A method is presented to define unique continuum states for the two-center Dirac Hamiltonian. In the spherical limit these states become the familiar angular-momentum eigenstates of the radial Coulomb potential. The different states for a fixed total energy ‖E‖>m may be distinguished by considering the asymptotic spin-angular distribution of states with unique scattering phases. The first numerical solutions of the two-center Dirac equation for continuum states are presented.

Charge-transfer and electron-detachment cross sections for collisions of ${\mathrm{H}}^{\mathrm{-}}$ and ${\mathrm{D}}^{\mathrm{-}}$ with Cs atoms have been measured in the energy range from 3 to 300 eV.

The photoionization cross section of Mgi Σ near threshold in the region between 160–100 nm was studied using synchrotron radiation, an atomic-beam technique, and a time-of-flight mass spectrometer for the selective detection of the ${\mathrm{Mg}}^{+}$ photoions. Special attention was given to the signals of the double electron resonances 3pns and 3pnd ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ in comparison with theoretical calculations and recent experiments.

Kα $L^1$ x-ray satellite spectra of P, S, Cl, K, and Ca excited by Rh x rays are studied employing a plane crystal spectrometer. Aberg’s theoretical I(${\mathrm{KL}}^1$)/I(${\mathrm{KL}}^0$) intensity ratios based on a free Ne core are found to be overestimations for Z>15. The average ${\mathrm{KL}}^1$ satellite group energy shifts from that of ${\mathrm{KL}}^0$ of P, S, and Cl are found to be in general agreement with Dirac-Fock evaluations of Maurer et al. up to 3 to 4 M electrons.

We have calculated the partial autoionization widths of the O v 1s2$s^2$2p ${}^3$$P^0$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^0{}_{}{}^{}$ states which were measured in recent experiments by Bruch et al. In order to account for electron correlation and interchannel coupling we have used the state-specific theory of autoionizing states with complex coordinates. Our results correspond to well-defined regions of stability of the widths and yield ratios for the partial widths which, contrary to existing theory, are in agreement with experiment.

For the first time, ratios of partial autoionization widths have been measured for highly ionized oxygen projectiles. In particular we have studied the Auger decay of the Be-like (1s2$s^2$2p${)}^3$$P^0$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^0{}_{}{}^{}$ states in $0^{4+}$ decaying to the 1$s^2$2p and 1$s^2$2s final ionic states. Our experimental intensity ratios for the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_{}^0{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^0{}_{}{}^{}$ states are 1.7 and 16, respectively. We also present new results on the partial and total autoionization widths of these states from calculations which include electron correlation and interchannel coupling via the application of the many-electron theory of ionic resonances in terms of complex coordinates.

A Fokker-Planck code in which the Rosenbluth potentials are calculated from the anisotropic distribution function has been employed to study the isotropization of initially highly anisotropic velocity distributions. The results show a fundamental difference between the isotropization of oblate and prolate distributions.

We study the elastic scattering of two neutral composite systems, both of which have states nearly degenerate with the initial state. A novel consequence of the degeneracy is the appearance of a term in the scattering amplitude f, coming from the second Born approximation, which behaves as i($Q^2$/p)ln($Q^2$/$p^2$) for small momentum transfer Q and fixed momentum p. This term strongly affects the behavior of the partial-wave amplitudes $f_l$ for l>1. Possible applications to experiment, especially to the scattering of excited-state positronium from molecules, are indicated.

The boundary-layer equations are solved to calculate the velocity profiles for a gas obeying the non-Newtonian viscosity formula which predicts a vanishing viscosity as the Knudsen number or the shear rate increases. An analytic formula is obtained for the reduced velocity profile which shows a trend of a diminishing boundary-layer thickness as the density decreases, and it eventually becomes almost a step function which makes the velocity look as though it is slipping. This is in contrast to the prediction obtained using the Chapman-Enskog viscosity which yields a reduced velocity profile invariant to change in the gas density or the Knudsen number.

We report a numerical computation of the energy-level spacing distribution, the spectral rigidity, and the transition-amplitude distribution for an ensemble of Hamiltonians with a single ‘‘bright’’ state interacting with a manifold of ‘‘bath’’ states. The bath states are assumed to be part of a regular spectrum so that the initial states have Poisson statistics. Despite very strong average coupling of all bath states to the bright state (which may represent coupling to another electronic surface), statistical properties of the final spectrum are more like those of the initial bath distribution than those of the Gaussian orthogonal ensemble distribution expected for an irregular quantum spectrum. The level spacings show level repulsion, but with the nonlinear threshold behavior P(s)∼$s^{0.625}$.

The azimuthal anchoring of a nematic liquid crystal at a grooved interface is usually explained in terms of the well-known Berreman model. Recent experimental results, however, suggest that the original Berreman model must be modified in order to account for the presence of a reduced (or increased) order at the surface. A further important parameter, assumed to be infinite in the Berreman model, is the polar anchoring energy. In this paper we investigate how finite polar anchoring and the reduced (or increased) surface order affect the azimuthal anchoring, and we propose a more general expression for the azimuthal anchoring energy at a grooved interface. The main result of this paper is that the finite polar anchoring can lead to an important contribution to the azimuthal anchoring, while the effect of surface order can be usually neglected except in the case where the isotropic phase wets the substrate.

We point out that an expression derived recently by Gupta and Lawande for the growth rate of the Rayleigh-Taylor instability in spherical geometry is identical to the result of Binnie derived some time ago, a result that was subsequently shown to be in error by Plesset. We also comment on some of the differences between the results of Binnie and Gupta-Lawande and of Plesset.

We clarify the assumptions involved in our analysis of the Rayleigh-Taylor instability in a spherical geometry published recently.

Accurate wavelengths for the 4$p^6$5p ${}^2$$P_{1/2}$–4$p^5$5s5p ${}^4$$S_{3/2}$, 4$p^6$5p ${}^2$$P_{3/2}$–4$p^5$5s5p ${}^4$$S_{3/2}$, and 4$p^6$4d ${}^2$$D_{3/2}$,5/2–4$p^5$5s4d ${}^4$$P_{5/2}$ transitions of Rb i have been measured by photographing a sliding-spark light source with a 10.7-m normal-incidence vacuum spectrograph. The energies of the 4$p^5$5s5p ${}^4$$S_{3/2}$ and 4$p^5$5s4d ${}^4$$P_{5/2}$ levels were determined as 134 250.1±0.4 ${\mathrm{cm}}^{\mathrm{-}1}$ and 136 806.4±0.4 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively.

The role of a static field in the classical, stochastic ionization of the stretched hydrogen atom, interacting with a microwave field, is studied. Field-dependent action-angle variables are constructed and the effect of the microwave field is treated perturbatively in terms of the new canonical variables. The threshold for the onset of irregular motion is calculated using the Chirikov resonance-overlap criterion. This value for the critical ac field is seen to be nontrivially modified by the additional static field, though the change is inadequate to explain discrepancies with experiment.

We formulate a gauge-invariant approximation for treating the ionization of a one-electron atom by two lasers, one intense and of low frequency, the other weak and of high frequency. We have applied the approximation to hydrogen and obtained an intensity-dependent structure in the photoelectron angular distributions.

To explain the recent observation of resonances in 6p photoionization at and above the second (6$p_{3/2}$) threshold multiconfiguration Dirac-Fock and configuration-interaction calculations were performed for the low-energy excited states of atomic Pb. The results support the assumption that these are autoionization resonances arising from 6s6$p^3$ configurations.

Spectroscopic measurements of the electric-quadrupole-allowed 5$d^{10}$6s ${\mathrm{}}^2$$S_{1/2}$ to 5$d^9$6$s^2$ ${\mathrm{}}^2$$D_{5/2}$ transition near 282 nm on a single, laser-cooled ${\mathrm{Hg}}^{+}$ ion give a recoil-free absorption line (carrier) and well-resolved motional sidebands. From the intensity ratio of the sidebands to the carrier, the effective temperature of the ${\mathrm{Hg}}^{+}$ ion was determined to be near the theoretical minimum of 1.7 mK. A fractional resolution of better than 3×${10}^{\mathrm{-}11}$ for this ultraviolet transition is achieved.

We show that in addition to the length of the Bloch vector there exists an energylike integral of motion for the Maxwell-Bloch equations. This allows us to recast the semiclassical problem of atom-field interaction into a classical Hamiltonian problem of two coupled oscillators which contain both periodic and homoclinic orbits. Melnikov’s method is then used to demonstrate the existence of Smale horseshoe-shaped maps in its dynamics which preclude the existence of any further analytic integral of motion. The treatment yields a systematic procedure for understanding the mechanism of instability in this fundamental model of quantum optics.