We have calculated two linear polarization effects in the elastic scattering of x rays and γ rays (in the energy range a few keV to little above 1 MeV) from targets with Z ranging from 13 to 92, using Rayleigh scattering amplitudes obtained numerically with the procedures of Kissel et al. We consider both polarization of an unpolarized beam in scattering and asymmetries in the scattering of an initially polarized beam. Contrary to the simple form-factor predictions these polarization effects depend on atomic number (Z) and photon energy (E). Results obtained using only K-shell Rayleigh amplitudes are not too different from those using total-atom amplitudes. In some cases polarization properties are more sensitive to Delbrück amplitudes than the differential cross sections. Theoretical predictions are compared with the limited experimental data for both types of measurements.

We formulate a model describing migration of population from low-l to higher-l states of the same principal quantum number in the highly excited state of the atom. The physical mechanism is the degenerate, nearly resonant Raman coupling. Specific calculations are performed for the hydrogen atom. The laser light is modeled as a monochromatic coherent or chaotic colored noise.

The $M_4$,5${\mathrm{N}}_4$,5O Auger spectra of Xe have been measured and compared with theoretical profiles. Single-particle predictions are found to deviate strongly from experiment in the case of the $M_4$,5${\mathrm{N}}_4$,5${\mathrm{O}}_1$ transitions. Multiconfiguration calculations which take into account the mixing between the 4$d^9$5$s^1$5$p^6$ and 4$d^9$5$s^2$5$p^4$5$d^1$ configurations reproduce the observed structure reasonably well.

The cubic Schrödinger equation has singular solutions in two or more space dimensions. The singularities occur at points of intense self-focusing. In this paper we present numerical results that describe the nature of the focusing singularity.

We consider in detail our earlier classical model of multiple-photon excitation of molecular vibrations in an infrared laser field, wherein the observed fluence dependence of the process is associated with chaotic variations of a pumped mode. We focus our attention first on a quasicontinuum model of background vibrational modes, which provides us with a dynamical system in which the role of chaos is neatly interpretable. Then we consider a model with only a finite number of background modes in order to corroborate the conclusions drawn from the quasicontinuum model.

Optical nutation signals are found to be modified in interesting ways by a short, intense, excitation pulse. In particular, pulses of appropriate character are found to effect a time reversal of the optical nutation signal, i.e., the nutation signal after the short pulse evolves as the mirror image of the nutation signal before the pulse. In ideal cases, all dephasing effects resulting from inhomogeneous broadening are reversed, and a nutation echo occurs when all atoms rephase in the excited state. Various excitation conditions that lead to the time reversal of optical nutation signals are considered, and numerical simulations are compared with experimental results. Finally, the rephasing considered here is compared to that observed in the case of rotary echoes.

Stimulated emission by an unaccelerated electron beam propagating through a periodically modulated dielectric is studied. The laser gain in the low-gain regime is calculated for the case of a cold, tenuous electron beam by applying the Einstein-coefficient technique in the classical limit ħ→0. In the high-gain, strong-pump regime equations for the evolution of the electron beam dynamics and of the radiation are developed using a self-consistent, one-dimensional model for the interaction. Analytic calculations of the small-signal gain, and numerical computations of the nonlinear saturation characteristics, are presented.

We analyze how phase noise degrades or, sometimes, even creates temporal structures in the laser with an injected signal and in optical bistability. In the case of a laser with an injected signal, the influence of the phase fluctuations of the driving field is considerably larger than what one might expect on the basis of intuitive arguments.

The effect of finite momentum spread on the free-electron-laser amplifier is investigated for a configuration of a helical wiggler and an axial guide magnetic field. A set of coupled nonlinear differential equations in three dimensions describing the evolution of the electron trajectories and the radiation field is derived and solved numerically for several sets of parameters. The initial beam distribution is defined external to the wiggler field and the adiabatic injection of the beam into the wiggler is modeled by allowing the wiggler amplitude to increase slowly from zero to a constant amplitude over several wiggler periods. While the efficiency is found to decrease rapidly with increasing momentum spread, resonant effects between the wiggler and guide magnetic fields are found to substantially decrease the sensitivity of the saturation efficiency to the beam emittance. In addition, a relatively sharp change in the slope of the efficiency versus beam momentum spread is identified with the transition between the ‘‘cold’’ and ‘‘thermal’’ beam regimes.

We have studied the behavior of two symmetrically coupled Lorenz systems in the parameter space where the uncoupled systems correspond to the bad-cavity limit of the single-mode laser model. The differential equations show that the threshold for the appearance of a strange attractor in the coupled system drops below the uncoupled value, exhibits a minimum at β=0.5, and then increases rapidly as β→0.7. We have also studied the use of coupled logistic cusp maps for predicting the initial drop in the second laser threshold. The results show good qualitative agreement for low β values (β<0.25).

Mean switching time of a two-mode laser is calculated in two different approaches: the first-passage-time treatment by the generalized Kramers method and the lifetime approach by the two-dimensional WKB method. In both cases, the most probable path is generally curved. Numerical study in the WKB calculation reveals that the general curved most probable path can be replaced with high accuracy by the path specified by the minimum potential for many cases of practical interest, that is, when the mode-coupling constant ξ is within the range 1<ξ<5. Thus, we are able to carry out calculations of the mean switching time that are accurate (within the framework of the WKB approximation) throughout the entire range, above threshold, of the pump parameter a. Compared with the analytical formula which is valid only for large a, our result yields perfect agreement in the large-a limit, but shows that the previously used formula consistently overestimates the mean switching time when a is not large where the experiments are performed.

A review is given of correlation experiments in optics which explicitly distinguish between the predictions of classical and quantum theory. In particular, the Cauchy-Schwarz and Bell’s inequalities and the phenomena of photon antibunching and squeezing are discussed. The violation of classical inequalities is shown to be associated with the nonexistence of a positive Glauber-Sudarshan P function. An example is given of a dissipative system where a violation of Bell’s inequality occurs.

A parametric bistable resonance is shown to occur in a crystal irradiated with two optical beams of frequencies ${\omega }_1$ and ${\omega }_2$, respectively. The square of the amplitude of the lattice vibrations is resonantly enhanced when twice its frequency equals the difference frequency ${\omega }_1$-${\omega }_2$. It is shown that this process results in the generation of an optical wave at the combination frequency 2${\omega }_1$-${\omega }_2$ even when symmetry limitations prevent coherent anti-Stokes Raman spectroscopy. Thus it becomes possible to observe Raman-inactive modes in crystals via four-wave-mixing spectroscopy. The intensity of the generated optical field exhibits a bistable dependence on the intensities of the incident optical fields and on the difference frequency ${\omega }_1$-${\omega }_2$. .AE

Spectra of the n=3 to n=2 transitions in neonlike silver emitted from the Princeton Large Torus have been recorded with a high-resolution Bragg-crystal spectrometer. The measurements cover the wavelength region 3.3–4.1 Å and include the forbidden 3p→2p electric quadrupole lines. Transitions in the adjacent sodiumlike, magnesiumlike, and aluminumlike charge states of silver have also been observed and identified. The Ly-α spectra of hydrogenlike argon and iron, the Kα spectra of heliumlike argon, potassium, manganese, and iron, and the Kβ spectrum of heliumlike argon fall in the same wavelength region in first or second order and have been measured concurrently. These spectra provide a coherent set of wavelength reference data obtained with the same spectrometer and from the same tokamak. This set is used as a basis to compare wavelength predictions for one- and two-electron systems to each other and to determine the transition energies of the silver lines with great accuracy.

A model is considered for the interaction among charges resulting from thermal ionization balance in a structureless fluid. In the limit of nondegenerate electrons and negligible excluded volume the Debye-Hückel result is obtained, while the Fermi-Thomas result arises for degenerate electrons and substantial excluded volume. The free energy of the system implies a vapor-liquid phase transition; an insulator-metal transition is also obtained. Generalizations of the model, which include electron–neutral-atom interactions and self-consistent clustering effects, should be suitable for treating materials such as cesium, rubidium, and mercury and will be presented in future work. Numerical results are discussed.

In this paper we present new measurements of thermal transport in spherical geometry using time-resolved x-ray spectroscopy. We determine the time dependence of the mass-ablation rate by following the progress of the ablation surface through thin layers of material embedded at various depths below the surface of the target. These measurements, made with 6 and 12 uv (351 nm) beams from the OMEGA laser system of the Laboratory for Laser Energetics of the University of Rochester, are compared to previous thermal transport data and to detailed hydrodynamic code simulations. We find agreement with code results for the scaling of the mass-ablation rate with absorbed intensity, but measure larger absolute values than predicted. This finding is interpreted as being the result of irradiation nonuniformities on target. The sharp decrease in the mass-ablation rate after the peak of the laser pulse predicted in simulations is consistent with experimental observations.

Spectroscopic investigations of iron-ion line radiation (Fe x–xxiv) in the extreme-ultraviolet region have been carried out during ion cyclotron resonance frequency (ICRF) heating experiments in the JIPP T-II-U tokamak, at the Japanese Institute of Plasma Physics (JIPP), Nagoya University, Nagoya. The central-chord radiances of the iron-ion resonance lines were observed to increase during ICRF heating. After the radiance of the highest ionization stage attained the peak toward the end of ICRF heating, the radiances of the lower stages were observed to attain their peaks at successively later times. The rf-power injection was accompanied by strong gas puffing and additional current rise, which raised the electron density and concurrently lowered the central electron-temperature. The electron-temperature decrease in the central region, which was further promoted by radiative losses due to metallic impurities, continued after ICRF heating. The spectroscopic observations are interpreted as showing that the ICRF heating causes a large additional influx of iron into the plasma, and that the iron accumulates preferentially in the plasma interior with long confinement times. The sequential peaking of the radiation from the highest to the lowest ionization stages reflects the decrease in the electron temperature and the concurrent recombination into successively lower stages. These interpretations are supported by the soft-x-ray and bolometric measurements, and confirmed by the modeling calculations that reproduce the iron-ion line radiances.

This paper discusses the behavior of simple models of interacting, semiflexible, nonchiral polymers. The Ursell-Mayer expansion is used to systemize calculation of the properties of such systems and avoid problems encountered with the strong, bonding interactions in a direct analysis of this problem. The formation of a nematic phase is discussed within a field theory which is equivalent, order by order in perturbation theory, to the Ursell-Mayer expansion. Symmetry analysis allows two universality classes; one has the usual, tensor order parameter, the other has a vectorlike order parameter. The vectorlike order parameter has no direct physical interpretation (distinguishing the transition from a ferroelectric transition) but must be introduced because of the strong, bonding interactions between monomers on a polymer. The behavior of some physical observables near the critical point in systems with phase transitions controlled by the vectorlike order parameter is discussed, including a brief discussion of nonclassical critical behavior. Evidence is given that transitions with either type of order parameter are realizable in model systems.

We have carried out some more nonlinear molecular-dynamics calculations of the conductivity of the Lennard-Jones fluid near its triple point as a function both of the number of particles and of the external field. We have employed the subtraction technique and Evans-Gillan algorithm together with a recently proposed criterion to compare the strengths of impulsive and stationary perturbations. Accordingly, the gradients we used lie in the range 1.2×${10}^7$ to 5.8×${10}^9$ K/cm. Our results suggest that the response of the energy current at K=0 is linear for F̃ up to 5.8×${10}^8$ K/cm. We find nonlinearity for larger values of the perturbation. No dependence of the response on the particle number is found.

The steady-state climbing motion of isolated dislocations is studied both analytically and numerically in several two-dimensional model systems. Numerical measurements of the wave vector at which the defect is stationary are generally found to be in agreement with predicted values. Measurements of the climbing velocity as a function of wave vector reveal a basic difference between relaxational and nonrelaxational models, and this is understood in terms of a formalism which does not depend on a perturbation expansion near onset. Finally, the effects of coupling to a drift velocity generated by vertical vorticity are explicitly studied. The behavior in this case is of the same characteristic form as general nonrelaxational models without vorticity.