A time-differential level-crossing technique using laser excitation of a fast ion beam is presented. The fast beam thus oriented or aligned makes possible measurements of the Landé $g$ factors. They have been measured to be $g_{\frac{1}{2}}=0.672\mathrm{}\pm 0.006$ and $g_{\frac{3}{2}}=1.328\mathrm{}\pm 0.008$ for the $6p{}_{}{}^2{}_{}{}^{}P_J^{}{}_{}{}^{}$ levels in ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$II.

The calculation is given of the spectral distribution of resonance fluorescence from an atom that is driven near a $j=\frac{1}{2}$ to $j=\frac{1}{2}$ transition by a monochromatic, $x$-polarized electric field and that is subject to radiation damping only. In accordance with Mollow's results for a two-level system, the power spectrum of the scattered radiation corresponding to the $x$ component of the dipole moment contains a coherent part for all driving-field intensities. The coherent part is absent in the fluorescence radiation corresponding to the $y$ and $z$ components of the dipole moment. For weak driving fields all power scattered by these latter components is present in a narrow Lorentzian. Consistent with energy conservation, the width of the Lorentzian corresponds to the probability of excitation of the atom out of its ground level due to the presence of the driving field. For strong driving fields the power spectra corresponding to the $x$ component and to the $y$ or $z$ component mainly differ in the heights and widths of their central incoherent band and their Rabi-precession-frequency-shifted side bands.

Self-induced modulation of Raman-Stokes pulse amplification and laser pulse attenuation was observed in coherent Raman pulse propagation in ${\mathrm{H}}_2$ gas. This modulation is due to laser pulse depletion and transient response of the material polarization. Theoretical analysis of the propagation equations agrees well with experimental observations.

We present an experimental investigation of the third-order intensity correlations in the light from a single-mode intensity-stabilized He-Ne laser in the threshold region. Accurate results were obtained by utilizing a fast digital correlator operating in real time. The computation of the intrinsic third-order correlations requires also a knowledge of the second-order intensity correlation which was measured under the same operating conditions. The experimental data are in agreement with recent theoretical computations based on the usual Van der Pol model of the laser oscillator. A new definition of the intrinsic correlations, which seems more appropriate for the interpretation of the experimental results, is discussed.

The semiclassical interaction of a two-level system with a multimode electromagnetic field is discussed in detail. The equations of motion for the macroscopic density matrix are transformed into a single integral equation for the population density, with a complex kernel which can take into account both running- and standing-wave configurations. The iterative solution to this integral equation, which corresponds to an expansion in terms of powers of the electromagnetic field amplitudes, turns out to be divergent when the field amplitudes are greater than some critical value. In the case in which the kernel is a periodic or quasiperiodic function of time, a general solution to the integral equation is found, and this solution turns out to be unique and convergent for every value of the electromagnetic (e.m.) field amplitude. This solution is expressed through a continued-fraction expansion, whose terms are matrices, which can be readily obtained from the kernel. This solution is a generalization of the continued-fraction expansion usually met in the strong-signal semiclassical theory of lasers. Some particular cases are then treated in detail; among them, the solution to the multimode operation in laser devices is given in which the excited level lifetime is long compared with the period of the beating term between two adjacent modes. Although the theory presented is directed towards the steady-state regime in multimode lasers, it may also treat the transient regime of laser operations, or it may be extended to include phenomena where a strong e.m. field, made up of several modes equally spaced in frequency, interacts with a two-level system.

A complete and detailed treatment of a three-level atom interacting with two near-resonant monochromatic fields is presented. It is assumed that the only damping mechanism is radiative damping, and that the atoms all have the same resonance frequencies, as is the case in an atomic beam. Detailed analytic solutions as well as numerical examples are determined from quantum-electrodynamic equations of motion. In addition, approximation techniques are presented which allow one to get accurate quantitative predictions for strong applied fields from simple rate-equation-like arguments. Absorption and emission spectra are determined, as are the transient and steady-state response of the atom. Two of the more interesting predictions are an emission spectrum containing up to seven components and a prediction of steady-state populations larger than 0.5 in the second excited state.

Broadened Lyman-$\alpha$ x-ray lines from neon x and argon XVIII radiators, which are immersed in a hot, dense deuterium or deuterium-tritium plasma, are discussed. In particular, these lines are analyzed for several temperature-density cases, characteristic of laser-produced plasmas; special attention is paid to the relative importance of ion, electron, and Doppler effects.

We consider the multiply scattered electric field from a system of noninteracting Brownian scatterers. We show that the lowest-order contribution to the depolarized correlation spectrum of the scattered light is due to double scattering. We find an expression in the far-field approximation for the electric-field correlation function double scattered from a system of pointlike particles. We show that while the correlation function is slightly nonexponential, the $t=0\mathrm{}$ slope of the correlation function is independent of scattering angle and equal to the single scattered correlation time corresponding to a scattering angle of 180°. We then generalize our result to include larger, Rayleigh-Debye particles of radius $l\lesssim \lambda$ where $\lambda$ is the wavelength of the incident light. The results for larger particles are qualitatively similar to those for pointlike particles in that there is a lack of a large scattering-angle dependence in the correlation time of the double-scattered light, and the magnitude of the correlation time is slightly greater than the correlation time for single-scattered light at 180°. We then perform intensity autocorrelation experiments on two systems of polystyrene microspheres of radii $l=0.055\mathrm{} \mathrm{and} 0.117$ μm. We measure the depolarized scattered light correlation time and compare it to our theoretical results for double scattering. The agreement of theory and data is seen to be good. We discuss potential applications of this work to other phenomena.

Tokamak discharges are characterized by electron densities usually ∼(0.3-1.0)×${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and temperatures from a few hundred eV to several keV. In addition to the working gas (H or He), the plasma normally contains some light impurities (∼${10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$ O or C) that are completely stripped except at the outer periphery, and heavier elements from the vacuum wall and current-aperture limiter (Fe, Cr, Ni, W, Mo, and others, ∼${10}^{10}$-${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$) that remain partly stripped, hence relatively strongly radiating, throughout the discharge. Other elements, especially noble gases, may be deliberately added for diagnostic purposes. Resonance lines of Fe and Ar in the beryllium and lithium sequences, of Fe, Kr, and Mo in the magnesium and sodium sequences, and of Mo and Xe in the zinc and copper sequences have been used for rough determination of plasma composition. Since crucial plasma characteristics such as temperature and confinement time are sensitively affected by the local composition, it is essential to improve the available atomic data necessary for more accurate analysis: wavelengths, transition probabilities, excitation, ionization, and recombination rates, especially for the heavier elements.

The critical parameters of a gas with molecules interacting via the Lennard-Jones (6, 12) potential are calculated by analyzing the irreducible cluster integrals $b_l$ of the fugacity expansions. The method used here has earlier been successfully applied to the Ising model, and locates the critical temperature $T_c$ as the limit of the zeros of the cluster integrals $b_l$. The asymptotic behavior of $b_l$ at $T_c$ is numerically found, and from the fugacity series the critical pressure $p_c$ and the critical density ${\rho }_c$ are calculated. One can then deduce the correct shape of the critical isotherm $p-p_c\propto \mathrm{sgn}(\rho -{\rho }_c){|\rho -{\rho }_c|}^{\delta }$ and determine the related critical index $\delta$. The ratio $\frac{p_c}{{\rho }_c{k}_B{T}_c}$ is found to be $\frac{\zeta (2+{\delta }^{-1\mathrm{}})}{\zeta (1+{\delta }^{-1\mathrm{}})}$, where $\zeta \mathrm{}\left(x\right)\mathrm{}$ is the Riemann zeta function, and its universality is thus related to that of $\delta$. The numerical values of the critical parameters are then compared with the experimental data on rare gases; the agreement is good and represents an improvement over the methods based on equations of state. This can be taken to be a strong support for the numerical conjectures about the behavior of zeros of $b_l$ and the asymptotic behavior of $b_l\left(T_c\right)$.

The interaction between whistler solitons and an electron beam is considered. The rate of energy exchange between the beam particles and the soliton gas is given. It is shown that there can be efficient energy transfer from the solitons to the particles. The dependence of the energy exchange rate on various plasma parameters is examined. The results are shown to be relevant in the solar wind.

Marked pretransition softening is reported of the smectic layer elasticity near the weakly first-order smectic-$A$-nematic phase transition in $N-p-\mathrm{cyanobenzilidene}-p-n-\mathrm{octyloxyaniline}$ (CBOOA). In addition, it is demonstrated that CBOOA will not support a static layer strain. This new relaxation process is studied in detail.

Raman spectroscopy gives us a very useful tool for studying the vibrational dynamics of liquid crystals and, in particular, their structural phase changes. In order to investigate lattice mode nature of the structure sensitive low- and very low-frequency intermolecular modes of TBBA (terephtal-bis-butyl-aniline), we have studied the polarized Raman spectra from oriented monocrystals, and have followed the temperature dependence of such spectra from 90 to 300°K. The reported results are interpreted on the basis of a proposed dynamical model involving rotational Brownian motions in the crystalline phases.

Recent measurements of the x-ray and Auger line strengths in Li-like neon produced by 45-MeV ${\mathrm{Cl}}^{13+}$ → Ne collisions are compared with theoretical values. We assume that in the collision there is a statistical population of configurations and multiplet states. An overpopulation of the $1s2s^2$ and $1s2s2p$ configurations (relative to the $1s2p^2$) is observed. We suggest cascade feeding as a likely mechanism for producing this effect.

Using single-electron wave functions associated with the independent-particle-model potential of Green, Sellin, and Zachor, with parameters obtained by an ab initio variational procedure, one can obtain accurate atomic form factors for any atom and positive ion with $Z\le 54$. Sample results are found to compare favorably with available Hartree-Fock results. However, for most positive ions with $Z\le 54$, Hartree-Fock or Hartree-Fock-Slater calculations of the form factors do not presently exist.

Computational and experimental results for rotational excitation in atom-hydrogen-halide collision are just becoming available. Such processes are particularly important in many chemical lasers, and there is an obvious need for reliable estimates of the cross sections. One possible approach is to employ an information-theoretic synthesis. Previous applications of the information-theoretic approach to rotational energy transfer were largely confined to homonuclear diatomics. The recent accurate close-coupling computations of Collins and Lane for low-energy He-HF collisions provide a basis for a detailed comparison. We show that the magnitudes of the relative cross sections and their energy dependence, as computed by Collins and Lane, can be reproduced by the information-theoretic synthesis. We then proceed to predict similar results for HCl-rare-gas collisions.

The static electric dipole polarizability of the $4s^2\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ ground state of the calcium atom has been measured using an electric deflection technique. The result is normalized to the polarizability of lithium by comparing deflections of calcium and lithium beams passing through an inhomogeneous electrostatic field. We find (25 ± 2.5) × ${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^3$ for the polarizability of calcium.

We have calculated nuclear mass corrections of orders $\frac{{\alpha }^{2}m}{M}$, $\frac{{\alpha }^{3}m}{M}$, and $\frac{{\alpha }^2{m}^2}{M^2}$ to the $g_J$ factor for the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state of the helium atom. These corrections enable us to make a precise prediction for the isotopic $g$-factor ratio. The result is $\frac{g_J({}_{}{}^3{}_{}{}^{}\mathrm{He},2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{})}{g_J({}_{}{}^4{}_{}{}^{}\mathrm{He},2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{})}=1+2.8303\mathrm{}\times {10}^{-9\mathrm{}}$ with an estimated uncertainty of ± 0.0005 × ${10}^{-9\mathrm{}}$. This is an improvement upon a more approximate result reported previously by Grotch and Hegstrom.

It is shown that Born-approximation calculations for the excitation of atomic oxygen are in reasonable agreement with recent close-coupling calculations using a single-configuration ground state, when the one-electron orbitals are determined via an approximation to the central potential of Herman and Skillman. This is in contrast to Born-approximation calculations using the independent-particle model of Green, Sellin, and Zachor, where the cross sections tend to be larger than those of the close-coupling calculations, but closer to the multiconfiguration ground-state calculations than the single-configuration ground-state calculations.

Previous theoretical work by the author, on electron self-trapping, is extended to positronium in helium. Results are obtained for several values of the positronium-helium $s$-wave scattering length ($a$), in order to determine the validity of published analyses of data on the dependence of the ortho-positronium annihilation rate on the average density and temperature of its fluid helium host. It is concluded that the recently reported value of ${}_{}{}^1{}_{}{}^{}Z_{\mathrm{eff}}^{}{}_{}{}^{}=0.129\mathrm{}\pm 0.006$ is not "contaminated" by self-trapping effects or models, while the value reported for $a=0.77\mathrm{}\pm 0.04$ Å results from an invalid analysis. It is suggested that a value for $a$ in the neighborhood of 0.8 Å may be reasonably inferred from available experimental data and the calculations presented here, but that the precision of this determination is not very high.