Rate constants for the formation of ${\mathrm{O}}_2$${\mathrm{}}^{\mathrm{-}}$ ions in collisions between K(nd) Rydberg atoms and ${\mathrm{O}}_2$ have been measured for values of n in the range 8–12. The measured rate constants are small and decrease from ∼5×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^3$ ${\sec }^{\mathrm{-}1}$ at n=8 to ∼1×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^3$ ${\sec }^{\mathrm{-}1}$ at n=12. The data are consistent with a reaction model in which Rydberg electron attachment leads to the formation of short-lived excited ${\mathrm{O}}_2$${\mathrm{}}^{\mathrm{-}}$ ions, some of which are subsequently stabilized by an interaction with the core ion.

Positronium- (Ps) formation cross sections in positron-lithium scattering have been calculated by using the distorted-wave model of Khan and Ghosh. The results for the total and differential cross section are reported. The present Ps-formation cross section decreases with the increase of the incident positron energies. At about 100 eV, Ps formation is found to be negligible.

We show that the remarkably rapid convergence of the molecular expansion allows an accurate and fast determination of capture cross sections by multicharged ions.

Nonrelativistic single-configuration Hartree-Fock calculations have been used in the intermediate-coupling scheme to compute the intensities and energies of (1s3p${)}^{\mathrm{-}1}$→(2p3p${)}^{\mathrm{-}1}$ transitions in medium-Z elements from K (Z=19) to Ge (Z=32). The α’’ satellite, which previously could not be assigned to a 2p spectator-hole transition, can now be identified with the most intense line of the 3p spectator-hole complex.

We demonstrate numerically that external excitation of a nonlinear waveguide can produce sequential threshold behavior via multisoliton emission from the waveguide. This behavior is similar to that predicted to occur at a nonlinear interface.

A novel four-wave-mixing polarization bistability is predicted in a system consisting of an anisotropic crystal and one partial reflection mirror (retroreflection configuration). The polarization input-output characteristics at constant input power are shown to exhibit not only conventional ‘‘S’’ but also ‘‘seesaw’’- and ‘‘isola’’-type bistability. The global topological nature of this polarization bistability is depicted in the entire phase space in terms of the polarization state of light.

Two nonidentical two-level atoms are separated by a fixed distance R. The probabilities of the atoms being excited are calculated for both the A⋅p interaction and the E⋅r interaction in the electric dipole approximation. For the A⋅p interaction the second atom has a nonvanishing probability of being excited for times t<R/c, and so the interaction is not retarded. For the E⋅r interaction the second atom has zero probability of being excited for times t<R/c, and so the interaction is completely retarded. Reasons for the difference are discussed.

Measurements are performed in an attempt to control fluctuations of a collisionless magnetoplasma by using ferromagnetic material. A shaped ferromagnetic cylinder, which modifies an external uniform magnetic field to yield an l=3 helical configuration with magnetic shear, is realized and is set around the plasma column having drift-wave-type fluctuations. By increasing the region where the plasma column is surrounded by the ferromagnetic cylinder, the plasma fluctuations are demonstrated to be suppressed.

By assuming a nonisothermal equation of state for the electrons and considering the case for three-dimensional magnetized ions, a multidimensional modified Korteweg–de Vries equation is derived. The latter governs the dynamics of nonlinear ion-acoustic waves in a magnetized plasma. The possibility of narrow-width three-dimensional solitons is pointed out.

The iterates of the logistic map, $x_{n+1\mathrm{}}$=${\mathrm{rx}}_n$(1-$x_n$), exhibit chaotic behavior for many values of r∈[3.57,4.0]. However, if we treat x as an m-state discrete variable, as is done in any digital computer calculation, then the iterates of x invariably form short limit cycles of length ≊ √m for chaotic values of r. Although this result questions the validity of digital computer simulations of chaos, we nevertheless find that the statistical properties of the continuous equation, such as the invariant probability distribution and the Lyapunov exponent, are preserved in these cycles. We also find that the transition between periodic and chaotic behavior is still well defined in the discretized map.

The symmetry-breaking bifurcations for symmetric solutions of the standard mapping have been studied. The standard mapping is identified with the model for a rigid bar under periodic impact loads.

When a polymer is modeled by a long elastic rod of homogeneous and isotropic material, we find that its persistence length is proportional to the cube of the critical length for Euler buckling caused by the root-mean-square contraction force. The proportionality constant involves the radius of the circular cross section of the rod. For paramyosin, the persistence length is larger than the critical buckling length by a factor of about 3.

The diffusion-limited aggregation (DLA) model is extended to include a size-dependent sticking probability. By varying the size dependence, transition in the large-scale structure occurs from branching (DLA-like) aggregates to nonbranching aggregates. This is in contrast to constant sticking probability where branching is always predicted to occur in the large-scale limit. The model illustrates the competition of two diverging length scales and permits the study of changes in the geometrical form from variations of the growth processes. Examples of the transition from numerical simulation are presented.

Chaotic behavior in an anharmonically coupled exciton-biexciton two-oscillator model has been studied.

Obtaining ultracold atomic gases is now possible because of laser cooling, and it has been proposed to search for collective quantum effects in these gases. In this paper, I show that the observation of such effects seems unlikely in alkali-metal vapors, at least in a noncondensed phase. Finally, I discuss briefly the effects of radiative trapping and long-range resonance dipole interactions on laser cooling at high atomic densities, and I show that this phenomenon introduces severe limitations of the maximum attainable density, at least for the most elementary laser-cooling scheme.

Recently Ghosh and Parr [S. K. Ghosh and R. G. Parr, Phys. Rev. A 34, 785 (1986)] have developed a new nonlocal exchange functional given by K(ρ)=(π/2) F ${\rho }^2$(r)β(r)dr. Using the spherical average of the exchange hole, we redefine β(r) using $t_s$(r,ρ)=Tr(t→) where t→ is the kinetic energy density tensor matrix. We then obtain the same formula whatever the symmetry of the system.

Recently Arcuni [Phys. Rev. A 33, 105 (1986)] has given a quantitative treatment of the emission-angle-dependent post-collision interaction (PCI). We show that his formula for the PCI-induced energy shift can be improved such that it is much more accurate and at the same time is valid over the whole range of projectile velocities V. We compare the analytical formula with numerical calculations of the classical three-body problem and find excellent agreement.

Motivated by recent experiments on highly excited Rydberg atoms in electrically polarized states, we consider the semiclassical analysis of quantum systems defined on the interval $0<x<\mathrm{}\infty$. Caustics plague the conventional approach, and Maslov's technique cannot be used. Instead we introduce an affine phase-space formulation that leads to a global, uniform semiclassical approximation to the wave function which fully captures the effects of caustics.

The continuous electron emission in 150-, 450-, and 750-keV Kr-Kr collisions has been studied at a laboratory scattering angle of 15°. The analysis of the spectra in terms of quasimolecular Auger-electron emission superimposed on a "background" of directly ionized electrons indicates that many quasimolecular states are involved. This washes out structural details which have been expected from simple models.