Laser excitation of ${\mathrm{OH}}^{+}$ to rovibrational levels of the ${\mathrm{c}}^1$Π state results in predissociation to form ${\mathrm{O}}^{+}$+H and O+${\mathrm{H}}^{+}$. Rotational levels with parity ${\mathrm{-}\left(\mathrm{-}1\right)}^{\mathrm{J}}$ (f component) decay to form ${\mathrm{O}}^{+}$+H, whereas those of parity ${+\left(\mathrm{-}1\right)}^{\mathrm{J}}$ (e component) decay predominantly to form O+${\mathrm{H}}^{+}$. The branching is interpreted in terms of coupling of the c ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }$ state to nearby electronic states and dissociation continua. Signals are recorded by tunable dye laser irradiation of a fast mass-selected beam of ${\mathrm{OH}}^{+}$ and detection of ${\mathrm{O}}^{+}$ ions or neutral oxygen atoms.

The question is examined whether there may be scattering media which could generate spectral frequency shifts in radiation from sources that are at rest relative to the observer and yet would imitate Doppler shifts. Scattering kernels of media whose physical properties fluctuate randomly in both space and time are presented which achieve this to a good approximation. The frequency shifts produced in this manner are not necessarily small. The results might be of particular interest in connection with the long-standing controversy about the origin of some of the large redshifts observed in the spectra of certain quasars.

We report the first experimental observation of type-II intermittency in an optical system. The type-II intermittency is observed in the light intensity of a GaAs/GaAlAs semiconductor laser with external feedback in the regime of the coherence collapse. We conclude the occurrence of a time-inverted type-II intermittency from the time distribution of the observed intensity breakdowns, with the injection current as control parameter. This interpretation is confirmed by a reconstructed Poincaré plot exhibiting the spiraling behavior of the type-II intermittency.

Wave multiple scattering is responsible for making a random medium cloudy in appearance and opaque in the sense of structure delineation. For a randomly layered medium such as the earth’s subsurface, however, knowledge about the generic behavior of multiple scattering enables us to construct a theory of statistical inversion which can recover from a single data set the slowly varying mean character of a medium with signal amplitude only ${10}^{\mathrm{-}3}$ that of the multiple-scattering noise. Inversion accuracy improves systematically with the availability of statistically redundant data.

Measurements of frequencies and density perturbations of the large amplitude l=1, ${\mathrm{k}}_{\mathrm{z}}$=0 diocotron mode on pure electron plasma columns are presented. This mode is a nonlinear dynamical state of an off-axis electron column which is E×B drifting around the containment axis. At large amplitudes, the plasma column distorts into an oval cross section and the mode frequency shifts by an amount proportional to mode amplitude squared. The frequency shift arises because (1) the plasma is closer to its image than a linear model assumes, and (2) the shape distortion modifies the image charge distribution.

We report the first observation of field-reversed configurations with nearly force-free magnetic fields in the central region of the compact toroid. The field profiles and flux ratios suggest that some form of relaxation phenomenon is occurring in this high-β kinetic regime. The magnetic helicity of the translating plasmas arises from axial asymmetry, possibly through end-shorting Alfvén waves.

A first-principles molecular-dynamics study of metallic liquid silicon is presented. Our description of the local order is in excellent agreement with x-ray and neutron diffraction experiments. Analysis of the electronic charge density shows persistence of some covalent chemical bonds in the liquid. These bonds give rise in the power spectrum of the system dynamics to a well identifiable feature associated with stretching vibrations. The single-particle electronic density of states shows metallic behavior. The calculated electronic conductivity agrees well with the available experimental information.

Molecular-dynamics computations based on a simple tight-binding interatomic potential fitted to hcp zirconium confirm the Burgers mechanism for the temperature-dependent hcp-bcc phase transition. In our model the transverse ${\mathrm{T}}_1$ [110]-bcc N-point phonon, which is involved in the transition, is unstable in the quasiharmonic approximation but stable at high temperatures. In addition, we calculate the vibrational entropy difference Δ${\mathrm{S}}_{\mathrm{vib}}$ between the two phases. The small value of Δ${\mathrm{S}}_{\mathrm{vib}}$ proves the importance of the electronic contribution to the total ΔS in the case of Zr.

It is shown that quadrupole-perturbed nuclear magnetic resonance provides a powerful technique to determine the local-polarization distribution and its second moment, the Edwards-Anderson order parameter in proton and deuteron glasses. The experimentally determined local-polarization distributions show the characteristic features predicted by a deuteron-glass model with infinitely ranged random-bond interactions in the presence of quenched random fields. The measurements demonstrate that we deal in ${\mathrm{Rb}}_{0.56}$(${\mathrm{ND}}_4$${)}_{0.44}$${\mathrm{D}}_2$${\mathrm{PO}}_4$ with a random-field smearing of a random-bond-type pseudo-spin-glass transition and not with a random-field-type freezing or a classical random-bond-type spin-glass transition.

The delayed nuclear-resonant Bragg scattering of synchrotron radiation at a strong allowed charge scattering peak in α${\mathrm{-}}^{57}$${\mathrm{Fe}}_2$${\mathrm{O}}_3$ has been observed. The detailed time dependence has been measured and successfully interpreted theoretically. This result demonstrates that it is no longer necessary to extinguish crystallographically the normal charge scattering in synchrotron radiation studies of nuclear-resonant scattering.

Angle-resolved reflection inelastic electron scattering has been used to measure the dispersion of the surface plasmon for thick films of Na and K. The measured dispersion is negative at small momentum parallel to the surface as predicted by quantum-mechanical calculations of the dynamic response of the electrons at the surface of an interacting electron gas. A simple physical picture is presented to explain the origin of the negative surface-plasmon dispersion in simple metals.

A planar quantum-point-contact device is proposed capable of producing focused electron beams. This solid-state device consists of a tunnel barrier attached to a quantum point contact. By solving the time-independent and time-dependent Schrödinger equation of the 2D system, it is shown that the angular spread of the emitted beam can be reduced to less than 10°, is coherent and collimated in energy, and is able to produce interferences. By applying a magnetic field for which electron orbits have a radius of 2 μm, it should then be possible to focus the beam down to 0.15 μm.

We have studied conductance fluctuations δg in a mesoscopic quasi-one-dimensional modulation-doped GaAs/AlGaAs heterojunction as a function of Fermi energy ${\mathrm{E}}_{\mathrm{F}}$ and applied magnetic field B at 1.3 K. Such a system has no, or negligibly small, spin-orbit scattering. We observe a reduction of the variance scrV(g(${\mathrm{E}}_{\mathrm{f}}$)) by a net factor of 4. This factor-of-4 reduction is a novel observation which is absent in a system with strong spin-orbit interaction and is due to the effect of the Zeeman splitting in breaking the spin degeneracy.

For the first time, we have observed the fractional quantum Hall effect in a junction comprised of four one-dimensional ballistic constrictions, less than 100 nm wide, in a high-mobility two-dimensional electron gas. The four-terminal Hall resistance at 280 mK is quantized at (h/$e^2$)/i where i denotes certain odd-denominator fractions, and surprisingly for (h/${\mathit{e}}^2$)/0.55<${\mathit{R}}_{\mathit{xy}}$<(h/${\mathit{e}}^2$)/0.45, as well. 0.45, as well.

A photorefractive effect which results from the spatial separation of electrons and holes in compositionally graded semiconductors is demonstrated for the first time. Unlike conventional photorefractive effects, this nonlinear optical phenomenon requires only a single illumination beam, does not rely on the presence of traps, and is intrinsically very fast. The time evolution of the formation and decay of the space-charge electric field is measured via the electro-optic effect using picosecond pump and probe techniques.

The near-band-gap photoluminescence of AlGaAs:Si shows a slow intensity transient after cooling the sample in darkness to low temperatures. This transient correlates to the Si dopant concentration. By investigating the behavior for below- and above-band-gap illumination we show that the observed transients are caused by hole capture at the DX center (hole-capture cross section ${\mathrm{\delta }}_{\mathrm{DX}}$,h≥2×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$). Using the transient to determine the occupation of the DX state, we confirm the electron-capture barrier and the photoionization cross section for DX centers, observed by Mooney et al.

Optical investigations of ZnSe/${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$Se quantum wells reveal a system in which the magnetic-field-induced spin splitting of the valence band in the barrier layers overwhelms the zero-field, ‘‘natural’’ band offset. This results in type-I behavior for one spin state (holes confined to the wells), and type-II behavior for the other spin state (holes confined to the barriers). Strongly asymmetric spin splittings are observed, and the asymmetry associated with this spin segregation of the holes provides an accurate measure of the valence-band offset in this unique system.

We have studied the anisotropy in the band structure of a strongly coupled GaAs/AlAs superlattice by the application of a magnetic field. An anticrossing behavior between the Γ and X states in the superlattice dispersion is demonstrated by the presence of an energy gap in the Landau-level diagram for cyclotron motion through the layers. A simple model is developed to treat Γ-X mixing at interfaces in the envelope-function approximation and the observed mixing strength is examined in terms of theoretical predictions.

We demonstrate that the presence of a magnetic field H leads to an enhancement of multiphonon resonant Raman scattering in GaAs. Sharp lines at up to 7 times the LO-phonon Raman shift are observed. The Raman efficiency of multiphonon peaks shows resonant behavior as a function of H. From this an identification of the Landau levels which participate in the observed resonances is possible. Intraband and intralevel Fröhlich electron-phonon interaction is found to mediate the higher-order Raman processes.

We have investigated the interplay of fractional statistics and magnetic field for anyons on a lattice, by exact calculation of ground-state energies and wave functions for small systems. We find that the two types of ‘‘flux’’ (magnetic and statistical) are to some extent interchangeable in determining the ground-state properties, and thus that mean-field theories, which replace statistical phase shifts by magnetic fields, may be an appropriate method for treating fractional statistics.