A method to obtain exact lower bounds for the correlation energy of one-component plasmas is extended to neutral mixtures of charged hard spheres and to mixtures of point charges in a uniform background of opposite charges. In the former case, upper bounds for the correlation functions of charge densities are derived. These upper bounds, the semipositivity of these functions, and that of the charge-density-fluctuation spectrum are used to improve upon the known bound due to Onsager. In the latter case, the semipositivity of pair-distribution functions and generalizations of Mermin's inequality for the structure factor lead to bounds which improve upon known results in the domain of intermediate coupling.

The Landau-deGennes theory is used to study the isotropic-nematic phase transition of a semi-infinite system having a wall which induces local nematic order. Analytic solutions are obtained and it is found that (1) for sufficient surface order, the briefringence-induced optical phase shift diverges at the transition rather than at the supercooling limit, and (2) the regions of superheating and supercooling are severely restricted.

It is pointed out that with the sources and detector used in the above measurements, the values of 53.5 and 74.5 keV instead of 52.4 and 72.2 keV should have been used for the $K$ conversion x rays. The authors' claim of agreement with theory and other existing data within the quoted experimental errors is not correct when this slight change in energy is taken into proper account.

A comprehensive many-body theoretical calculation of the photoionization cross section along the neon isoelectronic sequence is reported. Our calculation has shown that the effect due to the ground-state configuration interaction diminishes rapidly as the degree of ionization increases; i.e., approximately 10% at $Z=20\mathrm{}$ and about 5% at $Z=30\mathrm{}$. In contrast, the many-body correction due to the final-state interchannel interaction seems to decrease at a slower rate and approach a constant value of approximately 10% at large $Z$.

Previous calculations on electron-impact excitation of argon and krypton have been extended to the analogous neon and xenon transitions. Strong-coupling effects are found to be important for the processes of interest.

The intensity ratio of the resonance line and the intercombination line in heliumlike C v has been measured in plasmas produced in a theta-pinch discharge and is compared with theoretical models. Electron density and temperature were obtained from laser scattering measurements. The results suggest a weaker temperature dependence of the laboratory low-density ratio than now obtained theoretically.

An infinite train of short light pulses is used for periodic creation of a coherent superposition of atomic substates. The coherence is sensitively detected by an optical probe beam using a polarization-selective technique. Narrow resonances are observed whenever the pulse repetition rate or its multiples coincide with an atomic Bohr frequency. Half-widths (full width at half maximum) of 20 kHz have been obtained for the Zeeman splitting of the sodium ground state.

Detailed electron-correlation calculations which include the mixing of closed channel with open channels, suggest that ${\mathrm{He}}^{-}$ has only two bound states: The $1s2s2p{}_{}{}^4{}_{}{}^{}P_{}^o{}_{}{}^{}$ and the $2p^3{}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}$. The $1s2p^2{}_{}{}^4{}_{}{}^{}P$ and ${}_{}{}^2{}_{}{}^{}P$ states are above the He $1s2p{}_{}{}^3{}_{}{}^{}P_{}^o{}_{}{}^{}$ threshold and therefore qualify only as core-excited shape resonances. This conclusion contradicts certain previous theoretical and experimental findings which implied the existence of bound ${\mathrm{He}}^{-}{}_{}{}^4{}_{}{}^{}P$ and ${}_{}{}^2{}_{}{}^{}P$ states. Both ${}_{}{}^4{}_{}{}^{}P_{}^o{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}$ autoionize to the adjacent continuum via spin-dependent forces—a well-known decay mechanism. However, there are two more decay mechanisms: nonrelativistic (for ${}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}$) and relativistic (for ${}_{}{}^4{}_{}{}^{}P_{}^o{}_{}{}^{}$) radiative autoionization (RA). The probability for RA of the ${}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}$ state in the Hartree-Fock approximation is computed to be 1.14×${10}^{10}$ ${\sec }^{-1\mathrm{}}$

We report the observation of Na${\mathrm{Li}}^{+}$ produced by crossed-beam collisions of Na and Li in the presence of a single laser field. The magnitude of the cross section and its dependence on laser intensity are in accord with a simple second-order perturbation theory.

The range of experimental confirmation of our scaling rule for electron loss from a hydrogen atom in collision with a heavy, highly stripped ion has been considerably broadened by new measurements on carbon-, niobium-, and leadion projectiles in molecular hydrogen.

It is shown that a current in the direction of the external magnetic field can give rise to an effective coupling between the magnetostatic modes which are then described by the Hasegawa-Mima equation. The implication of this result is discussed in the light of the many existing studies of the latter equation.

Parametric interaction of longitudinal and transverse waves is studied for a system consisting of a relativistic electron beam interpenetrating a molecular medium. The analysis suggests new possibilities for free-electron generation of coherent radiation.

Our recent treatment [Phys. Rev. A 18, 1066 (1978)] of the broadening and shift of alkali-metal-atom two-photon $S-S$ transitions is applied to the Na $3S-\mathrm{nS}$ transitions ($n=6-9\mathrm{}$) perturbed by Ar, Kr, and Xe rare gases. Comparison with recent experimental data [A. Flusberg et al., Phys. Rev. A 19, 1607 (1979)] on the Na-Ar pair for $n\le 7$ shows good agreement. However, for transitions to $n>\mathrm{}8\mathrm{}$ the calculated pressure effects no longer agree well with experimental data; presumably the pressure effects cannot be adequately described by a simple two-body long-range impact theory.

Starting from a completely relativistic single-particle theory and also from the full field theory we examine the radiative corrections to electron energy levels in the presence of a perfectly conducting wall. We also solve the nonrelativistic Schrödinger equation exactly for the potential $\frac{-e^2}{4z}+\frac{e^2}{4\pi m{z}^2}$. Our results agree with the conclusions of Shakeshaft and Spruch [Phys. Rev. A 22, 811 (1980)] concerning the size of the radiative level shift.

The relative merits of the complementary variational principles for the upper and lower bounds to the exact Thomas-Fermi energy of a neutral atom, and of a suggestion of Anderson, Arthurs, and Robinson for the determination of the parameters in a variational function from the minimization of the difference between the upper and lower bounds, are examined by applying these to two variational functions. It appears that the variational principle for the lower bound is of the most practical value. The suggestion of Anderson et al. appears to have little merit.

We have compared theoretical predictions for pair-production cross sections near threshold with recent experiments of Coquette and of En'yo, Numao, and Yamazaki, using results from numerical calculations in partial waves for the Hartree-Fock-Slater potential with the exchange term omitted. A majority of these experimental results for germanium are consistent with these calculations, but a residual discrepancy does persist in the middle portion of the lowest-energy range studied by Coquette.

It is shown that the formula for the drift velocity of electrons in gases presented by Braglia and Dallacasa, and also other more general formulas, can be derived from the Kubo-Greenwood relation without using Braglia and Dallacasa's assumptions about the angular distribution in the scattering and the form of the optical theorem. Several approximate drift-velocity formulas are tested numerically and compared with experimental data for the case of electrons in argon.

A discrepancy between the hydrodynamic limit for the Brillouin spectrum in a low to moderately dense gas and that calculated from a kinetic model of the generalized Enskog equation is removed.

The Born approximation and a closure technique are employed to obtain the single and double differential cross sections for the ejected electron produced by electron detachment of ${\mathrm{H}}^{-}$ ions in collisions with He. Comparison is made to the recent measurements of Menendez and Duncan at 0.5 MeV. The theoretical results show that the experimentally observed structure in the double differential cross sections in the forward direction can be explained in terms of single-electron-loss processes alone.

An expanding laser-produced plasma has been used as a source of a beam of slow, highly charged carbon ions. Bursts of collimated and energy-selected ${\mathrm{C}}^{q+}$ ions were directed through an atomic hydrogen gas target and charge analyzed by time-of-flight methods. Total electron-capture cross sections are reported for ${\mathrm{C}}^{q+}+\mathrm{H}$ collisions ($q=3,4,5,6\mathrm{}$) at energies ranging from 11 to 387 eV/amu and are generally in accord with heretofore untested low-energy theoretical calculations.