajectory Monte Carlo calculations have been performed for collisions of protons with state-selected hydrogenic Rydberg atoms. The examples investigated were Rydberg atoms in the $n=10\mathrm{}$, $l=9\mathrm{}$ level with $m_l=0\mathrm{} \mathrm{and} 9$. The collision velocity range was 0.1 to 1.0 a.u. (2.2×${10}^7$ to 2.2×${10}^8$ cm/s). The ionization cross sections were found to be relatively insensitive to changes in the $m_l$ levels. However, the charge-transfer cross sections showed considerable enhancement if the Rydberg electron is orientated in a plane which is parallel to the direction of the incident projectile.

The effect due to the polarization of the incident high-energy (∼GeV) proton beam on the ionization cross section of heavy atoms is investigated. In particular, with the use of Darwin wave functions for the atomic electron, the effect for hydrogenlike atoms is worked out. A numerical illustration shows that the polarization effect is completely insignificant. We also conclude that the $K$-shell ionization process is completely insensitive to the polarization of the incident beam.

The $1s\to \mathrm{ns}$ and $1s\to \mathrm{np}$ Coulomb-modified Glauber amplitudes for the excitation of hydrogenlike ions under the impact of a charged particle are reduced to simple one-dimensional integrals in real space that can be evaluated numerically with convenience irrespective of the magnitude of $n$.

We present differential cross sections of the elastic scattering of electrons from the metastable $2s$ state of atomic hydrogen at 100 and 200 eV. The present calculations are compared with the available theoretical results.

De Rújula has noted that if $K$ or $L$ capture is kinematically forbidden, enhancement of internal bremsstrahlung electron capture (IBEC) can occur. This enhancement may make IBEC useful in measurements of the neutrino mass. It is shown that a similar enhancement occurs for photon-induced electron capture. The cross section for the most promising case, ${}_{}{}^{163}{}_{}{}^{}\mathrm{Ho}$, is calculated. Unfortunately, one finds that the cross section is too small to be of use.

The inclusion of dissipative fluxes in the set of independent thermodynamic variables leads, among other features, to a modification of the equations of state. Here, we introduce into the van der Waals equation the generalized pressure obtained in extended irreversible thermodynamics and analyze the influence of a shear stress or a heat flux on the critical point of a fluid.

The theory of NMR quadrupolar splitting of deuterated cholesteric liquid crystals is presented. In particular, the NMR spectra of cholesteric blue phases having body-centered—cubic $O^5$ ($I432\mathrm{}$) and $O^8$ ($I{4}_{1}32$) and simple-cubic $O^2$ ($P{4}_{2}32$) structures are calculated and compared with those reported experimentally. It is found that the theoretical results for the $O^2$ structure are qualitatively in agreement with the measured spectra.

A two-dimensional hydrodynamic model of adsorbate behavior is modified by including the isothermal desorption. It is shown that for mobile adsorbates at reasonable temperatures and coverages it is the desorption which provides the dominant momentum nonconserving process and that the short-wavelength sound modes can propagate in the adsorbate provided desorption time is not too short.

Relativistic one-electron problems frequently suffer from variational instability associated with the absence of both upper and lower bounds to the eigenvalues of the Dirac Hamiltonian $H_D$. Such instability is trivially avoided if the square $H_D^2$ is used rather than $H_D$ to find energy eigenstates and eigenvalues. There are then no spurious roots, and true energy bounds and reliable wave functions are readily obtained. Sample calculations for H-like systems are presented.

Collisionally aided radiative excitation of atoms using a laser detuned to the blue is shown to lead to heating of the external degrees of freedom. The velocity distribution of the excited atoms is studied experimentally and compared with theoretical predictions.

Excitations of the atomic shell in heavy-ion collisions are influenced by the presence of a nuclear reaction. In the present Rapid Communication we point out the equivalence between a semiclassical description based on the nuclear autocorrelation function with an earlier model which employs a distribution of reaction times $f\left(T\right)\mathrm{}$. For the example of U+U collisions, results of coupled-channel calculations for positron creation and $K$-hole excitations are discussed for two schematic reaction models.

A frequency measurement of the fine-structure transition ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$, $\Delta m_j=0\mathrm{}$ in the first metastable triplet of ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ is reported. The experiment has been performed on a metastable atomic beam interacting with the submillimetric radiation ($\lambda \simeq 0.5$ mm) of a phase-locked backward wave oscillator. The transition is observed via the fluorescence emission at 4571 Å of the intercombination line ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$. The center frequency was evaluated to be $\nu =601\mathrm{}277.16\mp 0.05$ MHz with precision limited by first-order Doppler effect.

The first explicit solutions for laser-induced autoionization by a strong, smooth pulse are reported. Under certain "quantization conditions" for the area of the hyperbolic secant pulse, the photoelectron spectrum exhibits a novel coherence property: a multipeak structure.

Observations of steady-state hysteresis in absorptive bistability are reported for well-collimated atomic beams of sodium within a traveling-wave interferometer. The atomic medium approximates a system of homogeneously broadened "two-level" atoms, and effects due to standing waves are eliminated. The experiment thus represents the first demonstration of optical bistability under conditions that have been widely employed in theoretical models. Absolute measurements of switching intensities versus atomic cooperativity $C$ are reported, and a preliminary comparison with Gaussian-beam theory is made.