Absolute partial-ionization cross sections have been determined for inelastic interactions involving fully stripped atomic projectiles and noble-gas atoms. For helium bombarded with stripped H, He, Li, B, C, and O at reduced energies of 1.44 and 2.3 MeV/amu, the ratio of cross sections for double and single ionization $R=\frac{{\sigma }_{i2\mathrm{}}}{{\sigma }_{i1\mathrm{}}}$ shows a strong dependence on the incident projectile charge. Furthermore, striking differences between results for equivelocity electrons and protons are observed, the ratio of double-to-single ionization by electron impact (∼2 MeV/amu) is almost a factor of 2 greater than that for proton impact, and the ratios for both particles converge only for reduced energies ∼10 MeV/amu. For neon targets, a similar strong charge-state dependence for incident, bare nuclei is observed, while the electron-proton discrepancy is considerably less. In contrast, the heavier noble gases all show a considerably weaker dependence of the cross-section ratio on the incident charge state.

A new and accurate method for measuring lifetimes of excited ionic states is described. The method employs selective laser excitation and rapid Doppler tuning to convert the spontaneous exponential decay of the excited state into a spatial variation of fluorescence intensity. The technique has been tested on the $6p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^o{}_{}{}^{}$ Ba ii level and our final result of 6.35±0.09 nsec is in excellent agreement with previous high-precision measurements. The method should make it possible to routinely achieve 1 percent accuracy measurements of ionic lifetimes longer than a lower limit of a few nanoseconds, set by transit-time broadening and ion-beam velocity spread.

Recommended spin-orbit intervals in the ground state, $2p^5{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}-{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$, of fluorinelike ions are presented for ions of high nuclear charge ($Z\le 56$). The recommended values are based on the relativistic Hartree-Fock calculations with corrections deduced from known experimental values in the low -$Z$ side of the sequence.

The interaction of a diatomic molecule with two near-resonance laser fields is investigated with the use of a one-dimensional anharmonic oscillator model. Our study shows that, over a certain range of intensity of one of the laser fields, induced quenching of the absorption takes place.

The effect of vibrational averaging on the photoionization cross section and photoelectron asymmetry parameter for photoionization of C${\mathrm{O}}_2$ leading to the $C{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state of C${\mathrm{O}}_2^{+}$ has been studied. We have computed the photoionization continuum wave function in the frozen-core Hartree-Fock approximation using the iterative Schwinger variational method. Averaging over the symmetric stretching mode of C${\mathrm{O}}_2$ reduced the peak resonant cross section by about 15%. This is in contrast to the earlier published results using the continuum multiple-scattering method where a much larger reduction in the peak cross section was obtained. A similar difference between these two theoretical methods was seen in their predicted asymmetry parameters. The present results have also been compared to available experimental data.

The classical limit of the Einstein-coefficient method is used in the low-gain regime to calculate the stimulated emission from a tenuous relativistic electron beam propagating in the combined solenoidal and longitudinal wiggler fields $(B_0+\delta Bsin{k}_{0}z){\hat{e}}_z$ produced near the axis of a multiple-mirror (undulator) field configuration. Emission is found to occur at all harmonics of the wiggler wave number $k_0$ with Doppler upshifted output frequency given by $\omega =\frac{(l{k}_0{V}_b+{\omega }_{\mathrm{cb}})(1+\frac{V_b}{c}){\gamma }_b^2}{(1+\frac{{\gamma }_b^2{V}_{\perp }^2}{c^2})}$, where $l\ge 1$. The emission is compared to the low-gain cyclotron maser with $\delta B=0\mathrm{}$ and to the low-gain free-electron laser (operating at higher harmonics) utilizing a transverse linearly polarized wiggler field.

The collective interaction in a free-electron laser with combined helical wiggler and uniform axial guide fields is presented in the linearized regime. The analysis involves a perturbation of the Vlasov-Maxwell equations about the constant-velocity helical trajectories, and the general driving currents are derived for this configuration. The complete dispersion equation is then obtained for a monoenergetic beam. Analytic solutions are obtained in the strong pump and space-charge dominated regimes, and an extensive numerical analysis is presented for a wide range of operating parameters. The results indicate that substantial enhancements in the gain are possible when the relativistic axial gyrofrequency is comparable to the free-electron laser doppler upshift. In addition, there is a range of parameters for which the ponderomotive potential acts to destabilize the electron beam. In this regime, we find both unstable electrostatic beam modes and largely electromagnetic modes with broad bandwidths.

We analyze the structural relationship between microscopic and macroscopic tensors which characterize the linear- and nonlinear-optical properties of molecular crystals exhibiting a strong donor-acceptor intramolecular interaction, with particular reference to methyl-(2,4-dinitrophenyl)-aminopropanoate (MAP). The quasiplanar structure of the active part of these molecules results in a strong and characteristic anisotropy of the optical hyperpolarizabilities, which can be traced up to the macroscopic level when taking into account the crystal symmetry as well as the orientation of the molecules in the unit cell. The experimental data on MAP are thoroughly analyzed on this basis, and it is found that a two-dimensional model of the lowest-order hyperpolarizability tensor results in a structural relation between the macroscopic tensor components, which is in agreement with experimental data. In addition, the principal dielectric axes of this monoclinic crystal are determined by the orientation of the aromatic plane in the unit cell. The overall analysis also enables the determination of four independent components of the molecular hyperpolarizability tensor from experimental data only, and the results have been compared to those of a semiempirical intermediate neglect of differential overlap calculation. The anisotropy of this tensor reveals that the intramolecular charge transfer responsible for the large optical nonlinearity is predominantly from the amino group to the nitro group in para position, rather than towards the nitro group in ortho position. Finally the overall analysis provides a basis for discussing what should be the best orientation of the molecules in the unit cell for maximizing the crystal nonlinearity. The result is that phase-matchable nonlinear coefficients up to six times larger than in MAP could be observed in compounds with similar molecular hyperpolarizabilities but an optimum crystal structure.

Efficiency of three-wave interactions in molecular crystals depends on the conjugation of the molecular unit, which in turn is a one- or two-dimensional property. This strong anisotropy reduces the number of non-negligible molecular lowest-order hyperpolarizability coefficients to four. The lowest-order macroscopic optical nonlinearity can be expressed, in the absence of significant intermolecular effects, as the tensorial sum of molecular hyperpolarizabilities. This analysis is applied to the 17 relevant noncentrosymmetric crystal point groups, generalizing a previous analysis of nonlinear-optical properties of methyl-(2,4-dinitrophenyl)-aminopropanoate crystals. In several cases, the molecular unit anisotropy is shown to impose structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from the crystal point symmetry only. In such cases, nonlinear-optics experiments can be used for testing molecular anisotropy and molecular orientations within the unit cell in the absence of significant nonlinearity arising from intermolecular coupling. Similar relations can be derived between electro-optic coefficients, but limited to the case of weak contributions of intermolecular vibration to the electro-optic effect. We investigate for each point group the possibility of inferring hyperpolarizability coefficients from macroscopic nonlinear measurements, a complementary approach to that based on theoretical molecular calculations or electric-field-induced second-harmonic generation in solution. In the case of highly anisotropic one-dimensional charge-transfer systems (exemplified by $p$-nitroaniline), for each point group and a given molecular hyperpolarizability, the optimal orientation of the charge transfer axis, leading to the highest phase-matchable coefficient, is given. It is shown that crystal point groups 1,2,$m$, and $\mathrm{mm}2$ correspond to the highest possible value of this coefficient, while other crystal symmetry is less favorable. These considerations are applied to four available efficient molecular crystals and used either as a check of molecular orientations in a case of low crystalline symmetry or to estimate otherwise unavailable molecular nonlinear coefficients.

Relatively fundamental problems in physics hinder realization of large information-handling capability offered by optical switching. This paper examines the physics of optical switching in a way which exhibits those problems and which suggests strategies for their solution. The primary limits on optical switches arise from heat generated during the switching process. The principal barriers to easing those limits are the small value of interaction cross sections for atoms in solid-state materials and the lack of high-finesse microresonators for the optical field. The research most relevant to the solution of these problems appears to be that dealing with enhanced atom—optical-field interactions in solid-state microstructures. Short-optical-pulse generation and the short-time dynamics of solid-state microstructures are also highly relevant.

It is shown that the effects of a standing electromagnetic wave of cosinusoidal form on a neutral two-level atom can be represented by an effective static potential, which is obtained by a local unitary transformation to dress the atom by the field. The form of the effective potential is obtained self-consistently by minimizing fluctuations of the atomic kinetic energy induced by the locality of the dressing transformation. Moreover this effective potential is shown to depend on the atom-field detuning and on the field intensity, and to exhibit discontinuities in some ranges of these parameters. The band structure which characterizes the eigenvalue spectrum of the dressed Hamiltonian is discussed in the spirit of the tight-binding approximation, and the fluctuations of the kinetic energy near resonance are shown to induce splitting of the bands; a physical interpretation of the atomic dynamics associated with these splittings is presented. Off resonance the fluctuations are shown to induce exchange of real photons and variations of the atomic kinetic energy. The results are compared with those of previous theories, leading to an improved understanding of the dynamics of a two-level atom in a standing-wave field.

An intracavity two-photon absorber interacting with two coherently driven cavity modes is studied. The possible stable output configurations of the cavity-mode amplitudes are determined for both the absorptive and dispersive cases. Under certain conditions this system is shown to display optical bi- and tristability.

The density-functional theory (DFT) for a system of ions and electrons having overall charge neutrality is presented. For a given temperature and free-electron density, the theory self-consistently determines the various pair-distribution functions, bound states, and the effective ion charge $\overline{Z}=Z-n_b$, where $n_b$ is the mean number of bound electrons per ion. With the use of a sufficiently large correlation sphere to represent the "physically relevant" part of the plasma, a Kohn-Sham-type Schrödinger equation is solved for the electrons. The density-functional equations for the ions reduce to a Gibbs-Boltzmann equation containing an effective ion-correlation potential. The latter is obtained from the hypernetted-chain equation. The electron exchange-correlation potential is obtained from quantum many-body theory. The method is applied to an electron-proton plasma (EPP) for a number of physical situations. The machine-simulation (MS) results in the classical one-component-plasma limit, as well as the standard "proton-in-jellium" results, are correctly recovered in suitable limits. The MS results for the EPP plasma are limited by classical approximations, but are found to be in reasonable agreement with our DFT results which are probably more well founded. A shallow $1s$-like bound state appears with increase of temperature or decrease of density. The proton-pair distribution function in the EPP shows the onset of short-range order only when $\Gamma \gtrsim 5$ or more. The electron-proton distribution function is found to be relatively insensitive to the details of the ion distribution for the static properties studied in this paper.

During neutral-beam-heated, counter-injection discharges in the ISX-$B$ tokamak, impurities tend to collect in the interior of the plasmas. In these circumstances iron ions preferentially accumulate with long confinement times in regions of almost constant electron temperature. The concentrations of intermediate stages of iron (Fe XV, XVI, XVIII, and XIX) have been studied under these conditions to investigate the ratios of recombination and ionization coefficients. The dielectronic recombination coefficients are then deduced by assuming that the ionization and the radiative and charge-exchange recombination rates can be calculated closely from theory.

For a driven nonlinear semiconductor oscillator which shows a period-doubling pitchfork bifurcation route to chaos, we report an additional route to chaos: the Pomeau-Manneville intermittency route, characterized by a periodic (laminar) phase interrupted by bursts of aperiodic behavior. This occurs near a tangent bifurcation as the system driving parameter is reduced by $\epsilon$ from the threshold value for a periodic window. Data are presented for the dependence of the average laminar length $〈l〉$ on $\epsilon$, and also on additive random noise voltage. The results are in reasonable agreement with the intermittency theory of Hirsch, Huberman, and Scalapino. The distribution $P\left(l\right)\mathrm{}$ is also reported.

The theory of inhomogeneous fluids is applied to a $d$-dimensional system near its critical point to derive the probability of finding a particle at a distance $r$ from a pair separated by a distance $s$, given that $r\gg \xi \gg s$, where $\xi$ is the correlation length. When this result is used in the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, an approximation-free equation is obtained, from which it follows that the pair correlations for $r\gg \xi$ satisfy the Ornstein-Zernike differential equation.

A recently developed variational principle is applied to the solution of a self-consistent repeated-ring kinetic theory of the Lorentz gas in one, two, and three dimensions. Calculated values of the diffusion constant $D$, are in excellent agreement with molecular-dynamics simulation results for $d=2\mathrm{} \mathrm{and} 3$. The theory predicts the existence of "critical scatterer densities," ${\rho }_c^{*}$, above which $D=0\mathrm{}$; for $d=1-3\mathrm{}$, ${\rho }_c^{*}=0,{\pi }^{-1\mathrm{}},\mathrm{and}\frac{3}{2\pi }$, respectively. The theory behaves well above ${\rho }_c^{*}$. The behavior of $D$ near ${\rho }_c^{*}$ is examined, as is the "long-time tail" at densities close to ${\rho }_c^{*}$, and a comparison of the results to those of other authors is given.

We study the growth of Rayleigh-Taylor instabilities at the interfaces of any number $N$ of stratified fluids forming an arbitrary density profile. Using the linear theory, we show that there are $2(N-1\mathrm{})\mathrm{}$ exponential growth rates which can be found by calculating the eigenvalues of an $\mathrm{}(N-1\mathrm{})\mathrm{}\times \mathrm{}(N-1\mathrm{})\mathrm{}$ band matrix. We illustrate analytically the case $N=3\mathrm{}$. For general $N$ we state and outline the proofs of two theorems: the first refers to the invariance of the spectrum of growth modes under inversion (${\rho }_i\to \frac{1}{{\rho }_{N+1-i}}$, $t_i\to t_{N+1-i}$), and the second relates the spectrum of any inversion-invariant density profile having free boundaries to the spectrum of the same profile between fixed boundaries. We compare and illustrate the results of our numerical code with the case of a continuous density profile $\rho ={\rho }_0{e}^{\beta y}$, and, setting $N=12\mathrm{}$, we apply our technique to solving a particular problem in designing a multishell target for intertial confinement fusion.

The fast-ion-velocity distributions for polyethylene plasmas produced by spherical laser illumination were observed by a Thomson parabola spectrometer. It has been found that the velocity distributions have undulations, sharp truncations, and broad shelf structure because of multi-ion-species effects. The spherical symmetric plasma expansions including multi-ion-species and two-electron temperature effects have been simulated by a newly developed computer code. The simulation results indicate that protons are accelerated near the front edge of carbon ions, and the maximum velocity is determined predominantly by the ions which run in the region ahead of the carbon-ion front. Consequently, the proton-expansion front has a structure slipped out from the carbon-ion front towards vacuum. Since such protons compensate the accelerating electric field, the maximum velocities of proton and carbon ions become higher and lower, respectively, than in the case of single-ion species. These expansion dynamics reduced by the simulation can explain well the detailed structure of the observed velocity distribution.

The Wigner method in quantum statistical mechanics, which allows the derivation of the quantum-mechanical behavior of the properties of an "almost classical" system, has been applied to the determination of series expansions with respect to $h$ for a general correlation function and the moments related to its spectrum. Explicit expressions of the terms up to the order $h^4$ are given for the zeroth, second, and fourth moment in the case of a many-body system with a Hamiltonian $\mathit{H}=\frac{{\mathit{P}}^2}{2m}+\Phi \left(\mathit{R}\right)$ and for variables which are functions only either of $\left\{\mathit{P}\right\}$ or $\left\{\mathit{R}\right\}$ coordinates. From these expressions the corrections to the classical behavior can be calculated via the classical molecular-dynamics simulation technique.