The full set of Weisskopf-Wigner equations describing the spontaneous radiative decay of a system of up to four excited two-level atoms and many field modes is integrated numerically. Measures of collimation and of photon-photon directional correlation are evaluated and demonstrate the tendency of the photons to form a single ray. Decay rates of the initial state are found to be proportional to the number of atoms, as expected from analytical work.

The Jaynes-Cummings model describes the interaction between a two-level atom and a single mode of the radiation field. This system has a conservation law which restricts its dynamics. On the basis of this conservation law alone one can make useful statements about the photon statistics of the fields which arise in the model. Two cases are considered. In the first the field is initially in a coherent state. Here it is found that the field statistics become less sub-Poissonian as the amplitude of the initial state increases. The case of an initial thermal state is also considered. In that situation, if the average number of photons present in the initial state is greater than 2.55, then the field statistics can never become sub-Poissonian.

The distinguishing feature of the polarization-symmetric (PS) laser is the decrease of its partial intensity linewidths with increasing pump rate far above threshold. This property is not found in the single-mode (SM) laser with fixed polarization. We study the transition from the PS-laser properties to the SM-laser ones if symmetries of the cavity and of the laser-active gas are broken more and more. We restrict ourselves to disturbances that affect the partial intensity distribution, either by influencing the partial threshold or the stimulated emission. In both cases we explicitly give the stationary distributions of the photons of various polarizations.

The effect of cross relaxation, i.e., the transfer of coherence between transitions coupled by inelastic collisions, on third-order nonlinear processes, namely, saturated absorption and four-wave mixing, is studied within the context of a model four-level system. Numerical results displaying changes in the resonant structures in four-wave-mixing signals for a range of collisional parameters are presented.

In this paper we study connections between multilevel systems of different complexity. We show that the structurally simplest multilevel configurations are serial and parallel. Using unitary transformations we seek for the change of representation which allows us to cast an arbitrary N-level Hamiltonian matrix in serial or parallel form. For three- and four-level systems we are able to construct the transformations in analytical form. These transformations make it possible to consider conditions by which three- and four-level systems can have decoupled subsystems and we apply this to study coherent population trapping. We also investigate unitary transformations which transform a parallel configuration to serial and vice versa. The dynamical features in these two special systems are compared in the light of the transformations. For example, we relate the recurrence dynamics of the parallel system to the population sloshing dynamics of the serial system. We also show that an equally coupled and detuned infinite serial system is capable of trapping a part of the population.

We present a numerical and analytical study of a multimode Jaynes-Cummings model describing the spontaneous decay of a single atom in a high-Q cavity. The theory of cavity-modified spontaneous emission is discussed in terms of quantities that have clear physical interpretations: the photon wave packet radiated by the atom, its reflection by the cavity boundaries, and its reabsorption by the atom. Multimode corrections to the single-mode Jaynes-Cummings model are calculated. The multimode corrections are formulated in terms of quantities that may be calculated by solving the single-mode equations of motion.

The assumed existence of a time-independent Kubo-type operator renders possible an analytic expression for the evolution operator of a quantized system. For a multilevel system dipole interacting with a continuous-wave laser, the realization of a suitable Kubo-type transformation through an algebraic algorithm may require that the Hamiltonian operator of the system be simplified through, say, the imposition of the rotating-wave approximation and/or the appeal to transition dipole selection rules that validate the neglect of unimportant couplings. The resultant sparse Hamiltonian operator is arbitrarily constructed on the basis of the near-resonant laser coupling of the ground state to the excited states, but without direct appeal to the magnitude of the dipole coupling strength parameters. The implementation and associated computational aspects of a resolvent method for evaluating the evolution operator is discussed and illustrated through the calculation of the transition probabilities for the excitation of an anharmonic oscillator model of the CH stretch in ${\mathrm{CD}}_3$H.

We investigate the self-oscillations of an induced absorber (CdS, photothermal effects) in a hybrid ring cavity. If the induced absorber is intrinsically bistable, we show that for a given set of system parameters different oscillation modes may exist depending on the initial conditions. In contrast to the behavior of an intrinsically bistable absorber, we find a bifurcation route to optical chaos if the induced absorber is not intrinsically bistable.

A new technique is introduced in which a canonical transformation is used to transform the atom-radiation Hamiltonian to one without counter-rotating terms. As well as allowing an exact treatment of a one-mode dipole-coupled photodetection model, the theory predicts the existence of squeezing induced by the detector coupling. A feature of the canonical transformation used here is that the resulting interaction Hamiltonian has a symmetric combination of the p⋅A and d⋅E interactions. This allows the resolution of long-standing questions about the correct interaction Hamiltonian to use in photodetector theory. The new theory is able to handle dipole-coupled radiative transitions over ultrafast time scales of the order of a single oscillation period, where the rotating-wave approximation is invalid. The coupling of the detector to the radiation field is predicted to vary as $f^{\mathrm{-}1/2}$ at high frequencies, and as $f^{+1/2\mathrm{}}$ at low frequencies.

The transverse current correlation function is studied in liquid rubidium by computer simulation, and the associated memory function is directly determined from the data, for a number of wave vectors. The usual phenomenological relaxation-time approximations for the memory function are shown to be inadequate, particularly in the wave-vector range where well-defined shear waves are supported by the liquid. A comparison of the results with new data obtained for a Lennard-Jones system is also made, and the dynamical processes contributing to the structure of the memory functions are assessed. The related generalized wave-vector-dependent shear viscosity, η(q), is derived and its importance to the applicability of a microscopic Stokes-Einstein relation finally discussed. It is shown that the details in η(q), which are revealed by the molecular-dynamics data, play a vital role in establishing this relation in liquid rubidium.

The stability properties of stationary, cylindrically symmetric solutions of the scalar and vectorial exponential nonlinear Schrödinger equation are analyzed in two spatial dimensions. Both equations are fundamental models describing envelope solitons, like Langmuir cavitons in plasma physics and laser pulses in nonlinear optics. The scalar as well as the vectorial equations have a saturating nonlinearity which allows us to treat finite amplitudes. Using a Liapunov functional for stability, we prove that scalar envelope solitons are two dimensionally completely stable. This has to be seen in contrast to previous results for the three-dimensional case where only large-amplitude solitons have been found to be stable. Localized solutions of the vectorial equation are stable to perturbations which only depend on the radius (longitudinal stability). Thus, no radial collapse of initially well-behaved plasma cavitons occurs in two spatial dimensions. For azimuthally dependent electrostatic perturbations we find instability. This indicates that vectorial envelope solitons suffer a symmetry-disturbing transversal collapse.

Spectra of heliumlike nickel, Ni xxvii, have been observed from Tokamak Fusion Test Reactor (TFTR) plasmas with a high-resolution crystal spectrometer. The experimental arrangement permits simultaneous observation of the heliumlike resonance line, the intercombination and forbidden lines, and all the associated satellites due to transitions 1$s^2$nl-1s2l’nl’’ with n≥2. Relative wavelengths and line intensities can thus be determined very accurately. The observed spectral data are in good agreement with results from the present Hartree-Fock-Slater atomic model calculations and predictions from the Z-expansion method.

The dynamics of planar plastic targets seeded with small amounts of sulfur as a diagnostic tracer is studied spectroscopically. The time-dependent x-ray emission of the hydrogenic and heliumlike sulfur is used to provide a measure of the plasma parameters. The experiment, which is designed to provide optically thin x-ray lines with small transverse gradients, is analyzed by the coupling of a kinetics model to a hydrodynamics simulation. The results of the analysis show that the time histories of the x-ray line intensities can be used as a plasma diagnostic to constrain the modeling. Intensity ratios between ionization stages cannot be accurately modeled even including time-dependent ionization effects. This disagreement appears to be due to detailed assumptions concerning electron distributions and transport.

The static correlations in highly charged colloidal and micellar suspensions, with and without added electrolyte, are examined using the hypernetted-chain approximation (HNC) for the macro-ion–macro-ion correlations and the mean-spherical approximation for the other correlations. By taking the point-ion limit for the counter-ions, an analytic solution for the counter-ion part of the problem can be obtained; this maps the macro-ion part of the problem onto a one-component problem where the macro-ions interact via a screened Coulomb potential with the Gouy-Chapman form for the screening length and an effective charge that depends on the macro-ion–macro-ion pair correlations. Numerical solutions of the effective one-component equation in the HNC approximation are presented, and in particular, the effects of macro-ion charge, nonadditive core diameters, and added electrolyte are examined. As we show, there can be a strong renormalization of the effective macro-ion charge and reentrant melting in colloidal crystals.

We report results from an experimental study of superthermal electron energy transport in planar layered disk targets irradiated at high intensities (I>3×${10}^{14}$ W/${\mathrm{cm}}^2$) by short (<100-psec) pulses of Nd-laser radiation. The Kα emission from a Ni fluor covered by an Al electron-transport layer has been recorded both spatially integrated and spatially resolved over the target surface. By analyzing the results, with the aid of a Monte Carlo electron-transport code, it has been found that electron transport within the target is dominated by a non-Maxwellian group of very energetic electrons ($E_e$≊50–200 keV) which carry about 3% of the absorbed laser energy deep within the targets. The spatial distribution of the electrons over the target surface underlines the effect of the coronal magnetic and electric fields on lateral energy transport by superthermal electrons.

In this paper we present an experimental study of quasiperiodicity in a hydrodynamical system. Partial results have already been reported. We present them now in more detail as well as the experiment itself. The system studied is Rayleigh-Bénard convection in mercury in a time-dependent state with one limit cycle. An external oscillation is coupled to this limit cycle by passing an ac electrical current through the mercury and embedding the system in a horizonal magnetic field. We have measured the fractal dimension of the complement of all locked states on the critical line. Local properties of this line, such as scaling indices, were also measured for two different irrational routes followed. The results are in good agreement with quantitative predictions of the circle map.

The critical behavior of the shear modulus of agarose gel was studied as a function of concentration and temperature. Experimental evidence is presented that the elasticity exponent agrees with the conductivity exponent of a percolating resistor network. It is found that the value of the elasticity exponent of agarose gel is independent of the path to the phase boundary. These results strongly suggest that the elastic behavior of gel in the critical region can be explained by the scalar elasticity model of percolation theory irrespective of the mechanism of gelation.

A theory is constructed for the nonequilibrium statistical mechanics of a many-particle system, including effects of multiparticle correlations. The particular system studied is a dense monatomic fluid, whose atoms interact by pairwise central forces. The nonequilibrium fluid possesses coupled position and momentum correlations, of which the two-particle correlations are treated explicitly, while three- and more-particle correlations are included in a mean-field approximation. A truncated gradient expansion is applied, representing the condition that the one- and two-particle probability densities vary by a relatively small amount under translation through a distance not larger than the correlation length. New concepts which are used in the theoretical construction include the following: a localized nonequilibrium potential of mean force; a nonequilibrium h function and the corresponding h currents; and interaction integrals, which are the counterpart of Boltzmann’s collision integral, and which are based on effective two-particle interactions that conserve particle number, momentum, and a statistically appropriate energy. The resulting theory consists of two coupled evolution equations, one for the one-particle probability density, and one for the two-particle correlation function. The evolution equations satisfy the continuum equations for conservation of particles, of momentum, and of energy; the evolution equations also satisfy an h theorem, whose source function is a Lyapunov functional, and whose equilibrium solution gives the correct equilibrium values for the one-particle probability density and the two-particle correlation function. The simplified theory that results from neglecting two-particle momentum correlations is also presented.

We solve the linearized problem for the onset of convection in binary fluid mixtures in a layer of infinite lateral extent for rigid-rigid, no-flux boundary conditions exactly. We present detailed data for the critical Rayleigh number and for the neutral frequency as a function of the separation ratio for the two experimentally most relevant systems: for ethanol-water and ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixtures. For both the oscillatory and the stationary onset we compare our results with those obtained for simple trial functions. The influence of the effect of rigid-rigid, no-flux boundary conditions on the location of the codimension-2 point is examined.

We present theoretical results for the critical temperature for Bose-Einstein condensation, condensate fraction, and heat capacity of a gas of Bose particles that are confined by a generic power-law potential trap. All three of these quantities are found to vary markedly with the shape of the potential. Both the ideal and the weakly interacting Bose gas are considered.