Threshold conditions for obtaining amplified spontaneous emission of x radiation, inclusive of strong line broadening and loss processes, are presented and evaluated for the case of pumping by ion-atom-ion-ion collisions. Quasistatic as well as pulsed operation of such x-ray lasers is seen to be realizable in terms of currently operative plasma systems.

We discuss here the intercollisional memory effects which have been detected in the low-frequency region of the absorption spectra induced by collisions in dilute solutions of para-${\mathrm{H}}_2$, He, and Ne in liquid argon. The theories of gaseous mixtures are shown to be inadequate to describe the spectra in liquids. Then we calculate the low-frequency part of the translational absorption spectrum for an argon crystal doped with impurities. The results of such a calculation for the solid are compared with the experimental data on liquid solutions and with the behavior predicted by the gaseous theories. We also show that from the induced-dipole-moment autocorrelation function, as obtained from the experimental data, it is possible to determine the short-time behavior of the force and velocity-autocorrelation functions of the impurity in the liquid. In particular we obtain the mean-square force $〈F^2〉$ acting on the impurity and $〈{\dot{F}}^2〉$. The derived values of $〈F^2〉$ and $〈{\dot{F}}^2〉$ are compared with those estimated with the help of the radial-distribution function of the liquid mixtures.

Theoretical and experimental studies of negative-ion processes in weakly ionized glow discharges have been conducted. Emphasis in these investigations has been directed towards analysis of gas mixtures in which negative ions are produced by dissociative electron attachment of C${\mathrm{O}}_2$, CO, or ${\mathrm{O}}_2$. It is shown that attachment, detachment, and clustering reactions normally occurring in discharges containing these species can significantly affect both the steady-state and transient characteristics of the plasma, even when an external source of ionization is provided. The magnitude and electron temperature dependence of the electron-molecule attachment coefficient is found to be particularly important. Specifically, analysis shows that when the electron attachment coefficient has a strong positive dependence on electron temperature, and a magnitude exceeding that of the ionization coefficient, an ionization instability can occur. This instability will occur under these circumstances when the negative-ion concentration is comparable to the electron density. Numerical evaluation of the conditions required for this electron-attachment—induced mode of ionization instability results in good agreement with experimentally determined conditions for the onset of striations in gas mixtures for which the charged-particle kinetics can be calculated in detail. In other cases, observed differences between theory and experiment have been related to uncertainties associated with the loss mechanisms of clustered negative ions in the discharge.

Carbon-13 NMR spectra have been obtained for $p$-methoxy benzylidene $p^{\prime }-n$-butylaniline (MBBA) in the isotropic, nematic, and solid phases. Sharp, resolved spectra were obtained in the nematic phase with high-power proton-spin decoupling. The isotropic-nematic transition is accompanied by a discontinuous change in ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ absorption frequencies due to the molecular ordering and anisotropic chemical shielding. It is concluded that the benzene rings reorient rapidly about the para axes. Employing average values from studies of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ chemical shielding tensors in model aromatic solids, absolute values of the order parameter $S$ are estimated and normalized values are compared with previous results. The nematic-solid transition is accompanied by sudden and severe line broadening due to anisotropic chemical shielding.

A semiclassical theory of two-photon Lamb dips is presented, when one of the radiations is in the radio-frequency region and the other is in the optical region. Perturbation solutions as well as an exact recurrence formula are discussed. For the latter case numerical results are given. The resonance line shape of the two-photon Lamb dip shows a variety of changes according to the experimental conditions. When both the optical and the rf fields are weak a line is of a dispersion form. When the rf field is increased, the line changes to a Lorentzian form. When the optical field is increased, the line is shifted and broadened. If the radio frequency is close to or less than the Doppler width of the gas, a two-photon Lamb dip is accompanied with satellite dips which correspond to molecules with nonzero velocity components along the propagation of the laser radiation. The intensity of a two-photon Lamb dip is expected to decrease rapidly with increasing radio frequency. Additional effects when the sample cell is placed inside the laser cavity are also discussed. Those results agree qualitatively with recent observations by Fruend, Römheld, and Oka.

Experimental measurements have been made of the reflection coefficient of phonons incident on a boundary between several dielectric crystals and liquid helium, gaseous helium, and solid neon. The measurements were made using the heat-pulse technique. A brief discussion of these results is given in terms of current theories of the Kapitza resistance.

An expression is derived for the optical-field-induced birefringence due to molecular reorientations in cubic plastic crystals. The effect is described by a nonlinear susceptibility tensor shown to be proportional to a tensorial correlation function. For most cubic point groups this correlation function has only two independent components. A decomposition into irreducible tensorial sets leads to the two physically meaningful quantities. One is labeled the tensor strength. It measures the short-range angular correlation of molecular pairs. It is a susceptibility that increases as this range increases, i.e., as the plastic-solid phase transition is approached. The second quantity is labeled the tensor anisotropy. It measures the long-range cubic anisotropy of the molecular orientations. It is related to an orientational order parameter of the plastic-liquid transition.

In this paper we present a coherent physical picture of four electronic transport regimes in expanded liquid mercury. We propose that the first two metallic regions, i.e., the propagation regime ($\rho >11.0\mathrm{}$ g ${\mathrm{cm}}^{-3\mathrm{}}$) and the diffusion regime ($9.2<\mathrm{}\rho <11.0\mathrm{}$ g ${\mathrm{cm}}^{-3\mathrm{}}$), are separated from the semiconductor regime ($\rho <7.8\mathrm{}$ g ${\mathrm{cm}}^{-3\mathrm{}}$) by a third conduction regime where the material is microscopically inhomogenous with regard to electron transport. Density fluctuations in a one-component system characterized by a mean interatomic spacing $2r_s$ were handled by considering the density-density-correlation function. The latter is characterized by the Ornstein-Zernike decay length $\xi$ and by the Debye short correlation length $b$. The necessary conditions $b>\mathrm{}2\mathrm{}{r}_s$ and $b>\mathrm{}\xi$ imply the existence of structual fluctuations which are independent when separated by more than $2b$. Local electronic structure and local response functions will exist in a disordered material provided that the phase coherence length $l$ of the electronic wave functions is appreciably shorter than $b$, and given that quantum corrections associated with the inhomogeneities, i.e., tunneling and kinetic-energy contributions, are small. The transport properties of such a system were handled by a general effective-medium theory, which was formally reduced to the problem of transport in a two-component system. Assuming that liquid mercury in the density range 9.3-8.0 (g ${\mathrm{cm}}^{-3\mathrm{}}$) is characterized by a sufficiently large value of $b$, we were able to provide a complete semiquantitative interpretation of the transport data accounting for the slow variation of the Hall mobility, and for the volume and temperature coefficients of the conductivity in that density region. The analysis of the conductivity data together with our statistical fluctuation theory results in a value of $b=15\mathrm{}$ Å for the average radius of the density fluctuation, whereupon the requirements of internal consistency of our picture are well satisfied. To establish the validity of the semiclassical percolation picture we have provided estimates of the magnitude and relative importance of tunneling and kinetic-energy effects, establishing that these corrections are quantitatively small for this high-temperature system. In the inhomogenous regime, density fluctuations lead to localization and percolation, and the metal-nonmetal transition should be envisioned in terms of a continuous change of the conductivity.

The energy positions for the higher ${}_{}{}^3{}_{}{}^{}P$ autoionization states of helium in the elastic scattering region are reported. These results are compared with previous calculations. Except for the lowest autoionization state calculated by Bhatia and Temkin, the result presented here is generally lower than that of previous calculations.

A systematic perturbation expansion with respect to the small parameter ${\left(\Delta \tau \omega \right)}^{-1\mathrm{}}$, $\Delta \tau$ being the duration of the pulse and $\omega$ the optical frequency, is introduced. This method is applied to the problem of self-induced transparency for sharp-line absorbers at exact resonance. Corrections to the McCall and Hahn solution are obtained which describe the propagation of the form-preserving pulse with modulated frequency and slightly changed shape, group velocity, and wave vector. Earlier results by Marth, Holmes, and Eberly on phase modulation and wave-vector shift are confirmed.

Some expressions for the time evolution of quantum-mechanical systems with Hamiltonians periodic in time, derivable from the work of Shirley and applied by Young, Deal, and Kestner and Haeberlen and Waugh—all for finite-basis-set systems—are derived for a general system (possibly infinite Hilbert space). These results suggest a new type of approximation to the time-evolution operator, one which is exact at multiples of the period of the Hamiltonian. Comparison is made to an exactly soluble problem, namely, a nonrelativistic hydrogen atom in a circularly polarized monochromatic field.

The $2s{}_{}{}^3{}_{}{}^{}S-2p{}_{}{}^3{}_{}{}^{}P$ transition of ${}_{}{}^{6,7}{}_{}{}^{}\mathrm{Li}$ II at 5485 Å, excited in a cooled hollow-cathode discharge, is observed in high resolution with a Hypeac interferometric spectrometer. Using separated isotopes, we have measured the absolute wavelengths of the 18 components of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ II and the 15 components of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ II. From these results we deduce (a) the hyperfine-structure constants for the ${}_{}{}^3{}_{}{}^{}S$ and ${}_{}{}^3{}_{}{}^{}P$ terms of both isotopes, (b) the fine-structure constants of the ${}_{}{}^3{}_{}{}^{}S$ term, and (c) the isotope shift and (d) the Lamb shift for the transition. Comparisons are made with recent many-body calculations and level-crossing and beam-foil experiments.

Experimental cross sections for $L$ -shell ionization by heavy charged particles, of atomic number small compared to the target atomic number and with velocities low compared to the $L$ -shell orbital velocities, can be smaller by more than an order of magnitude than the quantum-mechanical predictions in the plane-wave Born approximation (PWBA). The contribution of two effects, not contained in the PWBA, is derived. These effects are the binding of the $L$ -shell electrons to the moving particle during collision and the Coulomb deflection of the particle in the field of the target nucleus. As developed here for $L$ shells and previously for $K$ shells, the theory treats the excitation of inner shells from perturbed stationary states (PSS) and incorporates the effect of the Coulomb-distorted wave of the heavy charged projectile semiclassically. It agrees well with experimental $L$ -shell cross sections of 16 different target atoms for protons, and accounts for the isotope effect observed on various targets with protons and deuterosn of equal velocities.

By means of photoelectron spectroscopy, we have measured the branching ratios for production of the ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$, and ${}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ states of ${\mathrm{Hg}}^{+}$ by photoionization of Hg in the wavelength range 600-250 Å. These branching ratios are combined with earlier measurements of the total photoionization cross section to produce partial photoionization cross sections for the above processes. The wavelength dependence of the ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}:{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ branching ratio exhibits a systematic deviation relative to the statistical value of 3:2. That is, this ratio is greater than statistical where the respective partial cross sections are increasing with photon energy and is less than statistical when the partial cross sections are decreasing. This effect was previously predicted and is a consequence of spin-orbit coupling in the ionized subshell, in this case, the $5d$ subshell of Hg.

The spark chamber is used to study the following gas phenomena: drift of single electrons under the influence of an electric field, deexcitation of metastable rare-gas atoms in collisions with molecules, and ionization by high-energy particles of various energies. The method determines the average number of electrons from the spark efficiency and is capable of showing that a large fraction of the primary ionization is due to metastable atoms.

Differential cross sections for electron impact ionization of helium are calculated in the Born approximation for ejected-electron energies below the $n=2\mathrm{}$ threshold of the residual ${\mathrm{He}}^{+}$ ion. The resonant features associated with the lowest-lying ${}_{}{}^1{}_{}{}^{}S$, ${}_{}{}^1{}_{}{}^{}P$, and ${}_{}{}^1{}_{}{}^{}D$ helium autoionizing states are analyzed using the Fano line-shape formula. The angular correlation between the two outgoing electrons is expressed as a Legendre-polynomial expansion, and the coefficients are evaluated for representative energy losses and momentum transfers. For an incident-electron energy of 256.5 eV, the predicted angular distribution is in satisfactory agreement with the coincidence measurements of Ehrhardt et al. except near the recoil maximum.

We present a calculation of the photoionization cross section for the $4s$ subshell of neutral zinc using many-body perturbation theory. The cross section is strongly affected by correlations among $4s$ and $3d$ electrons. Resonances due to the $3d^{9}4s^2\mathrm{np}$ configurations dominate the cross section and are split by the spin-orbit interaction. We have also included interactions between resonances. Correlations reduce the Hartree-Fock cross section by an order of magnitude near threshold. Our results are in qualitative agreement with the experimental data of Marr and Austin.

Measurements of total and differential $L_{2,3}$ Auger cross sections for ${\mathrm{H}}^{+}$ + Ar collisions at 22 - 150 keV are presented. Additional values of these cross sections were calculated from data in earlier investigations by Volz and Rudd and by Crooks and Rudd up to 300 keV. Angular distributions are found to be isotropic. Over most of the energy range the total Auger cross sections are a factor of 2 smaller than predicted by Garcia's binary-encounter calculations.

Large-angle inelastic scattering is considered using the Glauber approximation for the scattered wave function, but without the additional approximation of purely transverse linear momentum transfer. At large angles, the electron-proton interaction dominates and has Rutherford $q^{-4\mathrm{}}$ behavior. Furthermore, with the direction of $\overrightarrow{\mathrm{q}}$ unrestricted the $\mathrm{L}\mathrm{y}-\alpha$ radiation is found to be, in general, elliptically polarized.

The spatial distribution of photoelectrons ejected from the $K$ shell of gold atoms by linearly polarized 1368-keV photons has been measured for photoelectron emission angles in the range 18°-82°. The experimental results are in good agreement with existing theoretical predictions when the predictions are modified to allow for distortion produced by scattering of the photo-electrons in the photoelectric target.