Near-ultraviolet emission bands have been observed in Li-Ar, Li-Kr, and Li-Xe mixtures. The bands are due to transitions from bound molecular Li—rare-gas states associated with the atomic Li $3d$, $3p$, and $3s$ states and ground-state rare-gas atoms. The transitions are all to the dissociative Li—rare-gas $2s\sigma$ molecular ground state. The Li-Ar, Li-Kr, and Li-Xe $3s\sigma -2s\sigma$ collision-induced bands are progressively shifted, for the heavier rare gases, further to the red of the asymptotic atomic Li $3s-2s$ energy difference. For Li-Xe this shift is a remarkable 4000 ${\mathrm{cm}}^{-1\mathrm{}}$. We also report rate coefficients for quenching, by all the rare gases, of the atomic Li $3d$ and $3p$ states.

The angular and spectral distributions of the radiation emitted by metastable $n=2\mathrm{}$ hydrogenic systems in arbitrary external electric and magnetic fields are analyzed. Special attention is given to level crossings as a means to enhance some of the final-state interactions which are possible only through the coupling with the radiative sector and the radiative reaction. A complete classification of the final-state interferences is given in terms of the possible scalar invariants and their symmetry with respect to time reversal. It is shown that the final-state interferences can dominate the radiative decay at one of the level crossings where the decoupling of the electronic and nuclear spin is important. The evaluation of the line shapes of these final-state interferences through the relevant level crossings allows one to consider their applications to lifetime measurements. Finally the presence of very small radiative corrections and recoil effects are analyzed and their physical consequences are discussed.

A new representation of the SU ($N$) coherence vector is introduced. $N$-level systems with two or more independent modes are found, using simple group-theoretical considerations. The Cook-Shore system is shown to contain $N$ independent modes, each providing an additional constant of motion.

We investigate the interaction of spontaneous collective radiation with incoherent pumping and damping mechanisms. In discussing a recent experiment of two-color superfluorescence, we allow for two optically active level pairs which have their upper level in common. Satisfactory agreement with the experimental results is achieved.

A newly developed numerical method of solving nonlinear Fokker-Planck equations is applied to study an acousto-optic bistability with additive noise. The time evolution of the probability distribution for the transmitted light is calculated for a double-well potential. The time-dependent moments are calculated.

The stability is investigated of helical trajectories of relativistic electrons in combined axial guide and helical wiggler magnetic fields appropriate for studies of free-electron lasers without using the frequently invoked assumption of evaluating the wiggler field on the laser axis. This is the first analysis of such trajectories which is free of this approximation. It is found that the helical trajectories are unstable with the possible exception of one value of the orbital radius. The instabilities are of two types corresponding to different properties of the eigenvalues of the matrix associated with the linearization of the equations of motion. These properties, in turn, depend upon the relative signs and magnitudes of the physical parameters that occur in the problem. At the exceptional radial value the trajectories can be either unstable or linearly stable depending upon the values of these parameters. For the linearly stable situations it is not known whether the trajectories are nonlinearly stable or unstable. We also prove that the trajectories of one of the instability types are of a particular kind called conditionally stable. This type of unstable trajectory has the property that it is approached by some (but not all) solutions of the equations of motion as time approaches positive or negative infinity. This type of instability is found to be more severe for larger values of the orbital radius. Even so, it still appears to be significant for small radii, of the order of $\frac{1}{100}$ of the wiggler wavelength.

In a previous paper [P. R. Berman and E. Giacobino, Phys. Rev. A 28, 2900 (1983)], a method was proposed for producing electronic-state coherences using radiatively-aided inelastic collisions (RAIC). Two atoms collide in the presence of two simultaneously applied laser pulses and the combined action of the fields and the collision leads to a final-state coherence ${\rho }_{\mathrm{ij}}$ between levels $i$ and $j$ in one of the atoms. In this paper a detailed calculation of the RAIC cross section is given, including effects of magnetic degeneracy and an average over all collision orientations. With an isotropic distribution of collision orientations and with one photon absorbed from each laser field in a single RAIC reaction, a final-state coherence ${\rho }_{\mathrm{ij}}$ is created only if states $i$ and $j$ have the same parity. In order to produce a final-state coherence between states of opposite parity, an anisotropic distribution of collision orientations is needed. Estimates of RAIC cross sections are obtained for both isotropic and axially symmetric distributions of collision orientations. For final states having opposite parity, the RAIC reaction can lead to the production of coherent radiation at the sum or difference frequency of the applied laser fields.

Measurements of microwave Čerenkov radiation in air show that the Čerenkov cone angle is broadened as predicted in earlier work. Extension of previous theory shows that for small Čerenkov angles, the broadening is asymmetric, giving an increase in the effective cone angle to a value in agreement with the experimental results. The shift in the radiation peak can be understood as a diffraction effect arising from the linearly varying phase of the radiation along the beam interaction length.

Phase noise of a one-dimensional laser with output coupling that is oscillating well above threshold is analyzed on the basis of a multimode formulation of the field. The phase diffusion, both in time and space, is calculated for the output beam taking into account the lowest-order moments of the thermal and the quantum noise forces. Thermal noise is introduced automatically without assuming any absorbing atoms or heatbaths, and is interpreted as the amplified, ambient thermal radiation that penetrates the laser cavity. Quantum noise is treated by the quantum Langevin method. The resulting linewidth formula differs from that of a conventional theory by factors that are determined by the degree of output coupling and by the nonuniform distribution of the saturated gain in the cavity.

The Schrödinger equation in the rotating-wave approximation has been solved nonperturbatively to study the temporal behavior of the occupation probabilities of different levels of a four-level atom interacting with four arbitrarily intense laser fields, such that the frequency of one of them equals the sum of the other three. The three-photon-excitation population dynamics, which is already complex, is strongly modified by one-photon excitation and detunings. Under certain conditions the three-photon excitation traps the population in the state initially populated; the population starts flowing into other states when one-photon excitation is also operative.

Propagation characteristics of Gaussian Schell-model fields through first-order optical systems and in free space are analyzed by the method of generalized rays. This allows the development of a simple geometrical description of these processes. The invariance of the degree of global coherence is established in full generality. Asymptotic behavior under free propagation and the emergence of a far-zone universal structure are analyzed. New invariants associated with incoherent superpositions of such fields are found.

A self-consistent, completely-quantum-mechanical formalism has been developed that characterizes the fundamental atomic properties of ions in dense plasmas. This theory is applied to neon plasmas under strongly coupled conditions and the results are compared with those obtained from the hypernetted-chain approximation employing a semiclassical two-body interaction. The comparisons point out the lack of validity of simple approximations for atomic calculations in strongly coupled plasmas and indicate that the self-consistent theory provides a method of obtaining meaningful results even when the plasma ion coupling parameter is of order 5.

Spectroscopy of line radiation from plasma impurity ions excited by charge-exchange recombination reactions with energetic neutral-beam atoms is rapidly becoming recognized as a powerful diagnostic for magnetically confined tokamak plasmas. Ion temperature, bulk plasma motion, impurity transport, and more exotic phenomena such as fast alpha-particle distributions can all be measured with this technique. In particular, it offers the capability of obtaining space- and time-resolved ion temperature and toroidal plasma rotation profiles with relatively simple optical systems. Cascade-corrected excitation rate coefficients for use in both fully stripped impurity density studies and ion-temperature measurements have been calculated for the principal $\Delta n=1\mathrm{}$ transitions of ${\mathrm{He}}^{+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{O}}^{7+}$ with neutral-beam energies of 5-100 keV/amu. Line intensities and profiles can be affected by atomic fine structure, $l$-mixing collisions, motional Stark effects, and product ions created in the neutral-beam region which drift into the viewing sightline. General estimates of the importance of these effects for the transitions of interest are provided, along with specific examples calculated for the PDX (Poloidal Divertor Experiment) and TFTR (Tokamak Fusion Test Reactor) tokamaks. A fiber optically coupled spectrometer system has been used on PDX to measure visible ${\mathrm{He}}^{+}$ radiation excited by charge exchange to illustrate some of these points. Central ion temperatures up to 2.4 keV and toroidal rotation speeds up to 1.5×${10}^7$ cm/s were observed.

The liquid-structure factors of fluid rubidium are calculated for a series of states along the saturated-vapor-pressure curve, ranging from temperatures close to the melting point up to the critical point. The calculation is based on the optimized random-phase approximation and on effective interatomic pair potentials derived from pseudopotential perturbation theory. The results are compared with the static-structure factors measured using elastic neutron scattering. Good agreement is found for temperatures up to $T=1400\mathrm{}$ K. From the divergence of $S\left(0\right)\mathrm{}$ the critical temperature is calculated to be $T_c=2120\mathrm{}\pm 5$ K, compared with an experimental value of $T_c=2090\mathrm{}\pm 20$ K. The calculation reproduces the strong increase of the long-wavelength scattering, but we find it to be confined to a narrower temperature interval below $T_c$. We bring evidence that this is due to the breakdown of the nearly-free-electron approximation in a regime where the mean free path of the electrons becomes smaller than the average interatomic distance.

The space-time memory function associated with the dynamical structure factor is expressed in terms of current-current and density-current correlation functions. We then apply the binary-collision expansion to evaluate these correlation functions. In this approximation, we include exactly all the effects of a single collision between a pair of particles. The dynamical structure factor is then evaluated from the memory function and compared with the neutron scattering data on dense krypton gas. The agreement is very good even for wave numbers below the first diffraction peak in the structure factor, indicating that the spatial correlations are included in this theory.

We report experimental measurements and calculations using a model on a driven, dissipative, dynamical system which shows chaotic behavior. The system is the diode resonator composed of the series combination of a generator, inductor, and a $p-n$-junction diode. It is studied where there are sudden transient changes in the strange attractor, phenomena called crises by Grebogi, Ott, and Yorke, for which a universal scaling law exists. We verify the scaling law both experimentally and with model calculations. Furthermore, the Lyapunov exponent, a measure of sensitivity to initial conditions, is shown by both methods to increase rapidly but continuously through the crisis region.

The stochastic wandering of magnetic field lines allows momentum of even perfectly magnetized electrons to be transported across the mean $\overrightarrow{\mathrm{B}}$. We include this effect, along with the usual acceleration and scattering terms, in a spatially one-dimensional Boltzmann equation for the electron distribution function. For an electric field $E_{\parallel }$ (along local $\overrightarrow{\mathrm{B}}$) which varies versus position normal to $\overrightarrow{\mathrm{B}}$, the momentum transport leads to a nonlocal electrical conductivity. We apply the formalism to sheared, force-free magnetoplasmas, in which the $E_{\parallel }$ gradient is caused by variable twisting of $\overrightarrow{\mathrm{B}}$ with respect to an externally applied uniform $\overrightarrow{\mathrm{E}}$. We examine in particular the experimentally documented phenomenon of field-aligned current density $j_{\parallel }>0\mathrm{}$ in regions of the sheared magnetic field where $E_{\parallel }\approx 0$ or even $E_{\parallel }<0\mathrm{}$. This phenomenon is in apparent violation of Ohm's law. Under suitable conditions of stochasticity and collisionality, we find that the spatial structure and temporal persistence of these force-free configurations can be directly caused by electron-momentum transport. This result is derived solely on the basis of electron dynamics. In contrast to fluid-turbulent models, our kinetic derivation requires no hypothetical "plasma dynamo" and no conjecture on the decay rates of magnetic helicity versus magnetic energy.

We suggest a new geometrical interpretation of the renormalization-group transformation near the critical point. Using the Weinhold-Ruppeiner metricization, conformal Killing symmetry is assumed and scaling laws are verified.

A theory developed by the author to describe macroscopic neocortical interactions demonstrates that empirical values of chemical and electrical parameters of synaptic interactions establish several minima of the path-integral Lagrangian as a function of excitatory and inhibitory columnar firings. The number of possible minima, their time scales of hysteresis and probable reverberations, and their nearest-neighbor columnar interactions are all consistent with well-established empirical rules of human short-term memory. Thus, aspects of conscious experience are derived from neuronal firing patterns, using modern methods of nonlinear nonequilibrium statistical mechanics to develop realistic explicit synaptic interactions.

We calculate the activation rates of metastable states of general one-dimensional Markov jump processes by calculating mean first-passage times. We employ methods of singular perturbation theory to derive expressions for these rates, utilizing the full Kramers-Moyal expansions for the forward and backward operators in the master equation. We discuss various boundary conditions for the first-passage-time problem, and present some examples. We also discuss the validity of various diffusion approximations to the master equation, and their limitations.