Cross sections for production of electrons which are moving with near-zero velocity in the projectile frame in pure-target-ionization and transfer-ionization events were measured for 12.7-MeV ${\mathrm{I}}^{13+}$ ions incident on He and ${\mathrm{H}}_2$ gas targets. These cusp-electron-production cross sections were found to be nearly equal for pure-target-ionization and transfer-ionization events in both targets. The cross section for cusp-electron production in transfer ionization from He targets comprised only 0.04% of the total transfer-ionization cross section and in pure target ionization comprised only 0.12% of the total pure-target-ionization cross section. Similar results were obtained for ${\mathrm{H}}_2$ targets except that all respective cross sections were larger than for He. The experiment shows that most of the free electrons released in pure-target- and transfer-ionization events for 0.1-MeV/u ${\mathrm{I}}^{13+}$ incident on He and ${\mathrm{H}}_2$ gas targets are not low-energy electrons in the projectile frame.

By use of a double-resonance technique, a spectrum of Rabi resonances has been observed. These resonances are enhancements in the dynamic response of a quantum system’s population to rapid changes in the phase of a perturbing electromagnetic field. Theoretically, it is shown that the appearance of a spectrum, as opposed to a single Rabi resonance, is due to the fact that the phase of the field changes discretely by π radians, rather than sinusoidally. In the presence of π-radian phase changes, the quantum system’s dynamic response is found to be similar to a damped harmonic oscillator driven by a δ-function force term. In this harmonic-oscillator approximation the Rabi-resonance spectrum arises naturally, resulting from a resonant enhancement of the atomic coherence.

The transfer of negative muons from the ground state of muonic hydrogen to argon and helium has been measured in gaseous mixtures at pressures between 10 and 15 bars and at different relative concentrations. The transfer rate to argon, reduced to liquid-hydrogen density [${\Lambda }_{p\mathrm{Ar}}$ =1.42(4)×${10}^{11}$ ${\mathrm{s}}^{\mathrm{-}1}$], agrees well with those obtained at ten times higher pressures. The transfer rate to helium [${\Lambda }_{p\mathrm{He}=0.88\mathrm{}\left(9\right)\mathrm{}\times {10}^8}$ ${\mathrm{s}}^{\mathrm{-}1}$], measured in a triple gas mixture, disagrees with the only other experimental value and is about two times higher than recent theoretical predictions. In addition, the muonic x-ray intensities of the Lyman series of argon resulting from muon transfer have been determined. They are well reproduced by a cascade calculation assuming a theoretical initial distribution over n and l states.

The dipole radial integral for an initial discrete 1s state and a final continuum state has been calculated under the screened hydrogenic model. In this model, single-electron hydrogenic wave functions are employed and the initial and the final states are treated by two different effective-charge parameters. Numerical values of differential oscillator strengths for transitions from 1$s^2$ ${}_{}{}^1{}_{}{}^{}S$ to the continuum for the helium sequence ions are obtained. Also calculated are the dipole polarizabilities, which are found to be in excellent agreement with the results of other authors.

We present a theory attempting an interpretation of existing data on three- and four-photon ionization of Sr in the wavelength range 557–575 nm and at 532 nm. Two-photon resonances with bound excited states and three-photon resonances with doubly excited autoionizing states are studied. ac Stark shifts and broadening of resonances, as well as singlet-triplet transitions, are shown to yield interesting new information on atomic structure. The absorption of a fourth photon above the threshold leads to excited final ionic states, with autoionizing states having served as intermediate three-photon resonances. The calculation is based on a configuration-interaction scheme, with singlet-triplet mixing taken into account either through jl coupling or phenomenologically by adjustment to some aspects of the data. The apparatus of resonant multiphoton theory is combined with the above scheme. Depending on the quantity in question, the agreement between theory and experiment ranges from good to semiquantitative.

The kinetic energy spectra of deuterium ions produced from ${\mathrm{D}}_2$ arising from collision-free subpicosecond irradiation at 248 nm with intensities spanning the ${10}^{13}$–${10}^{16}$-W/${\mathrm{cm}}^2$ range have been measured by time-of-flight analysis. The behaviors of the kinetic energy distributions of the fragments and the relative abundances of atomic (${\mathrm{D}}^{+}$) and molecular (${\mathrm{D}}_2^{+}$) ions reveal the presence of two mechanisms of multiphoton dissociative ionization. Calibration of the energy scale for ${\mathrm{D}}^{+}$ is facilitated by comparison with ${\mathrm{He}}^{2+}$. For intensities in the ${10}^{13}$–${10}^{15}$-W/${\mathrm{cm}}^2$ region, intermediate three-photon resonances and the optical Stark shift play important roles. At an intensity ∼${10}^{16}$ W/${\mathrm{cm}}^2$, a direct transition from the molecular ground state to the dissociative ionic level appears as a significant channel. No evidence of direct double ionization was observed.

A detailed theoretical analysis of the intensities of the pure rotational $R_0$(J) transitions in gaseous HD is presented. These transitions all manifest small intracollisional interference effects arising from the cross product of the allowed– and the induced–dipole-moment matrix elements. Using recent ab initio calculations of the induced dipole moment in an HD-HD pair, this interference is found to be constructive for most transitions, thus implying a density-dependent increase in intensity. For the $R_0$(0) transition, a small but significant additional contribution to the constructive intracollisional interference results from the mixing of rotational levels in a single molecule during collisions. Additionally, because of a near-resonance condition, simultaneous mixing of internal rotational levels in both molecules of a colliding pair leads to a large destructive interference affecting primarily the $R_0$(2) transition. The present theoretical values of the interference parameter $a_{\mathrm{theor}}$ are compared with experimental data and it is concluded that while the low-temperature data are in reasonable accord with the present theoretical results, significant differences still remain both between different experimental determinations, and between theory and experiment for the room-temperature data. Possible theoretical refinements to explain the observed large temperature dependence of the intracollisional interference are discussed briefly.

We examine, in a heuristic fashion, the analytic structure of the scattering amplitude for an electron interacting with an atomic potential in the presence of a radiation field. For each resonance pole of the amplitude there are infinitely many shadow poles lying on different unphysical energy sheets of the infinitely many sheeted Riemann surface. The pole having the dominant influence on the scattering amplitude is the one closest to the physical energy axis. All poles undergo significant movement when the field intensity is varied, and when a resonance pole which is dominant passes by a multiphoton ionization threshold a shadow pole usually moves closer to the physical energy axis and hence becomes the dominant pole. When this happens, an initially bound electron may jump eigenvalue curves, and in this way the electron can undergo a very large (ponderomotive) shift in its energy and still maintain the correct physical character of its wave function. The movement of resonance poles near thresholds has implications for the fate of autoionizing states and for population trapping. Consideration of threshold effects might also shed light on a puzzling result of a recent energy-shift measurement. We address the problem of how to determine, in numerical calculations, which sheet the energy eigenvalue is on so that the dominant pole can be identified. We illustrate some of our remarks by results of numerical calculations, and we also touch on the question of the completeness of a basis set in numerical calculations of resonance eigenvalues.

A systematic study of the $L_2$,3${\mathrm{M}}_1$ (3s-2p) transition in elements from Cl to Cr reveals a satellite of unusually large intensity (∼30% of parent line). Previous work on some of these elements led to the suggestion that the satellite is due to a two-electron shakeup (configuration-interaction) process. We compare our measurements of position, width, and intensity of the satellite as a function of atomic number with the predictions of this model and extend the theory to explain the apparent disappearance of the satellite outside this range.

The phase of molecular motion is modulated by the optical Stark effect due to high-intensity laser beams. This is shown to lead to modulation in coherent Raman spectra. The spectral modulation is more pronounced in stimulated Raman spectroscopy than in four-wave-mixing Raman spectroscopy. Increased damping reduces the phase modulation leading to the commonly known spectra.

The free-electron cyclotron resonance laser, based on conversion of synchrotron radiation to a coherent oscillation, operates at an extremely high harmonic with a frequency of about 0.39${\gamma }_{\perp }^3$${\omega }_c$, with ${\gamma }_{\perp }$ the transverse relativistic factor and ${\omega }_c$ the relativistic cyclotron angular frequency. By introduction of the field amplitude spontaneous modified coefficient and by local expansion of the interaction light field, the linear efficiency and the startup current are derived. It is shown that this device possesses a strikingly narrow width of the efficiency profile and so can successfully oscillate on a single longitudinal mode to produce light output of good coherence.

Propagators for coupled and driven coupled harmonic oscillators are evaluated exactly by the path-integral method. The propagators for coupled harmonic oscillators are used to obtain explicitly the energy expectation values.

This paper presents a general description of optical-frequency mixing of n Gaussian laser beams (n=1,2,3,...) with different confocal parameters focused at different positions in the gas medium. A detailed theoretical analysis of third-order sum- and difference-frequency mixing of the light of two laser beams indicates that under certain conditions the output power is enhanced by using unequal confocal beam parameters.

A lattice-gas model is constructed for oil-water-surfactant mixtures. The phase diagram of this model is obtained by using mean-field theory and Monte Carlo simulations aided by low-temperature expansions. Microstructures, structure factors, and mean droplet lifetimes are also determined in some phases. Both two and three dimensions are studied, the former in more detail than the latter. It is shown that it is natural to interpret the paramagnetic phase in our model as a microemulsion. Our model is found to exhibit various properties that are in qualitative agreement with experimental observations of oil-water-surfactant mixtures: (1) two- and three-phase coexistence occurs between oil-rich, water-rich, and microemulsion phases along first-order phase boundaries or a triple line in certain regions of the phase diagram of our model; (2) the triple line, which ends in a tricritical point, is short and this leads to low oil-microemulsion and water-microemulsion interfacial tensions; (3) microstructures (including bicontinuous ones in three dimensions) and structure factors are similar to some experimental ones; (4) droplets in our microemulsion phase are long lived like their experimental counterparts; (5) long-lived, metastable phases, including long-period, lamellar, and glasslike phases, appear at low temperatures. The limitations of our model are discussed. Our study is compared with other studies of models of oil-water-surfactant mixtures.

The long-standing disagreement over the shear viscosity coefficient of the Lennard-Jones fluid near the triple point is reexamined through a series of very extensive Monte Carlo molecular-dynamics calculations of this transport coefficient based on the Green-Kubo theory. The stress autocorrelation function is shown to exhibit a slow decay, principally in the kinetic-potential and the potential-potential terms, which is large compared with the kinetic-kinetic long-time tail predicted by simple mode-coupling theory. Nonetheless, the viscosity coefficient, exclusive of any correction for this tail for times greater than are accessible numerically, is found to agree with that of Schoen and Hoheisel (who discounted the existence of such a tail) as well as nonequilibrium molecular-dynamics calculations. The large value of the viscosity coefficient found by Levesque and co-workers for 864 particles is brought into statistical agreement with the present results by a modest, but not unrealistic, increase in its statistical uncertainty. The pressure is found to exhibit an anomalous dependence on the size of the system, but the viscosity as well as the self-diffusion constant appear to be linear in the inverse of the number of particles, within the precision of our calculations. The viscosity coefficient, including a long-time-tail contribution based on the extended mode-coupling theory is (3.796±0.068)σ(ɛ/m${)}^{1/2}$ for the Lennard-Jones potential, fitted to a cubic spline, and (3.345±0.068)σ(ɛ/m${)}^{1/2}$ for the potential truncated at 2.5σ.

The nature of the adsorption of fluids confined in pores in the proximity of the bulk critical point is investigated by means of Landau phenomenological models and nonlocal mean-field free-energy functionals. We also discuss the possibility of observing experimentally the asymptotic critical exponents for the adsorption in the case of finite-geometry systems.

The transport of passive impurities in nearly two-dimensional, time-periodic Rayleigh-Bénard convection is studied experimentally and numerically. The transport may be described as a one-dimensional diffusive process with a local effective diffusion constant $D^{\mathrm{*}}$(x) that is found to depend linearly on the local amplitude of the roll oscillation. The transport is independent of the molecular diffusion coefficient and is enhanced by 1–3 orders of magnitude over that for steady convective flows. The local amplitude of oscillation shows strong spatial variations, causing $D^{\mathrm{*}}$(x) to be highly nonuniform. Computer simulations of a simplified model show that the basic mechanism of transport is chaotic advection in the vicinity of oscillating roll boundaries. Numerical estimates of $D^{\mathrm{*}}$ are found to agree semiquantitatively with the experimental results. Chaotic advection is shown to provide a well-defined transition from the slow, diffusion-limited transport of time-independent cellular flows to the rapid transport of turbulent flows.

We explore some implications of the observed multifractal nature of the turbulent energy-dissipation field and of velocity derivatives of increasing order on the near-singularities of the Navier-Stokes equations and the singularities of Euler equations. Although these singularities occur on fractal sets of dimension close to (and only marginally less than) 3, it is shown that most of the energy dissipation is concentrated on a subset of fractal dimension about 2.87 and volume zero. Similar statements can be made with respect to velocity derivatives. In particular, it is shown that the higher the order of the velocity derivative, the less space filling the corresponding singularities become.

Thermal-transport coefficients have been determined for binary-liquid Lennard-Jones mixtures composed of strongly different component particles by equilibrium molecular-dynamics calculations. Two very unequal types of mixtures have been considered. The first type contained systems of particles of different interaction strength and equal ‘‘volume,’’ while the second type was comprised of systems of particles of different ‘‘volume’’ and equal interaction strength. For both mixture types, the 1-2 interaction potentials of the unlike pairs of particles have been varied systematically. We have chosen the Lennard-Jones potential parameters such that the mixtures with average 1-2 interaction showed nearly the same values for the mutual-diffusion coefficient, the shear and the bulk viscosity coefficients, as well as the thermal conductivity coefficient. This facilitated a comparison of the thermal diffusion coefficients of both types of systems. It was found that the latter coefficient differed drastically in magnitude and sign. On the other hand, the coefficient appeared to be rather insensitive to alterations of the 1-2 interaction, whereas the diffusion and viscosity coefficients showed considerable dependence on them.

A nonlinear Schrödinger equation is derived which governs the nonlinear interaction of the adiabatic particle motion with finite-amplitude high-frequency surface waves traveling on an unmagnetized homogeneous plasma layer. The evolution equation has been obtained from the nonlinear dispersion relation of the waves (symmetric and antisymmetric modes, respectively) under the assumption that the high-frequency wave propagation is modified by slow motion. It is found that in limited ranges of the wave frequencies and wave numbers and for appropriate values of the parameter σ=${\omega }_p$a/c (${\omega }_p$ being the electron plasma frequency, a the layer half thickness, and c the speed of light), the finite-amplitude antisymmetric modes can propagate as supersonic or subsonic bright-envelope solitons. Under the same conditions, the symmetric surface waves prove to be modulationally stable and the nonlinear waves which can exist are dark-envelope solitons (envelope holes).