Direct evidence for the photoionization of an autoionizing level of atomic oxygen is presented. Atomic oxygen is produced in the 1${\mathit{s}}^2$2${\mathit{s}}^2$2${\mathit{p}}^4$ ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ level by photodissociation of ${\mathrm{NO}}_2$ or ${\mathrm{N}}_2$O and excited to the ${(}^2$D°)3p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ and ${}_{}{}^1{}_{}{}^{}$${\mathit{F}}_3$ levels by two-photon absorption. These levels lie above the ${\mathrm{O}}^{+}$ ${}_{}{}^4{}_{}{}^{}$S${\mathrm{°}}_{3/2}$ ionization threshold but are forbidden to autoionize in LS coupling. Although the autoionization rates are thus very small, photoelectron spectra demonstrate that both levels do autoionize into the ${}_{}{}^4{}_{}{}^{}$S${\mathrm{°}}_{3/2}$ continuum. At laser intensities of ∼3×${10}^9$ W/${\mathrm{cm}}^2$, photoionization of the ${}_{}{}^1{}_{}{}^{}$${\mathit{F}}_3$ level competes effectively with autoionization. The implications of this work for high-intensity multiphoton ionization studies of above-threshold ionization and multiple ionization are discussed.

For neutral particle systems in a homogeneous magnetic field, there exists a constant of motion, the pseudomomentum, which allows a complete pseudoseparation of the center-of-mass motion. For ions such a separation is not possible, and the Landau orbit of the center-of-mass and internal (electronic) motion are intimately coupled. We investigate different physical situations for which this interaction becomes strong at laboratory magnetic-field strengths. In particular, we find that the Landau orbit itself can change substantially upon this interaction.

The exact solution is presented for the quantum-mechanical problem of a neutron interacting with the magnetic field of a linear conductor with current. It is shown that this problem has a supersymmetry. The energy spectrum and eigenfunctions are derived in a closed form using this supersymmetry.

The transverse resistive wall instability in the two-beam accelerator (TBA) is investigated analytically and numerically. Without any damping mechanism, we find one to four exponentiations in 100 m, depending on the design. It is found that Landau damping, due to energy spread within a beam slice, is not effective, due to rapid synchrotron oscillations in the free-electron laser ponderomotive well. Damping due to an energy sweep along the beam is also considered, and it is found that a small variation in energy along the beam, decreasing from head to tail, can significantly reduce growth. We conclude that the resistive wall instability is not a severe design constraint on a TBA.

This paper presents a theoretical study of angular distribution of spin-resolved electrons ejected by interaction of light in the nonrelativistic, electric dipole approximation with nonlinear molecules oriented in space. A theory, formulated in an earlier paper [N. Chandra, Phys. Rev. A 40, 752 (1989)] by taking full account of the group-theoretical transformation properties of a molecular target, is used to develop explicit expressions for the four parameters [namely, the spin-unresolved differential cross section ${\mathit{d}}^2$σ(${\mathit{m}}_{\mathit{r}}$)/dk^dω for photoionization and the three polarization parameters χ(${\mathit{m}}_{\mathit{r}}$,k,ω), κ(${\mathit{m}}_{\mathit{r}}$,k,ω), and ζ(${\mathit{m}}_{\mathit{r}}$,k,ω) that depend upon the state of polarization ${\mathit{m}}_{\mathit{r}}$ for the ionizing radiation, propagation vector k, and the orientation of the molecule in space] needed to describe the angularly distributed, spin-polarized photocurrent from linear as well as nonlinear molecules of any symmetry corresponding to one of the 32 point groups, oriented in space in their gaseous phase or on liquid (solid) surfaces.

These formulas have been applied to angle- and spin-resolved photoelectron spectroscopy of the ${\mathit{a}}_1$ orbital in those molecules that transform like the ${\mathit{T}}_{\mathit{d}}$ point symmetry group. The spin polarization of photoelectrons in this case is due directly to the spin-orbit interaction in the molecular continua. Detailed expressions for the four parameters with the electric vector in the radiation beam both parallel and perpendicular to the fixed axis of the oriented molecule are derived, probably in their simplest possible form, using the concepts of the extended (or spin-double) group. The properties of these parameters, which are influenced, inter alia, also by the spin-orbit interaction, are analyzed. Such angle- and spin-resolved studies of photoionization in oriented molecules will therefore provide more stringent tests for theoretical models, probes into the effects of target orientation on photoionization dynamics, and a measure of the influence of the spin-orbit interaction on the continuum part of the spectrum.

The procedure adopted and the formulation presented herein set a methodology and a framework for the analysis of the measurements and calculations of angle- and spin-resolved photoelectron spectra of those polyatomics in general, and ${\mathit{T}}_{\mathit{d}}$ molecules, in particular, which have a fixed orientation in space. The specific examples considered in this paper are photoionization in 4${\mathit{a}}_1^2$, 6${\mathit{a}}_1^2$, and 7${\mathit{a}}_1^2$ orbitals of oriented ${\mathrm{CF}}_4$, ${\mathrm{CCl}}_4$, and ${\mathrm{SiCl}}_4$, respectively. Without performing any dynamical calculations, the variations of various parameters with respect to the energy of the incident radiation and to the phases involved have semiempirically been studied in detail for two different orientations of the axis of the fixed molecule. This study has helped also in predicting the values of the polarization parameters in the energy range of the Cooper minimum for angle- and spin-resolved ${\mathrm{CCl}}_4$(6${\mathit{a}}_1^2$${)}^{\mathrm{-}1}$ and ${\mathrm{SiCl}}_4$(7${\mathit{a}}_1^2$${)}^{\mathrm{-}1}$ photoelectron spectra in oriented molecular targets.

The basic sixfold differential cross section ${\mathit{d}}^6$${\mathrm{\sigma }}^{2+}$/${\mathit{d}}^3$k${\mathit{d}}^3$k’ (SDCS) of the double photoionization process is derived within a ‘‘wave-function approach’’ (WFA) using a highly correlated ground-state wave function and a double-continuum final state approximated by a symmetrized product of one-electron Coulomb waves with k and k’ asymptotic momenta. Relevant integrations of the SDCS are shown to provide a complete description of the (γ,2e) process. The theory is illustrated by calculations for helium targets: total cross sections, kinetic energy spectra, and angular plots of photoelectrons are presented. Considering the total cross section, it is found first that the agreement between the length and velocity result is worse than expected from the previous studies. In addition, the present model provides kinetic energy distributions of photoelectrons having the symmetry required by the Pauli principle. This is in contrast with earlier formulations of the WFA. For the SDCS, calculations have been done with final states where Coulomb waves were provided with either fixed charge (Z=2: unscreened nucleus) or variable effective charges. In the former case, it is found that the event where electrons escape with the same energy, along the polarization direction, has a significant probability. This deficiency is removed by use of angle-dependent effective charges since final-state correlation is partly accounted for in this way.

A photodetector’s electronic bandwidth is the frequency range over which beats between different frequencies in the input can be observed in the output. The output of a balanced homodyne detector, the prototypical device for detecting squeezed light, consists of beats between a signal to be measured and a local oscillator. Beats between the signal and quantum local-oscillator fluctuations lead to noise that degrades the observed squeezing. Through wide-band calculations, we show that this noise contains contributions from the entire signal bandwidth, even when this bandwidth is orders of magnitude larger than the detector’s electronic bandwidth. This allows us to place an upper limit on the total power that can enter the detector’s signal port (both signal and extraneous light) within the detector’s optical bandwidth in order for a given level of squeezing to be observable.

We consider the behavior of a two-level atom interacting with a quasiperiodic, resonant field from the perspective of dynamical systems theory. In particular, we analyze the geometry of the phase-space attractor for the atomic system and find that there are basically only two types of attractor geometry, even though the atom’s temporal evolution is very complicated. The dynamics giving rise to these two geometries are differentiated by the degree of adiabaticity associated with the field’s variations, and in the regime of adiabatic dynamics, the attractor shows evidence of a noninteger scaling dimension. This result was unexpected, since IlinearR dynamical systems are not known to give rise to strange attractors. We attribute the attractor’s fractal nature to the scaling behavior of Bloch vector trajectories in the rotating reference frame.

The Jaynes-Cummings model with cavity damping is investigated in the rotated-wave approximation. First we introduce six appropriate combinations of the matrix elements of the density operator, which are still operators with respect to the light field. With the help of the s-parametrized quasiprobability distributions of Cahill and Glauber [Phys. Rev. 177, 1882 (1969)] the equations of motion for the density operator transform to six coupled partial-differential equations. By expanding the quasiprobability distributions into two suitable sets, we obtain six tridiagonally coupled differential equations for the expansion coefficients, which are solved by a Runge-Kutta method. Starting with an initial coherent state of the cavity field and the atom in its upper state, we find that the initially one-peak quasiprobability function splits into two peaked functions counterrotating in the complex plane and, depending on the damping constant, spiraling into the origin. Revivals of the inversion oscillation are found for those times, when the two peaks collide. The time dependence of the inversion and the intensity as well as some special distributions of interest are also discussed.

A quantum one-particle model for electron delocalization and transfer induced by a time-dependent potential is studied with a formalism based on the Lippman-Schwinger approach and fractional as well as operational calculus methods. The potential examined consists of a line of an arbitrary number of short-range (δ function) potentials whose amplitudes have an arbitrary functional dependence on time. The temporal evolution of the ionization probability, the energy spectrum, and the time of transfer are obtained analytically and calculated numerically through a stable and accuracy-controllable technique without any assumption regarding the strength of the perturbing potential.

In this paper we study the dynamics of a nonlinear quantum-optical system subjected to a periodic modulation in the form of kicks. The system consists of a nonabsorbing anharmonic medium related to optical bistability, a degenerate parametric amplifier with a classical pump field that is modulated by a periodic sequence of kicks, and the initial state is taken to be a squeezed vacuum state. Using the related SU(1,1) coherent states (which are squeezed vacuum states), we study and compare the quantum and classical dynamics of the system. The ‘‘classical’’ motion in this case means the motion projected onto the phase space defined by the parameter labeling the generalized coherent states. For certain values of the coupling constants, we obtain both regular and chaotic motion in the classical phase space. In the quantum dynamics of the system, the manifestation of the classical chaos turns out to be rather weak, at least in the regime numerically accessible to us. We study the time evolution of the initial-state population probability, the average photon number in the field, and the squeezing of the field. We also briefly discuss the level statistics of the evolution operator.

Several six-wave mixing processes generating extreme-ultraviolet and infrared radiation have been observed in argon gas excited by high-power subpicosecond 248-nm radiation. The processes involve waves over a very large spectral range, spanning the region between 63 nm and 4.3 μm. In addition, two nonparametric transitions are observed whose origin has not been determined

A complete description of the electric field in the Jaynes-Cummings model is presented using the exact operator solutions for the field variables. Two-time field correlation functions are calculated which display the destruction and subsequent partial revival of field coherence. It is shown that squeezing of the field can be enhanced for small n¯ when the atom is initially in a coherent superposition state. Amplitude-squared squeezing of both field quadratures occurs for all values of n¯ with these initial conditions. The squeezing is not related to collapse and revival phenomena.

We have solved Schrödinger’s equation numerically to obtain fully photon-dressed wave functions for an electron in a strong radiation field while it undergoes a collision with a model one-dimensional hydrogen atom. We have investigated a regime in which the incoming electron wave packet can be resonantly captured by the atom to form a model ${\mathrm{H}}^{\mathrm{-}}$ negative ion. We discuss the time dependence of the capture rate as well as the scattered energy spectra, which in the case of resonant capture contain additional structure due to the subsequent photodetachment of the captured electron.

We have made a quantitative study of quantum electrodynamic corrections to the energy of some simple atoms near a metallic surface. (i) The two-level atom provides a basic framework within which we discuss the van der Waals, Casimir, and resonant radiative level shifts. In this discussion we show that the level shifts of an excited atom are substantially different from those of a classical dipole antenna. (ii) We calculate the possible cavity QED corrections to the n=2 and n=1 hydrogen Lamb-shift measurements and obtain results that disagree with the published literature. (iii) We find a general expression for the shifts of the lowest S and P levels of alkali-metal atoms near a conducting surface. These might be studied in the laboratory as a test of cavity QED.

In this paper we examine the possibility of generating displaced and squeezed number states out of number states by the general driven time-dependent oscillator. Using the evolution-operator method developed by Cheng and Fung [J. Phys. A 21, 4115 (1988)], we investigate the evolution of a number state of a general driven time-dependent oscillator as well as its squeezing property. Our analyses indicate that the wave function of the time-dependent oscillator starts as a number state at t=0 and evolves as a displaced and squeezed number state at a later time.

Dynamics of vector solitons are studied within the framework of the general model of coupled nonlinear Schrödinger equations. The analysis is based upon the perturbation theory for the case when the system is close to the exactly integrable Manakov form. Evolution of the soliton’s polarization, coupled by the linear-birefringence terms (which take account of the difference in the group velocity between two linearly polarized modes) to the positional degree of freedom of the soliton, is studied. A four-dimensional dynamical system for the two coupled degrees of freedom integrates to a two-dimensional conservative system. Depending on the value of an arbitrary integration constant, there are four different types of the phase portrait of the latter system. For each value of the polarization angle, there exist two stationary vector solitons, at least one of them being stable. Generic trajectories on the two-dimensional phase plane correspond to oscillations of the polarization coupled to oscillations of the position of the soliton. A generalized model including the polarization-rotating linear coupling is also analyzed. Next, interaction of two slightly overlapping vector solitons is considered, and it is demonstrated that a stable bound state is possible. A stable periodic chain of the slightly overlapping solitons is also found. Finally, radiative decay of a vector soliton is investigated for the case when it has a large component in one subsystem and a small component in another subsystem.

We consider three nonlinear optical models that display a two-photon process and exhibit a Hopf bifurcation corresponding to a phase instability. The three models describe second-harmonic generation, two-photon optical bistability, and degenerate four-wave mixing in resonant cavities. In all three cases we consider only the resonant configuration and analyze the attractors which emerge when the driving field is increased beyond the Hopf bifurcation threshold. At a finite distance from threshold, we find a hysteresis domain that involves a pair of periodic attractors. For all three models, each of the two solutions has the same ‘‘field portrait’’ i.e., the same representation in a plane whose axes are the imaginary and the real parts of the electric field. This suggests that the scenario is generic for this class of systems. On the other hand, the models for two-photon optical bistability and degenerate four-wave mixing in resonant cavities exhibit additional nongeneric attractors when the driving field is further increased.

We study the physics of photosensitive-fiber-grating formation in optical fibers, assuming a local bleaching model, and demonstrate that sustained grating growth is possible. Good agreement with our experimental results is obtained after including fiber heating effects and assuming a reasonable two-photon absorption cross section. The grating is predicted to be chirped in space.

We study multiphoton ionization of hydrogen by a linearly polarized 248-nm field (the photons have an energy of 5.0 eV) up to an intensity of ${10}^{15}$ W/${\mathrm{cm}}^2$. We compare two very different computational methods for the calculation of the total ionization rate: on the one hand, the full numerical solution of the time-dependent Schrödinger equation; on the other hand, results obtained within the Floquet formalism, which assumes a quasiperiodic time dependence. We obtain very good agreement between the two sets of calculations throughout the whole intensity range. The calculated rates also agree well with third-order perturbation theory up to ${10}^{13}$ W/${\mathrm{cm}}^2$, but are orders of magnitude lower at ${10}^{15}$ W/${\mathrm{cm}}^2$. The shortfall at high intensities is attributed to the change from a nonresonant third-order process to a resonant fourth-order process caused by ac Stark-induced shifts in the energy-level structure of hydrogen. In fact, the ionization rate between 2 and 6×${10}^{14}$ W/${\mathrm{cm}}^2$ is almost constant, giving rise to a plateau feature in the rate versus intensity curve.