The photodetachment cross section of ${\mathrm{F}}^{\mathrm{-}}$ was obtained experimentally from a stationary arc plasma of ${\mathrm{SF}}_6$ at atmospheric pressure. The temperature was determined by spectroscopy. The electron number density was measured by two-wavelength laser interferometry and by emission spectroscopy. Departures from local thermodynamic equilibrium were taken into account. The variations of the photodetachment cross section of ${\mathrm{F}}^{\mathrm{-}}$ versus wavelength were obtained by measuring the absolute intensity of the continuum radiation emitted by the plasma and are compared with results from the literature.

Scalar and tensor polarizabilities of cesium 5f and 7d states were measured by three-color multiphoton excitation techniques. The low-field results were quadratic and were in good agreement with calculated values. The oscillator strength for the 5f-5g transition was measured to be 0.0336±0.0003, in close agreement with theory. At higher electric fields, we clearly resolved Autler-Townes avoided crossings between 5f levels and dressed 7d states. The observed doublet-component separations scaled linearly with Rabi frequency for the 7d-5f transitions, supporting a two-level model for the four-level experiment. The results are interesting because the observed spectra were independent of Rabi frequencies for the 6s-6p and 6p-7d transitions. Angular momentum forbidden transitions from 7d, $J_z$=(1/2) to 5f, $J_z$=(5/2) were observed at higher dc fields. These are attributed to dc-field-induced mixing between nearly degenerate 5f and 5g states.

Three-channel quantum-defect theory (3QDT) provides a simple and transparent description of how perturbations affect energies, widths, and photoabsorption line shapes in a Rydberg series of autoionizing resonances. As long as the resonances in the perturbed series do not overlap, very simple formulas can be derived for the positions, widths, and line-shape parameters without any approximation other than neglecting the energy dependence of the 3QDT parameters. We illustrate the applicability of the formulas in the example of a hydrogen atom in a magnetic field, where we present first ab initio calculations of photoabsorption spectra in the regime of field strengths where Rydberg series of resonances belonging to different Landau channels interfere.

A model is presented to investigate the problem of interaction between an N-level atom and N-1 modes of the field. The model includes detuning. Constants of motion are obtained. The evolution operator is calculated, and the probability distribution function for the photon numbers is computed for different initial atomic states. The characteristic functions are computed. Different statistical quantities concerning the photons or the atomic system are given. The case of a three-level atom and two modes is considered for its different configurations. The phenomenon of collapses and revivals is discussed for squeezed light, and the effect of squeezing is shown in this phenomenon.

Competition between amplified spontaneous emission (ASE) and the four-wave-mixing (FWM) process has been observed under conditions of two-photon resonant excitation of the sodium 3d level. The nature of the competition is that the FWM process is able to prevent the occurrence of ASE, even though the gain of the ASE process calculated in the absence of competition effects is much larger than that of FWM. The ASE is suppressed because the fields generated by the FWM process create a new excitation pathway connecting the ground and 3d levels, and under quite general conditions this pathway interferes destructively with that due solely to the applied laser field. These effects are modeled theoretically by solving perturbatively the density-matrix equations of the atomic system, thereby determining the population in the upper level and the nonlinear polarization of the medium. The coupling between the various optical fields due to the nonlinear polarization is described by coupled amplitude equations. The solution to these equations predicts that when the wave-vector mismatch is not too large the fields evolve spatially to reach steady-state values, and that the population excited to the 3d level by the total steady-state optical field is much smaller than that due to the incident laser field alone. We have observed experimentally the suppression of ASE by FWM and have observed that this suppression does not occur when the medium is excited with counterpropagating beams that cannot efficiently excite the FWM process. In addition, we have conducted a series of experiments that shows that the degree of suppression of ASE depends on the intensity and focusing characteristics of the incident laser as expected on the basis of our theoretical model.

We consider a generalized form of parametric amplification which produces k-photon correlations. We show numerically that this process is well-defined quantum mechanically, and we explain the quantum phase-space structures produced by such parametric amplification.

In this paper we propose an algebraic method to study the optical propagation in a nonhomogeneous medium with a quadratic profile of the refractive index. We use the Wei-Norman algebraic procedure establishing an analogy between the evolution operator and the optical propagation matrix.

We show that the phenomenon of above-threshold ionization can be caused by a modulation of continuum energy levels in a strong laser field. The dependence of above-threshold ionization peaks on laser intensity and frequency is analyzed. Calculated ionization rates are compared with recent experimental data for noble gases.

A Lorentz-covariant localization for light waves is presented. The unitary representation for the electromagnetic four-potential is constructed for a monochromatic light wave. A model for covariant superposition is constructed for light waves with different frequencies. It is therefore possible to construct a wave function for light waves carrying a covariant probability interpretation. It is shown that the time-energy uncertainty relation (Δt)(Δω)≃1 for light waves is a Lorentz-invariant relation. The connection between photons and localized light waves is examined critically.

A scheme for simultaneous laser acceleration of electrons and positrons to high energies is discussed. The acceleration is based on a cyclotron autoresonance between the particles and a linearly polarized radiation, propagating along an axial magnetic field. The nonlinear dynamics of an electron and a positron in the system is investigated for homogeneous fields and luminous radiation. The energy of the accelerated particles scales asymptotically as 0.82{[$B_z$(100 kG)/λ(10 μm)${]}^{1/2}$${\alpha }_x$z(m)${\mathrm{\}}}^{2/3}$ GeV, where z and $B_z$ are the distance and strength of the guide magnetic field and ${\alpha }_x$=eE/(mωc) and λ are the normalized radiation field amplitude and its wavelength, respectively. It is shown that high-current ‘‘quasineutral’’ beams (plasmas) can be accelerated to high energies with low radiation losses.

Transverse displacement of the electron beam at input to the wiggler induces betatron oscillation, imparting a sinusoidal transverse displacement to the gain medium seen by the optical beam. As the light propagates down this ‘‘rippled’’ medium it is guided by the dispersion, thereby acquiring some transverse sinusoidal displacement. At output from the wiggler the optical beam in general has acquired some transverse component of velocity, that is, it propagates off axis. As the e-beam displacement fluctuates statistically with every pulse, the optical output direction jitters around the nominal axis. The jitter is estimated with a model that includes off-axis optical propagation through a rippled gain medium but neglects feedback of the off-axis light on the e beam. The model is solved in perturbation theory. The jitter is calculated at the final aperture of the optical system beyond the wiggler. The result is (normalized to the final aperture diffraction) (rms focused jitter)/(diffraction width)${\mathit{\simeq }\mathit{\Vert }\mathit{n}}_{\mathrm{i}}$‖(w${\lambda }_{\mathrm{\beta }}$a${\lambda }_{\mathrm{bb}}$Δ where $r_b$ is the e-beam radius, w is the optical beam radius (1/$e^2$ intensity), λ is the optical wavelength, $n_i$ is the imaginary optical index on axis in e beam, ${\lambda }_{\beta }$ is the betatron wavelength, a is the rms e-beam transverse displacement, L is the wiggler length, and Δ(L) is a damped oscillating function of L. For the proposed Paladin free-electron laser at λ=10 μm on the Advanced Test Accelerator (ATA), this implies (jitter)/(diffraction)<0.25a/$r_b$.

We consider a two-state system kicked quasiperiodically by an external force. When the two kicking frequencies assumed for the force are incommensurate, there can be quantum chaos in the sense that (a) the autocorrelation function of the state vector decays, (b) the power spectrum of the state vector is broadband, and (c) the motion on the Bloch sphere is ergodic. The time evolution of the state vector is nevertheless dynamically stable in the sense that memory of the initial state is retained. We also consider briefly the kicked quantum rotator and find, in agreement with Shepelyansky [Physica 8D, 208 (1983)], that the quantum localization effect is greatly weakened by the presence of two incommensurate driving frequencies.

A full density-matrix theory for quantum beats in optical transmission-correlation or pump-probe type of experiments is developed. The model used is a three-level system with split excited-state levels. It is demonstrated on the basis of perturbation theory that quantum beats should be observable in such experiments. Theoretical and numerical results are compared with the complex details, including damped sinusoidal decays, observed in recent femtosecond relaxation studies of large organic dye molecules. It is also shown that there is no separately identifiable decay component corresponding to any homogeneous or inhomogeneous optical dephasing time in such experiments. Similar beating should also be observable in ultrafast spectroscopy using incoherent light and in Raman-type energy-level configurations where the ground state contains split levels.

Quantum tunneling times for the model of Drummond and Walls describing dispersive optical bistability are investigated for small cavity damping. Without damping the system can be described by an appropriate Hamilton operator. By expanding the density operator in eigenstates of this Hamilton operator the stationary solution as well as the lowest eigenvalues are obtained from a Pauli master equation for the diagonal elements of the density matrix for small cavity damping. The tunneling time follows from the lowest nonzero eigenvalue of this master equation. Expectation values as well as the Q function for the stationary case are also presented.

We investigate the properties of a material with excitation-induced increase of absorption in a hybrid ring resonator. The material is CdS at room temperature, showing photothermal optical nonlinearities. After characterizing the nonlinear medium itself, we investigate the behavior in a short Fabry-Perot resonator and then present experimental as well as theoretical data for this induced absorber in the ring cavity. We show experimental indications for the appearance of the Farey-tree structure, which was predicted theoretically in a recent paper. The dependence of the oscillations on various parameters is discussed.

Vortex structures in a hot plasma with cold impurity ions are investigated. It is shown that a new scalar nonlinearity resembling the well-known temperature gradient nonlinearity can appear.

We have measured the angular distribution of light scattered from two critical binary mixtures which are being continuously and turbulently stirred at a fixed temperature T close to the critical point. At temperatures above $T_c$, it is observed that the structure factor S(k) is strongly modified at small k values. S(k) exhibits a scaling form over a range of temperatures which span the critical temperature of the unstirred system. This functional form, however, changes as the stirring rate is varied. The present experimental results are consistent with earlier turbidity measurements, but the angular distribution data show no evidence of a well-defined transition from the ordered to disordered state as the temperature is decreased. Our results will be compared with a theory of Onuki [Phys. Lett. 101A, 286 (1984)].

A lattice model which has recently been developed to aid the study of microemulsions is briefly reviewed. The local-density mean-field equations are presented and the interfacial profiles and surface tensions are computed using a variational method. These density profiles describing the interface between oil rich and water rich phases, both of which are isotropic, are structured and nonmonotonic. Some comments about a perturbation expansion which confirms these conclusions are made. It is possible to compute the surface tension to high numerical accuracy using the variational procedure. This permits discussion of the question of wetting of the oil-water interface by a microemulsion phase. The interfacial tensions along the oil-water-microemulsion coexistence line are ultra-low. The oil-water interface is not wet by microemulsion throughout most of the bicontinuous regime.

We study numerically unsteady, one-dimensional flow between parallel plates in the hexagonal lattice-gas automaton. An initial tangential velocity instability is created at one plate and its propagation into the system is investigated. This propagation depends on the boundary condition at the opposite plate. From the observed fluid behavior, viscosity is estimated.

A theoretical examination is made of the mechanism of formation of cathode- and anode-directed streamers and the resultant ionized channel in ${\mathrm{SF}}_6$, at 100 kPa, with a uniform applied field. The evolution of positive and negative ions, and electrons, is described by one-dimensional continuity equations, with the space-charged electric field determined by the method of disks. The main features of streamer formation may be understood by noting that any change in the electric field in ${\mathrm{SF}}_6$ causes abrupt changes in the electron density $N_e$ and current density J due to either strong net attachment or strong net ionization which balance at the critical field $E^{\mathrm{*}}$. In the streamer head the net charge enhances the field ahead of the streamer and depresses the field behind, causing a rapid increase in $N_e$ and J, followed by a rapid fall. The cathode-directed streamer propagates because photoionization releases electrons ahead of the streamer, while the anode-directed streamer can propagate, without photoionization, via electrons from the streamer head. The streamer channel is left with a uniform electric field $E_Q$>$E^{\mathrm{*}}$ determined by a dynamic balance between conduction and displacement currents to maintain the total current.