A theoretical prediction of the photoionization spectrum of atomic calcium from the 4p threshold up to nearly the 6s threshold is obtained by an eigenchannel R-matrix calculation of the final-state multichannel quantum-defect parameters in LS coupling. The calculation shows how strong interactions are distributed among a large number (10–16) of channels and predicts an intricate spectrum of overlapping Rydberg series. Further calculations show that the ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^0{}_{}{}^{}$ Wannier-ridge states up to 11s11p obey a two-electron Rydberg formula, but that they are unusually difficult to excite directly from the ground state.

The angular distribution for Ar 2p photoionization has been measured from just above threshold to 400 eV photon energy, and calculated in the same energy range using the relativistic random-phase approximation. The present experimental and theoretical results are in good agreement, but disagree somewhat with earlier Hartree-Fock (HF) calculations. The HF values are found to be significantly higher in the near-threshold region. Possible reasons for this discrepancy are discussed with relevance to the general understanding of inner-shell photoionization phenomena.

The radiative lifetimes of the ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ metastable states of ${\mathrm{Kr}}^{2+}$ and ${\mathrm{Xe}}^{2+}$ have been measured to be 13.1±0.6 and 4.5±0.3 msec, respectively. The ions were created by electron bombardment on the neutral parent gas and contained in a cylindrical radio-frequency ion trap. A single-photon-counting photomultiplier tube was used to observe directly the subsequent spontaneous emission. The transitions observed were, for ${\mathrm{Kr}}^{2+}$, the M1 (magnetic dipole) transition at 350.5 nm from ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ to ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ in the 4$p^4$ ground configuration, and, for ${\mathrm{Xe}}^{2+}$, the M1 transition at 380.1 nm from ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ to ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ in the 5$p^4$ ground configuration. The experimental results are in fair agreement with theoretical predictions.

Certain nonclassical effects in four-wave mixing are studied within the framework of a quantum theory of multiwave mixing. It is shown that the Cauchy-Schwarz and Bell’s inequalities are violated for large detuning of the pump frequency from the side-mode frequencies.

We present results of a numerical model of the optical piston driven by laser-induced drift (LID). The model is based on a drift-velocity formulation of Lawandy, derived from an asymmetric-random-walk argument, and includes resonance-fluorescence effects. The model is applied primarily to the experimental conditions of H. G. C. Werij and co-workers [Phys. Rev. A 33, 3270 (1986)]. It is shown that both the velocity and the shape of the propagating density wave change continuously throughout the traverse of a (finite) diffusion tube under these conditions. In addition, we have numerically verified the contention of G. Nienhuis [Phys. Rev. A 31, 1636 (1985)] that a ‘‘self-preserving’’ form of the density wave is indeed possible for the case of a ‘‘finite number of absorbers in an infinite medium,’’ both with and without saturation. Finally, we show that resonance fluorescence slows the piston velocity, broadens the density distribution, and decreases its peak height. It is concluded that resonance fluorescence can play a significant role in determining the time evolution and detailed shape of the optical-piston density distribution in sodium.

We study experimentally the entrainment effects produced by an applied external signal on an acoustooptic bistable device with delayed feedback which shows the Feigenbaum route to chaos. We demonstrate the existence of normal entrainment in the system, as well as phenomena that are related to the presence of higher-order modes in the system. In the presence of an external signal we are able to force the system to change from a chaotic state to a periodic state.

For the model of Drummond and Walls describing dispersive optical bistability, stationary solutions as well as quantum-tunneling rates are calculated by solving a pseudo-Fokker-Planck equation (PFPE), i.e., a Fokker-Planck-like equation with a non-positive-definite diffusion matrix, in terms of matrix continued fractions. Due to the quantum nature of the model, an interesting oscillating variation of the decay rates of the PFPE as a function of the scaling factor for the photon number is found for small cavity damping.

Distribution of waiting-time intervals between the arrivals of successive photons on a photocathode illuminated by a beam of light is discussed. Analytic expressions for the conditional and unconditional distributions for squeezed light are derived in the high degeneracy limit. Results for binomial and thermocoherent states are also given. Curves are presented to illustrate the behavior.

We generalize Lamb’s quantum theory of lasers to treat the off-resonance operation of two-transition lasers in inhomogeneously broadened media. The master equation is derived from the density-matrix equation of motion in the Doppler limit. We first find the stationary-state solution from which we then obtain such operation characteristics as the threshold condition, the mean photon number, and the half-width of photon distribution. Compared with the one-transition laser, it is found that in the two-transition laser it is much easier to start oscillation and to obtain higher gain. Furthermore, we find that the two-transition laser is less sensitive to the detuning effect and its deviation from the coherent state is smaller than that of the one-transition laser. The remarkably different hole-burning phenomenon is also discussed.

Propagation of thermoconvective waves (TCW) in an unbounded incompressible viscous heat-conducting fluid is investigated when the fluid rotates about a horizontal axis with uniform angular velocity Ω and is subjected to a constant temperature gradient β perpendicular to the axis of rotation. A novel result of the analysis is that, unlike the nonrotating case, undamped TCW can propagate (with β>0 such that there is no thermal instability) if 2$P^2$S<(1+P)Pγ<2S, where P=ν/κ, S=4(ν/$g^2$${)}^{2/3}$${\Omega }^2$, and γ=(${\nu }^2$/g${)}^{1/3}$αβ. Here, ν, κ, α, and g denote the kinematic viscosity, the thermal diffusivity, the fluid volume-expansion coefficient, and gravitational acceleration, respectively. It is also shown that, unlike the nonrotating case where weakly damped low-frequency TCW can propagate when the fluid is heated from below, such waves in a rotating fluid are possible if β>0 and Pγ>S. It turns out that when a layer of finite thickness d rotates about a horizontal axis and is subjected to a constant temperature gradient β perpendicular to the rotation axis, weakly damped low-frequency TCW can propagate without violating the condition of thermal stability if 8/3${\mathrm{\pi }}^4$≤R-${\mathit{S}}_2$<27/4${\mathrm{\pi }}^4$, where R=gαβ${\mathit{d}}^4$/κν and ${\mathit{S}}_1$=4${\mathrm{\Omega }}^2$${\mathit{d}}^4$/${\nu }^2$.

A Monte Carlo model is used to simulate the morphological evolution of crystals growing from a vapor phase. The model combines nutrient diffusion, based on a modified diffusion-limited aggregation (DLA) process, with anisotropic surface-attachment kinetics and surface diffusion. First- and second-nearest-neighbor interactions are taken into account. Through a systematic variation of the simulation (growth) parameters, the whole range of growth morphologies observed in nature is recovered. Depending on the imposed temperature, supersaturation, and bond energies, the model crystals are found to grow either fully faceted or with (dendritic) protrusions which, in turn, can either be branchless or develop side branches with smooth or split tips. The successive transitions from compact faceted to open dendritic morphologies are seen to arise as a collective result of nutrient diffusion and interface kinetics. The scale invariance exhibited by DLA does not generally hold in (dendritic) crystal growth. Thus, this work is not only an extension of previous DLA-type simulations, but affords physical insight into the nature of the interaction of crystal morphology with surface and volume processes.

The properties of the liquid-vapor interface for three simple liquids (water, carbon tetrachloride, and methanol) have been measured using x-ray reflectivity. The measured surface roughness is interpreted using a model that combines the effects of thermally induced capillary waves and the dimensions of the constituent molecules.

Various laser diagnostics are used to study the cathode-fall and negative-glow regions of a He glow discharge with a cold Al cathode. The electric field and absolute metastable densities are mapped and the gas temperature is measured over a range of current densities from a near-normal (173 V) to a highly abnormal (600 V) cathode fall. These measurements are analyzed to yield the current balance at the cathode surface, the ionization rate in the cathode-fall region, and the metastable production rate in the cathode-fall and negative-glow regions. The experimental results compare favorably with the results of Monte Carlo simulations. The density and temperature of the low-energy electron gas in the negative glow is determined by combining information from the experiments and Monte Carlo simulations.

In this paper we report on calculations of flow profiles for cylindrical Couette flow of a Lennard-Jones fluid. The flow is subjected to a temperature gradient and thermoviscous effects are taken into consideration. We apply the generalized fluid dynamic equations which are provided by the modified moment method for the Boltzmann equation reported previously. The results of calculations are in good agreement with the Monte Carlo direct simulation method by K. Nanbu [Phys. Fluids 27, 2632 (1984)] for most of Knudsen numbers for which the simulation data are available.

Resonant photoexcitation in a two-component plasma is demonstrated for the first time in the x-ray range. The 1s-3p resonance line of Al xiii at 6.053 Å is used to pump the 2${\mathit{p}}^{\mathrm{*}}$${(}^1$${\mathit{S}}_0$)-2${\mathit{p}}^5$3d ${(}^3$${\mathit{D}}_1$) line of Sr xxix at 6.059 Å. Absorption of the aluminum pump line by the strontium plasma is shown unambiguously. The resulting increase in the x-ray fluorescence intensity of the pumped level is measured in a time-resolved experiment.

We study coagulation in the flow field of a time-periodic deterministic chaotic flow and focus on the simplest case: point particles convected without diffusion and allowed to coagulate with probability 1 when the distance is less than d. An analysis of the underlying physics is presented. Under ‘‘well-mixed’’ conditions the system behaves as if the particles were moved by Brownian motion, and a simple kinetic model describes the main results. The poorly mixed case is considerably more complex. Spatial inhomogeneities result from competition between the rate of coagulation and mixing, and trapping and leaking of clusters due to Kolmogorov-Arnold-Moser surfaces.

An exact Volkov state solution of the minimally coupled Dirac equation is used to calculate the transition rate dR of an electron scattering via a stationary ion in the presence of a very intense laser field. A consistent picture of the scattering is presented in which the electrons’ initial and final states are quasifree states. Accordingly, a modified transition rate ${\mathrm{dR}}^{\mathrm{*}}$ and a modified Maxwell-Boltzmann distribution are developed. They are used to calculate the heating rate W of the isotropic part of a quasifree neutral plasma in the presence of very intense laser light. In order to simplify the expression for the heating rate W, an important transformation, which changes an infinite sum over Bessel functions into a finite integral, is introduced. It is then shown that the leading term of the heating rate W is similar to the expression of Osborn (with corrections) for intensity I${<10\mathrm{}}^{16}$ W/${\mathrm{cm}}^2$ and $k_B$T<1 keV. A new correction factor is defined to show the effect of a very intense laser field when the intensity I${>10\mathrm{}}^{16}$ W/${\mathrm{cm}}^2$. For $k_B$T>1 keV, a spin-dependent term of order $k_B$T/${\mathrm{mc}}^2$ is also discovered. This represents a new term not previously known. We show that the effect of this term on the heating rate is substantial and it would be possible to measure its effect with present-day laser systems.

The Boussinesq equations for thermohaline convection in a finite two-dimensional box and with stress-free boundaries are considered. There are critical values of the aspect ratio at which the conduction state becomes unstable to two different roll patterns simultaneously. Near such a critical value a center manifold reduction allows us to reduce the dynamical behavior of the Boussinesq equations to a standard normal form equation that describes the interaction of two stationary modes. We present explicit analytical expressions for the linear and nonlinear coefficients on which the normal form depends. A numerical investigation of these coefficients leads to a division of the space of parameters (Prandtl number, solute Rayleigh number, Lewis number) into various regions that give rise to qualitatively different bifurcation behavior. Besides those encountered in ordinary convection, a variety of further phenomena is found, in particular in a vicinity of double tricritical points.

Using a relaxation theory of ion broadening we have performed calculations for ${\mathrm{L}}_{\mathrm{\alpha }}$, ${\mathrm{H}}_{\mathrm{\alpha }}$, ${\mathrm{P}}_{\mathrm{\alpha }}$, and ${\mathrm{L}}_{\gamma }$ for neutral hydrogen radiators. Our calculations span a wide range of electron density, temperature, and μ, the reduced mass of the radiator and ion perturber. We identify four regions of qualitatively different influence of ion motion on the half widths at half maximum of hydrogenic line profiles. At sufficiently small μ, the ion dynamics are impact in nature, while at very large μ they are quasistatic. The half width increases with μ in the impact region, and decreases with μ as the quasistatic region is approached. In between these latter two regions, the half width reaches a maximum. We observe a similar dependence of the reduced width as a function of the plasma density. The temperature dependence is discussed as well. The ion dynamic influence on the half width is determined by the ratio of the relevant collision duration to the collisional coherence time for the transition. We discuss validity criteria for the ion-impact and quasistatic limits.

The physics of beam-plasma interaction, which has been investigated for a long time mostly in relation with solar bursts, is now more widely invoked in various astrophysical contexts such as pulsars, active galactic nuclei, close binaries, cataclysmic variables, γ bursters, and so on. In these situations the interaction is more likely in the spirit of strong Langmuir turbulence rather than in the spirit of quasilinear theory. Many investigations have been done for two opposite extremes, namely, in very weak and in very strong magnetic fields. Very few properties of the strong Langmuir turbulence are known in the most usual astrophysical situation where the magnetic field plays a significant role but is not strong enough to force the electrons into one-dimensional motion. For this case, we analyze the dynamics of Langmuir wave packets and provide new results about the stability of the solitons against transverse perturbations. It turns out that both the averaged Lagrangian method and the adiabatic perturbation method derived from the inverse scattering transform give exactly the same results (which is not obvious in soliton perturbation theory). In particular, they predict the stability of the solitons as long as the electron gyrofrequency is greater than the plasma frequency (strong magnetic field) and their instability against transverse self-modulation in the opposite case (weak magnetic field); moreover, they allow one to deduce the self-similar collapsing oblate cavitons in the latter case. The laws governing the collapse of the wave packets determine the relaxation of the beam in the surrounding medium and we derive a useful formula giving the power loss of the beam. We outline the astrophysical consequences of this investigation.