Symmetric ion-atom reactions in xenon of the type ${\mathrm{Xe}}^{+}$ + Xe→${\mathrm{Xe}}^{+}$ + Xe* have been found to occur with large cross sections at collision energies below 50 eV (c.m.). The reaction cross section for production of the two lowest Xe resonance levels $6s{\left[\frac{3}{2}\right]}_1^o$ and $6s^{\prime }{\left[\frac{1}{2}\right]}_1^o$ at collision energies of 7-50 eV have been measured by monitoring the 147.0- and 129.6-nm XeI lines. All reactions involving ${\mathrm{Xe}}^{+}\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2},\frac{1}{2}}^o{}_{}{}^{}\right)$ impacting ions display kinetic-energy thresholds corresponding to the transfer of more than 11 eV of translational energy into internal energy. Experimental kinetic-energy thresholds exceed the thermochemical energy difference between the reactants and products evaluated under infinite separation conditions. Excitation of the lowest resonance level of Xe, $6s{\left[\frac{3}{2}\right]}_1^o$, is found to depend strongly upon the electronic state ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^o{}_{}{}^{}$ or ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^o{}_{}{}^{}$, of the reacting xenon ions.

The high-resolution fluorescence-excitation spectrum of ${\mathrm{H}}_2$ from 850 to 720 Å allows us to identify fluorescent levels lying above the $\mathrm{H}\mathrm{}(n=1\mathrm{})+\mathrm{H}\mathrm{}(n=2\mathrm{})\mathrm{}$ dissociation threshold and even above the ionization threshold. Below the ionization limit ($804 \AA <\lambda$) fluorescence remains a competitive decay channel for nearly all the excited levels except the $3p\pi {}_{}{}^1{}_{}{}^{}\Pi _u^{+}{}_{}{}^{}$ and $4p\sigma {}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$. Above the ionization limit we still observe fluorescence from many levels mainly of ${}_{}{}^1{}_{}{}^{}\Pi _u^{-}{}_{}{}^{}$ symmetry. The whole vibrational progression of the $3p\pi {}_{}{}^1{}_{}{}^{}\Pi _u^{-}{}_{}{}^{}$ up to $v^{\prime }=17\mathrm{}$ is observed. In addition fluorescence is observed from few levels lying above the $\mathrm{H}\mathrm{}(n=3\mathrm{})+\mathrm{H}\mathrm{}(n=1\mathrm{})\mathrm{}$ dissociation limit.

Asymptotic formulas and large-order perturbation calculations are reported for the energy levels arising from the $3s-3d$ degenerate levels of hydrogen in a magnetic field.

A model has been developed which predicts the relative abundances of the differently charged ions and the x-ray emission spectra resulting from the cascade decay of an arbitrary distribution of inner-shell vacancies created in atoms by energetic charged particles or x rays. The multiple-ionization and x-ray production cross sections are defined in terms of the populations of the single- and multiple-vacancy states occurring in the atomic reorganization process. All allowed radiative, Auger, and Coster-Kronig transitions are taken into account in the determination of these populations. The x-ray spectra are classified in terms of characteristic lines which are due to the radiative decay of single vacancies and satellites which are associated with transitions from multiple-vacancy states. Results of calculations are presented for the creation of the initial vacancy distribution by single inner-shell electron ionization of neutral Fe with electrons and photons. Multiple ionization is found to represent only 10% of the total for electron-impact ionization but is predicted to be predominant during $K$- and $L$-shell photoionization, in agreement with experimental results obtained for the rare gases. The intensity of the $L\alpha$ satellites is always less than the $L\alpha$ characteristic line intensity for electron-impact ionization but can substantially exceed the characteristic line intensity during $K$-shell photoionization, in agreement with previous results obtained for medium-$Z$ elements.

We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3S_{\frac{1}{2}}\to 3P_{\frac{1}{2}}\to 4D_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3S_{\frac{1}{2}}\to 3P_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited $3P_{\frac{1}{2}}$ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-Störer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the $3P_{\frac{1}{2}}$ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium $3P_{\frac{1}{2}}$ atoms and helium, neon, and krypton perturbers.

The theory of electron-atom scattering in a radiation field, either spontaneously produced or external, is formulated in such a way that the effectively strong interaction of the electron with soft photons (those with frequencies lying below some cutoff ${\omega }_s$) is properly accounted for at the outset. Effects of the residual interaction can then be included using standard perturbation theory. This approach follows that taken some years ago by Bloch and Nordsieck in their fundamental treatment of infrared radiation in potential scattering. The fact that the target is taken here to be a compound system which itself interacts with the field introduces additional complications. A gauge transformation is introduced which helps considerably in the analysis of these multiparticle effects. Particularly simple results are obtained by treating both the cutoff frequency ${\omega }_s$ and the residual interaction strength as quantities of first order and neglecting second-order corrections. The cross section obtained in this way, when summed over all final states containing soft photons, is expressed in terms of the cross section for scattering in the absence of any interaction with the field. The first-order correction term, not present in the earlier Bloch-Nordsieck version, can be interpreted in terms of the classical bremsstrahlung radiation emitted as the result of an instantaneous collision. The low-frequency approximation for the scattering amplitude derived here contains as a limiting case the result obtained by Low and others for single-photon bremsstrahlung. It also reduces in the appropriate limit to the recently derived approximation for electron-atom scattering in a low-frequency laser field.

$1s-2p$ excitation in hydrogen has been studied by observing the angular correlation of Lyman $\alpha$ photons detected in coincidence with inelastically scattered electrons at 54.4 eV incident energy. The electron scattering angles ranged from 10° to 133°; the results at scattering angles larger than 20° cannot be explained by currently available theories.

The magnitude and sign of laser saturation resonances induced in atomic transitions with level degeneracy can be dramatically altered by the presence of optical pumping. This paper presents a detailed theoretical treatment of optical-pumping Lamb dips and crossover resonances, and their experimental observation in the 553-nm barium resonance transition. The physical principles underlying the laser optical-pumping saturation effect are also discussed.

High-resolution spectra of laser-induced collisional energy transfer between europium and strontium are obtained for collisional interactions which are dipole-dipole in nature. A universal profile is found for this laser-induced excitation. For the first time a precise comparison with available calculations is possible. The line core agrees well with a theory assuming a dipole-dipole coupling, but the quasistatic wing deviates from the asymptotic behavior expected for a pure dipole-dipole interaction.

Ab initio calculations of the $3d$ photoionization cross section of ${\mathrm{Ni}}^{+}$ ($3p^{6}3d^9$ ground-state configuration) have been performed using $R$-matrix theory. Their results are discussed in terms of interpretation of $M_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ experimental spectra obtained by several techniques and validity of theoretical approximations. They point out the existence of more or less autoionizing sharp lines series converging to the various $3p$ ionization thresholds, in addition to a broad asymmetric Fano profile. Also, the connection of such calculations with the problem of Auger spectra is emphasized since they bring direct evidence of the importance of coupling the channels when treating the relaxation and broadening effects involved in the interpretation of x-ray photoemission spectra. Our conclusions are in favor of a $3d-4s$ shake-up process involved in the final ionic state $3p^{6}3d^{7}4s$ to explain the existence of the strong satellite (6 eV below the Fermi level) in solid nickel photoemission spectra.

The Čerenkov radiation generated in a cylinder and its surrounding medium by a particle moving along the axis of the cylinder is calculated. Owing to interference phenomena in the generation and reflection of the radiation inside the cylinder, the Čerenkov spectrum may be strongly modified. An effective refractive index defined on the basis of the simple Tamm-Frank formula exhibits large variations with the frequency of the radiation as well as with the velocity of the particle, in particular for wavelengths of the same order of magnitude as the radius of the cylinder.

A new semiclassical model of the active medium of a gas laser and a description of its interaction with a classical electromagnetic field are reported. The model is based upon an exact microscopic formulation of the density of the active medium in a suitably defined semiclassical state space. Field-medium interaction mechanisms are studied by coupling the equation describing the dynamics of this density with Maxwell's field equations and the Schrödinger wave equation. Coupled stochastic nonlinear equations are formulated and quasilinear techniques are employed to effect their solutions. Wave-dipole correlations, the dominant nonlinearity, is shown to effect stability, produce the phenomena of hole burning, mode coupling, and frequency conversion, and to provide the mechanism whereby nonresonant pump field energy is converted into resonant laser oscillations.

Coupled equations describing the propagation of the laser field (Maxwell equation) and the evolution of the electron distribution (generalized pendulum equations) are derived classically for the case of ultrarelativistic electrons moving in a helical magnet with an arbitrary slowly varying amplitude. The equations apply to coherent transient phenomena, such as picosecond pulse propagation, as well as to continuous-wave laser operation.

Generalized conservation laws and gain-spread relations in the free-electron laser are derived for variable wiggler configurations. The derivation follows from the general equations of paper I in this series. It is suggested that bandwidth limitations on the optimization of the small-signal gain-to-spread ratio of a storage ring FEL might be overcome by transverse velocity filtering of the electrons.

The influence of fluctuations on two models relevant in solid-state laser theory is discussed. One model is the usual solid-state laser model, the other describes a laser with an unstable saturable absorber. For the usual solid-state laser it is shown that fluctuations act as singular perturbations of a bifurcation. Also derived is a simplified master equation whose stationary solution is in near-perfect agreement with the known exact stationary properties of the laser. For the laser with absorber the fluctuations are shown to act as singular perturbations for some solutions and singular destruction for other solutions. A simplified master equation is also derived for this model, and its stationary solution is compared with the known exact stationary distribution. Here too very good agreement is found whenever comparison is possible.

A systematic method is presented for averaging the integral equations of motion for the atomic density matrix operator and its quantum correlation functions over the fluctuations of Markovian exciting fields with arbitrary bandwidth. The theory takes into account the statistics of the field to all orders. The method is applied to the investigation of resonance fluorescence in the presence of three different Markovian fields: (i) a phase-diffusion field, (ii) a chaotic field, and (iii) a Gaussian-amplitude field. It is shown that in the case of a resonant, intense phase-diffusion field a center line dip develops in the spectrum of resonance fluorescence when the Rabi frequency is approximately equal to the bandwidth of the field. In the case of amplitude fluctuations the sidebands of the on-resonance spectrum tend to reproduce the probability distribution for the amplitude of the exciting field.

A new fundamental theory of composite particles in nonrelativistic many-body systems is developed. The theory is constructed making use solely of the physical Fock space for the basic elementary particles which comprise the many-body system. In this manner exchange symmetry in the elementary particles is exact. Physical composite particle creation and annihilation operators are introduced and these operators satisfy exact Bose (Fermi) commutation (anticommutati$\stackrel{\mathrm{̇}}{\mathrm{o}}$n) relations depending on whether the composites correspond to bosons or to fermions. Commuting physical occupation number operators for composite particles are then constructed in the usual manner from the composite creation and annihilation operators. These number operators are highly dressed in terms of the basic elementary particle operators. Finally, the foundations of statistical mechanics for bound composite particles are formulated in terms of the appropriate occupation number operators.

The exchange energy, exchange potential, and the exchange contribution to the interaction function are evaluated for all nonzero temperatures. These results are the finite temperature versions of the ${\rho }^{\frac{1}{3}}$-theory of Kohn-Sham. The applications of the results reported here are indicated. The paramagnetic susceptibility of an interacting electron gas as a function of temperature exhibits an interesting peak structure.

A model for the development of electrical breakdown in dense gases is presented. It describes the initial phase of breakdown in the regime where the Townsend avalanche mechanism does not apply. The main features of the model are as follows: (1) It gives a continuous picture of the development both in the structure of the breakdown and the physics of the processes, and (2) it is based on electron kinetics, so that the theory is general in scope. In light of this model a brief discussion of experimental results is given.

A recently studied autocatalytic chemical reaction model for a nonequilibrium first-order phase transition is applied to fluctuations in surface density at the phase boundary. The model is exactly soluble for the dynamics of small fluctuations. The analysis also carries through for fluctuations from solitary solutions.