The causes of thermal instability in high-pressure-low-temperature molecular discharges have been investigated with special emphasis placed on externally sustained CO laser discharges. Analysis shows that cw discharges of the type used in high-power CO lasers are inherently unstable, having thermal instability growth times on the order of 0.1 msec. The single most important factor affecting the instability growth rate at a specific power-density level was found to be electron-ion recombination. Changes in the recombination process as a consequence of positive-ion kinetics were found to result in order-of-magnitude variations in the instability growth rate. For this reason ion clustering reactions and water ion chemistry were investigated for conditions generally similar to those of high-pressure laser discharges. Analysis of high-pressure ion chemistry showed that conversion of clustered ions to molecular ions can have a significant impact on both the steady-state and stability characteristics of molecular discharges.

The nonadditivity contribution to the surface energy (excess internal energy per unit area of the surface) is derived from statistical mechanics as an integral involving intermolecular potentials and molecular distribution functions. Using Fowler's step-function density profile for the vapor-liquid system, Kirkwood's superposition approximation for the triplet-correlation function, Axilrod-Teller's triple-dipole potential for the three-body interaction potential $u_{123}$, and neutron-diffraction data of Yarnell et al. for the radial distribution function, one can calculate a sextuple integral to obtain the three-body correction $U_3^{\mathrm{}\left(s\right)\mathrm{}}$, which is about -2.4±0.1 erg/${\mathrm{cm}}^2$ for liquid Ar at 85°K (near the triple point). This contribution is opposite in sign to the correction needed to improve the two-body contribution $U_2^{\mathrm{}\left(s\right)\mathrm{}}$ obtained from Fowler's formula with the same $g\left(r\right)\mathrm{}$ data and Barker's realistic potential. Thus the combined value 23.9 erg/${\mathrm{cm}}^2$ of $U_2^{\mathrm{}\left(s\right)\mathrm{}}$ and $U_3^{\mathrm{}\left(s\right)\mathrm{}}$ is in worse disagreement with the experimental value of 34.9 erg/${\mathrm{cm}}^2$ than $U_2^{\mathrm{}\left(s\right)\mathrm{}}$ itself.

An analysis of the coherent scattering function $S(q, \omega )$ for simple classical liquids is presented. The analysis is based on a generalization of the modified theory of Singwi, Tosi, Land, and Sjölander involving the polarization potential $\psi \mathrm{}\left(q\right)\mathrm{}$ and the response function corresponding to the true self-motion of the atoms. The polarization potential has been determined through the modified self-consistent scheme proposed recently by Ailawadi, Miller, and Naghizadeh, thereby implying that both the static structure and dynamics of classical liquids can be understood in one single framework. The first two moments of $S(q, \omega )$ are automatically satisfied, whereas the fourth moment is only approximately satisfied. Numerical predictions for $S(q, \omega )$, the full width at half maximum, and other related functions in liquid rubidium are compared with the neutron scattering measurements of Copley and Rowe. The theory yields agreement with the data for momentum transfers $q\ge 1.5$ Å${\mathrm{A}}^{-1\mathrm{}}$. Possible improvements for $q\le 1.5$ Å${\mathrm{A}}^{-1\mathrm{}}$ and in the hydrodynamic regime are discussed.

The statistical properties of a classical electromagnetic field in interaction with matter are investigated. To this end a nonlinear extension of a model proposed elsewhere is studied by numerically solving the Newton-Maxwell equations of motion. The time-average energy distribution of the electromagnetic normal modes is also computed. It is shown that nonlinearity, no matter how large, does not completely destroy the dependence of the final energy distribution on initial conditions. One is therefore led to the conclusion that, as far as one is concerned with electrodynamical systems of finite total energy, no statistical behavior is to be expected. In particular, the Rayleigh-Jeans distribution law is not a general consequence of classical physics. The dependence on initial conditions can be removed, however, by the introduction of white noise delivering an infinite amount of energy to the radiation field. In this case equipartition of energy is reached, but in accordance with an old conjecture by Jeans, this process takes place at a nonuniform rate, the energy transfer being slower at higher frequencies.

The results of a kinetic theory of spectral-line broadening are applied to the calculation of Stark broadening for the Lyman-$\alpha$ line in the classical-path approximation. In the small-plasma-parameter limit, the width and shift operator reduces to an average binary electron-atom collision operator with screened interactions and dynamic correlations. The effects of screening, particularly the variation with frequency, are indicated for a simplified model and compared with the corresponding model without screening but with an impact-parameter cutoff in the collision integral. Comparison with the dynamically screened second-order (Born approximation) collision integral is also given.

The convergence properties of the Fermi-hypernetted-chain method as originated by Fantoni and Rosati are investigated. Numerical results are reported for liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and two model fermion liquids. It turns out that for not too high densities and not too long-ranged correlation functions the convergence to an upper bound for the ground-state energy is excellent, but that for higher densities and/or long-ranged correlation functions, it is easily possible to underestimate the upper bound if one does not apply certain convergence criteria and associated error estimates.

The formation of ion acoustic solitons in an argon-helium plasma is studied as a function of the light-ion concentration. It is found that an initial perturbation does not break into solitons for $1\%\lesssim \alpha \lesssim 66\%$. This fact can be related to an increase in linear Landau damping and to nonlinear resonant particles, i.e., reflected ions in front of the potential hump and eventually trapped ions between two solitons. In the concentration range where solitons are formed (essentially for $\alpha \gtrsim 66\%$) the soliton's Mach number and width follow qualitatively the well-known theoretical dependence on amplitude. Detailed studies show, however, that the measured Mach number cannot be explained by fluid theories (namely the Korteweg-de Vries equation and Sakanaka's theory). Stationary solutions from theory which takes into account trapped electrons and reflected ions adequately describe the Mach number versus amplitude and light-ion concentration dependence.

We consider the nonlinear interaction between ion quasimodes and a large-amplitude electron whistler wave propagating at an angle to the external magnetic field. It is found that this interaction leads to an instability. The growth rate and threshold are calculated.

Four elliptically polarized waves acting in a cold and unmagnetized plasma are shown to be responsible for the parametric evolution in time of the rotation of the principal polarization axes (the precession frequency) and some other nonlinear effects in two evolving waves by two other powerful waves. The investigation requires the transition from the macroscopic to a new type of more general and vivid microscopic study of parametric processes due to transverse waves in continuous media. The parametric precession frequency (PPF), when complex, is a new source of instability in plasmas. Complex PPF and complex parametric frequency shift occur in some near-resonant interactions in which the sum or difference between two frequencies is close to the characteristic frequency of electrons in a plasma. It is found that when the plasma is subjected to two strong fields, sharp bands of monochromatic noise transform into growing continuous spectra.

Results are reported on the angular dependence of the ${\mathrm{H}}^{+}$ translational kinetic-energy distributions resulting from electron-induced dissociative ionization of ${\mathrm{H}}_2$. These measurements confirm a fact clearly implied by earlier measurements, namely, that considerable variation of the energy distributions with angle can occur even for electron energies as much as 40 eV above threshold, and bring into question conclusions previously drawn from comparison of ion and/or high-Rydberg-excited fragment energy distributions measured at widely separated angles.

The $K$-shell excitation of a hydrogenic atom by atomic projectiles is treated in the first plane-wave Born approximation including the screening effect of projectile electrons. Screening causes a large decrease in cross sections, in accordance with recently reported data of Hopkins et al.

A proposed interpretation in terms of outer-electron shakeoff is disputed.

The difficulties in interpretation of the $K$ x-ray shifts are acknowledged. A new experiment confirms, with higher accuracy, the previously reported data and excludes the possibility of a chemical shift.

A general method is given to determine the steady-state level populations for $N$-level systems interacting with $N-1\mathrm{}$ strong resonant laser beams. For a three-level system it reproduces recent results of Whitley and Stroud. For a four-level system it leads to simple conditions for the population inversion between the highest and the lowest level. However, the population of the highest level can never exceed 0.5.

This paper examines two theoretical aspects of the measurement of polarization of the quenched radiation which comes from the application of a weak electric field to the $2s$ state of hydrogen. Specifically, it outlines the justification of the Bethe-Lamb phenomenological analysis of this problem from the external field approximation of quantum electrodynamics. Further, it includes the lowest-order effects of the $J$-breaking part of the hyperfine structure in Rayleigh-Schrödinger perturbation theory. While it was expected that these terms would contribute to the polarization fraction a correction to the lowest order of the relative size of the ratio of the hyperfine splittings to the fine-structure splitting of the $n=2\mathrm{}$ state or about one part in ${10}^3$, cancellations occur and these $J$-breaking terms give a correction in the fourth place of the polarization fraction.

We report results for $n=2\mathrm{}$ and $n=3\mathrm{}$ excitation and ionization in proton-hydrogen collisions. The calculations use a single-center (target) basis with $s$, $p$, and $d$ angular momentum states, and include all terms in the Born series. For projectile energies between 50 and 200 keV the results are in excellent agreement with recent experiments.

Recently a method of calculating inner-shell ionization has been formulated in which Hartree-Fock wave functions are employed and all terms in the Born series retained. Results have so far been presented only in the energy region where projectile deflection effects are of little importance. In this paper it is shown how the time-dependent problem can be solved just as conveniently for hyperbolic classical projectile paths that result from the nuclear repulsion. The method is applied to the impact-parameter dependence of $K$-shell ionization by protons on copper in the energy range 0.5 to 2 MeV. Excellent agreement is found at the lower energy with experiments of Andersen et al. Poorer agreement is obtained in the higher-energy regime.

The Cohen-Lekner theory of excess electron mobility in liquid rare gases is applied to liquid krypton at the triple point. A serious disagreement between the theory and experiment is pointed out and discussed.