Dynamic dipole polarizabilities ${\alpha }_d$(ω) within and beyond the normal dispersion region for the neutral atoms Si, P, and S in their ground and valence excited states have been calculated using the linearized version of time-dependent coupled Hartree-Fock theory. Roothaan formalism has been adopted to treat the open-shell atoms. Excitation energies, oscillator strengths, and quantum defects have been estimated for several optically allowed transitions, and analytic representations of the singly excited Rydberg orbitals have been found. The results are compared with existing theoretical and spectroscopic data.

A correction to the 1/N-expansion method in quantum mechanics is presented. It actually improves the convergence when compared to previous versions of this technique, including the 1/N shifted version. By properly modifying the definition of the shifted parameter, the results are clearly better in the first approximation and the method becomes reliable for a much wider class of spherically symmetric potentials.

Using relativistic many-body perturbation theory, we have investigated the valence, exchange core polarization, and correlation contributions to the magnetic hyperfine constants in the ground states of the lithiumlike ions ${\mathrm{Be}}^{+}$, ${\mathrm{B}}^{2+}$, ${\mathrm{C}}^{3+}$, ${\mathrm{N}}^{4+}$, ${\mathrm{O}}^{5+}$, ${\mathrm{F}}^{6+}$, ${\mathrm{Ne}}^{7+}$, and ${\mathrm{Bi}}^{80+}$. Radiative corrections have also been investigated. Both the exchange core polarization and the correlation contributions as fractions of the valence-electron contribution decrease rapidly as one goes to more highly charged ions, the decrease being more drastic for the correlation effect. The radiative effect, on the other hand, increases very rapidly with increasing charge, becoming of the same order of magnitude as the correlation effect in ${\mathrm{O}}^{5+}$, ${\mathrm{F}}^{6+}$, and ${\mathrm{Ne}}^{7+}$. For ${\mathrm{Bi}}^{80+}$ the radiative effect is larger than the correlation contribution, being about 0.3% of the hyperfine field from the valence 2s electron. The significance of these results and trends will be discussed, and comparisons will be made between the net hyperfine fields obtained in the present work and available experimental results as well as with the results of earlier calculations.

In recent work a Stern-Gerlach interferometer (SGI) was considered in which a polarized beam of spin-(1/2 particles is split by a Stern-Gerlach apparatus into two partial beams, and then subsequent Stern-Gerlach deflecting magnets are used to reconstitute these two beams into one. In these studies it was shown that when such a coherent polarized beam passed through a SGI, some spin coherence is inevitably lost. In this regard, folk wisdom concerning irreversibility provides something of a guide to the present problem, since we all know that when Humpty-Dumpty had his great fall nobody could put him together again. In the present paper we consider the fate of our spin-(1/2 Humpty-Dumpty when a detector is present that is sensitive to the passage of particles along one trajectory, but not the other. It is not surprising that coherence is destroyed as soon as one is able to tell along which path the atom traveled. However, there seems to be no general agreement about the mechanism of coherence loss. Our conclusion is that the loss of coherence in measurements on quantum systems can always be traced to the dynamics of correlations between the measuring apparatus and the system being observed.

The three-particle quantum-mechanical Coulomb problem for general masses is treated based on the large-dimensional method. This extends the theory to anisotropic systems. A perspective of the bonded configurations of diverse three-particle systems is provided specified by radial and angular correlations. It is shown that proper dissociation behavior is embedded, being a culmination of appropriate treatment of mass polarization and recoil effects. Vibrational effects rooted in interparticle correlations and which materialize beyond the leading approximation are delineated. The accompanying force constants, knowledge of which is a prerequisite for determining the wave functions and spectral features, are investigated. A special premium is placed on exotic atoms which display enhanced anisotropic features and serve to illuminate the underlying physical aspects.

A multiple-scattering expansion is developed in this paper for the description of (e,2e) collisions in the presence of a laser field. The dressed states of the incident electron, the scattered electron, the ejected electron, and the target atom due to the laser field are considered and then the scattering matrix is expanded as a series of multiple-scattering terms based on the time-dependent multiple-scattering theory, such as ${\mathit{S}}_{\mathit{fi}}$=${\mathit{S}}_{\mathit{M}}^{\mathrm{}\left(1\right)\mathrm{}}$+${\mathit{S}}_{\mathit{M}}^{\mathrm{}\left(2\right)\mathrm{}}$+.... In the present approach, the effects of $S_M^{\mathrm{}\left(1\right)\mathrm{}}$ are almost equivalent to those of the first three orders of the Born expansion. Some aspects concerning the practical calculations of $S_M^{\mathrm{}\left(1\right)\mathrm{}}$ are also discussed at the end.

Vacuum-ultraviolet (vuv) polarization data are presented for the integrated radiation emitted following electron-impact excitation of He and Ne in the energy range from threshold to 500 eV. The data represent an improvement both in accuracy and energy range over those available previously. They can be used as secondary standards for calibrating vuv polarization analyzers and other optical devices.

Measurements of absolute total cross sections for single-electron capture in collisions of ${\mathrm{N}}^{3+}$, ${\mathrm{N}}^{4+}$, and ${\mathrm{N}}^{5+}$ ions with ground-state atomic hydrogen are reported in the energy region 0.9–1400 eV/amu. Good agreement is obtained with available theoretical calculations and with previous experimental results at the higher energies. None of the systems studied shows a clear signature of the induced-dipole attractive force in enhancing the cross section.

The two-potential approach is used to study the differential and integrated cross sections, the alignment and orientation parameters (λ,χ,〈$L_y$〉,γ), and the coherence of excitation in the resonant scattering of positrons with sodium atoms at three intermediate energies 12.1, 22.1, and 30 eV. These parameters are compared with the corresponding parameters in electron-sodium resonant scattering. Such a comparison between electron and positron scattering leads to useful information about the role of different interaction potentials in the scattering process.

Coupling effects among the channels for collisional excitation of 2$s^2$2$p^4$, 2s2$p^5$, and 2$p^6$ configurations of oxygenlike krypton have been reinvestigated and found to be small. In an earlier publication, it was reported that coupling to the 2s2$p^5$ ${}_{}{}^3{}_{}{}^{}°$ state has a strong influence on the cross section σ(2${\mathit{s}}^2$2${\mathit{p}}^4$ ${}_{}{}^3{}_{}{}^{}$P→2${\mathit{p}}^6$ ${}_{}{}^1{}_{}{}^{}$S). That apparent coupling is shown to be due to an inconsistency involving orthogonality and correlation functions. We solve the correct set of coupled integrodifferential equations using the niem, Σ, and rmatrix codes. All three calculations are performed using the same atomic states and the same configurations to describe the target. We find very good agreement among the cross sections obtained from all three codes. We also calculated distorted-wave cross sections for the same set of transitions. The distorted-wave results are nearly identical to the results obtained with the close-coupling codes.

Gauge-consistency aspects in multiphoton ionization by a strong single-mode laser field are considered. Differential and total cross sections of multiphoton ionization of the hydrogen atom are calculated in both the radiation (A gauge) and the electric field gauges (E gauge) within the framework of an S-matrix treatment, in which the wave function of the ejected electron is approximated by a Volkov-Coulomb wave function. The coupling between the initial atomic state and all the partial waves entering the expansion of the Coulomb function, resulting in a natural way from the requirements of gauge consistency, affects strongly the angular distribution of the ejected electrons, as compared with cases in which hybrid procedures are adopted.

By combining the quantum theory of radiative transition with multichannel quantum-defect theory, we have derived a general formula to calculate the lifetimes of highly excited states for atoms. As an example, we have evaluated the lifetimes for the perturbed 6snd ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ (n=8–40) and 6snd ${}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$ (n=10–26) Rydberg levels, and the perturbers of the ytterbium atom. We have not only obtained results that are in perfect agreement with the recent measurements, but also predicted the lifetimes of 18 other highly excited states. The results indicate that the dramatic variation for lifetimes of Rydberg states in the vicinity of perturbing states is due to the strong channel interaction.

Near-threshold photodetachment in strong laser fields has been discussed in the case of the presence of an autoionizinglike state in the ionic continuum. The role of a real negative pole of the propagator is stressed. The corresponding terms in transition amplitudes give rise to a new structure of the photoelectron spectrum and to the population trapping.

We consider a Hamiltonian trilinear in boson operators, which describes several physical processes, such as the interaction of N two-level atoms with a single-mode resonant radiation field, parametric amplification, Raman and Brillouin scattering, and frequency conversion. The dynamics of systems obeying this trilinear Hamiltonian is analyzed and the equations of motion are solved exactly. So the time development of the boson operators is described by means of Laplace transforms of suitable functionals. The integration method is based on iteration techniques and makes use of the Manley-Rowe relations. Finally, the results are applied to describe the time evolution of the boson operators when one of the three modes is initially in a high state of excitation.

We use amplitude-modulation techniques to investigate the response of a multimode dye laser when its pump intensity is sinusoidally amplitude modulated. A theoretical model is presented based on a Floquet analysis to obtain the stationary-state first-harmonic response of the population inversion and that of the laser intensity as a function of modulation frequency. We perform experiments in multimode continuous-wave dye lasers to demonstrate the validity of this theory in describing the behavior of multimode lasers operating near the lasing threshold.

Analytical traveling-wave self-pulsing solutions of a homogeneously broadened ring laser are presented in the limit that the dipole relaxation rate is much greater than the atomic inversion relaxation rate. The phase velocity v(Λ) as a function of the normalized pump Λ is given explicitly. This function has a simple relation with the upper boundary ${\alpha }_{\max (\mathrm{\Lambda }}$) of the Risken-Nummedal-Graham-Haken (RNGH) instability domain in the (α,Λ) plane, where α is the wave vector of the perturbations. The self-pulsing may appear in different ways, depending on how the stationary solution becomes unstable. If the instability of the stationary solution is caused by an unstable mode touching upon the upper boundary of the RNGH-instability domain in the (α,Λ) plane, the self-pulsing is supercritical; if the unstable mode lies on the lower boundary, a subcritical self-pulsing arises, and the system is bistable. This rule provides new signatures of the self-pulsing phenomena and will be helpful to experimental identification of the self-pulsing arising from the RNGH instability. The linear stability analysis reveals two kinds of instabilities for these self-pulsing solutions. One kind of instability is rooted in the RNGH instability of the stationary solution and may occur even when the amplitude of the self-pulsing solution approaches zero. The other kind of instability occurs when the oscillating amplitude of the self-pulsing solution becomes large. Above this large-amplitude instability threshold no traveling-wave self-pulsing can be stable any longer. The results for the corresponding Lorenz model are also presented.

We study the effect of the presence of a phase-conjugate mirror on the evolution of a classical radiating dipole and of a quantum-mechanical two-state atom. In both cases the modified evolution equations are solved and explicit solutions are presented. The evolution of the classical dipole and of the atomic coherences depends on the initial relative phase of the atomic dipole and the phase-conjugate mirror.

An analytical function H is derived in optical bistable systems, which serves as a criterion to characterize the multimode instability. Based on the criterion, the multimode instability boundary can be readily drawn. It is proved and verified by numerical results that in long-cavity systems, multimode instability always arises before the single-mode instability.

We develop a semiclassical theory of the polarization properties of phase conjugation by two-photon resonant degenerate four-wave mixing. The theory includes the effects of saturation by the pump waves. We solve the density-matrix equations of motion in steady state for a nonlinear medium consisting of stationary atoms with a ground and excited state connected by two-photon transitions. As an illustration of the general results, we consider an $S_0$→$S_0$ two-photon transition, which is known to lead to perfect polarization conjugation in the limit of third-order theory. We show that the fidelity of the polarization-conjugation process is degraded for excessively large pump intensities. The degradation can occur both due to transfer of population to the excited state and due to nonresonant Stark shifts. Theoretical results are compared to those of a recent experiment [Malcuit, Gauthier, and Boyd, Opt. Lett. 13, 663 (1988)].

The electromagnetic radiation emitted when a beam of electrons crosses a periodic stratified medium with a constant velocity is discussed. The treatment is valid for radiation wavelengths of the order of the period of the structure, particularly when Bragg reflections can take place. It accounts globally for the Čerenkov emission, the resonant transition radiation, and the Bragg resonances.