We report on an improved measurement of absolute cross sections for photoionization from an excited Ba i state. In a barium oven irons are produced using a two-step pulsed-laser excitation and ionization and are detected by applying the thermionic diode technique. The measured total cross sections show pronounced structures near the ionization threshold, which can partly be associated with recent calculations performed by Bartschat and McLaughlin [J. Phys. B 23, L439 (1990)] and which are in excellent agreement with the calculation of Greene and Theodosiou [Phys. Rev. A 42, 5773 (1990)]. The absolute scales are also comparable. Our revised threshold value of 80 Mb is in sufficient agreement with their theoretical values, 120 and 100 Mb, respectively.

Circular dichroism in photoelectron angular distributions (CDAD) is used to examine the alignment of the 7${\mathit{P}}_{3/2}$ and 7${\mathit{P}}_{1/2}$ levels of cesium, prepared by absorption of linearly polarized laser radiation. As expected, the 7${\mathit{P}}_{1/2}$ level exhibits no CDAD, whereas the 7${\mathit{P}}_{3/2}$ level shows a significant CDAD signal, although smaller than that predicted by theory. The difference between experiment and theory is attributed to ${\mathit{m}}_{\mathit{J}}$ mixing due to hyperfine coupling during the finite ionization time of the laser pulse.

The S 2${\mathit{p}}_{3/2}$–S2 ${\mathit{p}}_{1/2}$ spin-orbit splitting is measured for the free ${\mathrm{H}}_2$S molecule using both high-resolution x-ray photoelectron spectroscopy (XPS) and high-resolution photon-excited Auger-electron spectroscopy. It is found that the spin-orbit splitting differs in the two experiments by as much as 50 meV. In contrast, the Ar 2${\mathit{p}}_{3/2}$–Ar 2${\mathit{p}}_{1/2}$ spin-orbit splitting is practically the same when comparing Auger and XPS data. The origin of the shift for ${\mathrm{H}}_2$S is discussed with respect to molecular field splitting and state interference effects.

The coherent interaction of light with a Bose-condensed gas via atomic electric dipole transitions produces strong mixing of photons and excited atoms. Consequent optical properties near resonance include substantial reflection from the edge of the gas.

A spectroscopic measurement of the 1${\mathit{s}}^2$2s7I-1${\mathit{s}}^2$2s8K Rydberg transition energy in berylliumlike S xiii is presented as a high-Z test of long-range electron-ion interactions. The observed deviation from the Dirac energy (7×${10}^{\mathrm{-}4}$) is compared with atomic core-polarization models and with relativistic atomic structure calculations.

An intense single-mode correlated-spontaneous-emission laser or maser can be realized by driving the active atoms coherently with an injected external field. The scheme involves single-photon transitions unlike the two-photon or quantum-beat correlated-emission lasers that utilize correlations between successive photon-cascade emissions or between simultaneous emissions into modes of the field. Quenching of both the amplitude and phase noise and, in certain cases, squeezing of the amplitude fluctuations are found.

Energy sharing among the globally coupled cavity modes of a chaotic laser is characterized by probability distributions of the output intensity. While the distribution for an individual mode intensity is highly non-Gaussian, the distribution for the total intensity is determined to be approximately Gaussian. Numerical integration of the nonlinear coupled differential equations that describe the laser dynamics yields results that agree very well with experimental observations.

Nonunitary time evolution of an initially highly squeezed state under continuous photon-number measurement is investigated using the quasiprobability distribution. It is found that the measured quantum state bifurcates into two macroscopically distinguishable states upon the detection of a single photon, while preserving purity by feeding back real-time readout information.

This paper examines the general percolation problem of cutting randomly centered insulating holes in a two-dimensional conducting sheet, and explores how the electrical conductivity σ decreases with the remaining area fraction. This problem has been studied in the past for circular, square, and needlelike holes, using both computer simulations and analog experiments. In this paper, we extend these studies by examining cases where the insulating hole is of arbitrary shape, using digital-image-based numerical techniques in conjunction with the Y-∇ algorithm. We find that, within computational uncertainty, the scaled percolation threshold, ${\mathit{x}}_{\mathit{c}}$=${\mathit{n}}_{\mathit{c}}$〈${\mathit{L}}_{\mathrm{eff}}^2$〉=5.9±0.4, is a universal quantity for all the cases studied, where ${\mathit{n}}_{\mathit{c}}$ is the critical value at percolation of the number of holes per unit area n, and 〈${\mathit{L}}_{\mathrm{eff}}^2$〉 is a measure of ${\mathit{n}}_{\mathit{I}}^{\mathrm{-}1}$, the initial slope of the σ(n) curve, calculated in the few-hole limit and averaged over the different shapes and sizes of the holes used. For elliptical holes, ${\mathit{L}}_{\mathrm{eff}}$=2(a+b), where a and b are the semimajor and semiminor axes, respectively. All results are well described by the universal conductivity curve: σ/${\mathrm{\sigma }}_0$=[(1-x/5.90)(1+x/5.90-${\mathit{x}}^2$/24.97)(1+x/3.31${)}^{\mathrm{-}1}$${]}^{1.3}$, where x=${\mathit{nL}}_{\mathrm{eff}}^2$, and ${\mathrm{\sigma }}_0$ is the conductivity of the sheet before any holes are introduced.

Experimental results are reported concerning the possibility of reducing and even suppressing chaoticity in a bistable magnetoelastic beam system by means of parametric periodic perturbations. The experimental parameters are chosen such that a strange attractor is observed. Then a parametric perturbation is added. When its frequency approaches some resonant value, laminar phases are observed of increasing duration up to complete regularization of the motion at exact resonance.

A theory for the solubility of small particles in static structures has been developed. The distribution function of the solute in a frozen solid has been derived in analytical form for the quantum and the quasiclassical cases. The solubility at infinitesimal gas pressure (Henry’s constant) as well as the pressure dependence of the solute concentration at elevated pressures has been found from the statistical equilibrium between the solute in the static matrix and the ideal-gas phase. The distribution function of a solute containing different particles has been evaluated in closed form. An application of the theory to the sorption of methane in the computed structures of glassy polycarbonate has resulted in a satisfactory agreement with experimental data.

The bifurcation sets of driven strictly dissipative nonlinear oscillators are compared in terms of phase diagrams and fixed-point diagrams. The comparison reveals distinctive bifurcation patterns that occur for all models. In particular, two subpatterns of bifurcation curves within the period-doubling hierarchy can be identified. The results suggest that there exists a universal bifurcation structure for oscillators of the type investigated.

This paper investigates the influence of geometric anisotropy of two-dimensional percolation clusters on diffusion. Monte Carlo simulation indicates that the ensemble of all random walks starting from any given origin is typically anisotropic as determined by the local geometry of the cluster. Single-particle (or tracer) diffusion is described by the position-correlation matrix (PCM). A mean-field equation is developed for the PCM of the random walker in terms of the moment of inertia tensor of the cluster as a function of the chemical distance from the origin. In contrast, the ensemble of all random walks starting from all possible origins, weighted by the stationary probabilities, leads to essentially isotropic diffusion due to self-averaging over many regions within the cluster. The diffusion of an ensemble of particles in the latter case may be characterized by the velocity-correlation matrix (VCM). It is proved that the VCM is symmetric when a detailed balance condition holds and stationary initial conditions are used. An evolution equation for the VCM is constructed using a generalized Langevin equation with a power-law friction kernel. The kernel is a phenomenological characterization of the diffusion process in a fractal medium.

A local analysis for simple curves is performed using the geometric notions of size and deviation of an arc. For typical parametrized curves, the rate of decrease of these averaged measurements is related to the value of the fractal dimension. A method for calculating this dimension is derived, generalizing the variation method used for graphs of continuous functions to a large family of curves. Our method, the constant-deviation variable-step method, is tested on various types of curves (fractal Brownian motion, Gosper curve, and geographical coastline). A comparison is made with other methods such as the divider-step method, variation method, and diameter method. We show that our method offers the same level of performance while exhibiting much more general applications.

The stability of the solution of the hypernetted-chain (HNC) integral equation for the pair-correlation function of the fluid, with respect to its defining diagrammatic iteration loop, is investigated in one, two, and three dimensions for the inverse-power potentials characterized by a single dimensionless coupling parameter Γ. The Onsager limit and the solution of the HNC equation belong to the same basin of attraction with respect to the diagrammatic iterative map. A connection is established between (1) convergence properties of the diagrammatic low-density Mayer expansion, (2) the asymptotic strong-coupling, Γ→∞ (‘‘Onsager’’) limit of the HNC integral equation, and (3) the freezing density of simple liquids.

We add an on-site potential to the integrable lattice nonlinear Schrödinger equation and show how a number of interesting and novel features can be understood with the help of a simple soliton collective variable approximation. Results include: trapping of a soliton in a linear potential and on a maximum of a smooth potential; trapping of a soliton on a repulsive impurity and breaking into two solitons beyond a critical impurity strength; and a crossover from a soliton state to a local impurity mode upon increasing the strength of an attractive potential. In addition, we prove and illustrate the complete integrability of the system for a linear on-site potential. Results are compared with those for a nonintegrable discretization of the cubic Schrödinger equation.

We study analytically the evolution in the structure of a reacting, nonhomogeneous system. Under incomplete mixing, the reactants often display arrays of alternating striations, whose width and structure change during the course of the reaction. The problem can be modeled using results from the theory of zero-level crossings for stationary Gaussian processes. This allows us to obtain analytical expressions for the distribution of striation thicknesses. At large times this distribution turns out to be universal, depending only on the dimensionless width ζ=x/ √8Dt . We support our findings by direct numerical simulations of the reacting system.

The dissipative standard map is studied in both the regular and chaotic regimes using a numerical technique that allows us to obtain the stable and unstable periodic orbits of this map in a systematic way. The method applies in all limits of the map, including the Hamiltonian limit (where the dissipation parameter is b=1) and the strongly dissipative limit (b=0), where the map becomes the one-dimensional circle map. Using our technique we calculate the phase diagram of the map both below the critical line, where the dynamics is dominated by mode locking to stable periodic orbits, and above it, where chaotic behavior appears. We then study the scaling properties of the Arnold tongues as the coefficient of the nonlinear term, V, increases from zero. We find a scaling relation between the width of the Arnold tongue (with w=p/q) and the Liapunov exponent of the most stable orbit in that tongue, which both scale like ${\mathit{V}}^{\mathit{q}}$. This result is obtained numerically and confirmed by an analytic argument.