We have observed coherence between the photoionization of two different inner-shell vacancy states of argon, the energies of which differ by more than 75 eV. The method is based on the occurrence of interferences between photoelectrons and Auger electrons resulting from the photoionization of the two states concerned. The measured line shape of the interference structure can be well explained using the theory of van den Brink et al. on state-state interference. From the measurements we have derived a value of 1.9±0.2 for the width of the Ar 2${\mathit{s}}^{\mathrm{-}1}$${(}^2$${\mathit{S}}_{1/2}$) state.

The emission pattern of loosely bound projectile electrons is strongly dependent on the target species. For 0.5 MeV hydrogen colliding with krypton we observe large variations in the intensity of these electrons as a function of emission angle. There are also large changes in the energy and width of the electron loss peak associated with these intensity variations. These features are closely related to the Ramsauer-Townsend scattering of free electrons and can be interpreted within the electron impact approximation, a model which successfully combines free electron scattering by the heavy target with the Compton profile of the initially bound projectile electron.

The generalized Ramsauer-Townsend minima which occur, at certain scattering angles, in the intensity of electrons elastically scattered by atoms have been a subject of interest in atomic physics for over sixty years. While quantum mechanical calculations predict these minima with great accuracy, no clear, simple, intuitively appealing description of the underlying scattering processes has been given. It is shown here for the first time that simple semiclassical calculations provide such a description.

The n=2 fine-structure transitions in positronium have been measured by observation of increased Lyman-α radiation at microwave induced resonances. We used an accelerator based slow positron beam to produce positronium in the $2^3$${\mathit{S}}_1$ excited state inside a waveguide. Our results are ${\nu }_0$=18 499.65±1.20±4.00 MHz, ${\nu }_1$=13 012.42±0.67±1.54 MHz, and ${\nu }_2$=8 624.38±0.54±1.40 MHz for the $2^3$${\mathit{S}}_1$→k$2^3$${\mathit{P}}_{0,1,2}$ transition frequencies, respectively. The first error is statistical and the second is systematic. This is in agreement with recent bound state QED calculations.

The transfer operator developed recently by Bogomolny is used to calculate approximate semiclassical energy eigenvalues for a chaotic system, the wedge billiard. Only four classical trajectories, starting and ending on the Poincaré surface of section and crossing it once in between, are required to calculate an element of the T matrix. A 100×100T matrix is shown to give excellent results for the first 20 energy eigenvalues. It is also shown how the actions of the shortest periodic orbits of the 49° wedge can be obtained by calculating the Fourier transforms of Tr(${\mathit{T}}^{\mathit{m}}$).

We study how weak broadband noise affects 30.36 GHz 10%-‘‘ionization’’ thresholds F(10%) of ${\mathit{n}}_0$=54,...,72 H atoms. Calculations confirm that increased noise lowers F(10%) for most ${\mathit{n}}_0$’s raises some, but affects others little. We show noise to be a useful spectroscopic tool for uncovering novel quantal phenomena such as scarred states, which we link semiclassically to approximately adiabatic evolution near the separatrix of a nonlinear resonance island. Our results apply generally to the quantal behavior of classical mixed phase space systems.

In the chaotic region the spectra of hydrogen are shown to display universality in the response of the energy levels to magnetic field. Several autocorrelation functions are measured and shown to be in close agreement with recent theoretical predictions. These results represent the first experimental verification of universality beyond distributions of level spacings.

We have found several cases in our numerical simulations where Richtmyer’s prescription fails to give the correct growth rate for the Richtmyer-Meshkov instability in the linear regime. Another expression, due to Fraley [Phys. Fluids 29, 376 (1986)], agrees with our simulations. We discuss recent experiments and report two new types of perturbation freeze-out, single-shock and double-interface, which are quite distinct from the previously reported double-shock freeze-out.

A theory is presented for the electrohydrodynamic deformation of a thin fluid stream and applied to the experiments of Rhodes, Synder, and Roberts on colloidal dispersions. The resulting transverse velocity field has a symmetric four-lobe structure and, when the conductivity of the sample is greater than that of the surrounding fluid, the sample deforms into a ribbon whose axis is parallel to the incident field. The ribbon axis is perpendicular to the field if the conductivities are reversed. Electrohydrodynamic velocities are of the order of microns per second and the theory agrees with the qualitative and quantitative features of existing experimental data.

The Maxwell-Boltzmann (‘‘thermal’’) distribution constitutes the natural thermodynamic equilibrium state for a charged particle beam, and knowledge of its properties is therfore of fundamental importance. The Boltzmann relation for the particle density has a nonanalytic form when the space-charge force is included. We use numerical integration to determine the transverse and longitudinal density profiles for a relativistic beam in a linear focusing system at different temperatures ${\mathit{T}}_{\mathrm{\perp }}$ and ${\mathit{T}}_{\mathrm{\parallel }}$. The calculated profiles are related to space-charge tune despression, rms width, perveance, and emittance of the beam.

The two plasmon decay instability in a uniform ${\mathrm{CO}}_2$-laser produced plasma is studied by Thomson scattering of ruby laser radiation. Initially the instability grows as predicted by linear theory as decay plasma wave pair of expected wave vectors. During saturation unstable plasma wave activity spreads over wide regions in k space extending in places beyond the Landau cutoff contour. Comparing the saturated plasma wave intensity distribution in k space and measurements of the angular variation of 3/2-harmonic emission confirms the mechanism in which the radiation is generated by scattering of pump photons off two plasmon decay waves.

High-intensity periodic variations of the potential fall across the cathode sheath are observed in a low-pressure magnetic-field-free hollow-cathode discharge. These potential variations cause modulation of the discharge current, reaching a depth of almost 100%. Typical frequencies are tens of MHz, slightly lower than the ion plasma frequency near the cathode-sheath boundary and associated with the ion transit time through the sheath. The measured pressure dependence of the current threshold for the oscillations indicates that the mechanism of the instability is collisionless.

A scanning tunneling microscopy (STM) method for studying surface diffusion is developed based on measurements of the displacement distribution of adsorbates by ‘‘image-anneal-image’’ cycles which allow direct observation of the diffusion process while avoiding potential STM tip effects. The method is used to study the anisotropic diffusion of Sb dimers on Si(001). The energy barrier and the prefactor for the faster diffusion across the substrate dimer rows are measured. On the other hand, the diffusion observed by the ‘‘image-while-hot’’ method appears nearly isotropic and also much faster. It is shown that this discrepancy is due to the STM tip influence present in the latter method.

We derive relations linking the conductivity ${\mathrm{\sigma }}_{\mathrm{*}}$ and elastic moduli of any two-dimensional, isotropic composite material. Specifically, upper and lower bounds are derived on the effective bulk modulus ${\mathrm{\kappa }}_{\mathrm{*}}$ in terms of ${\mathrm{\sigma }}_{\mathrm{*}}$ and on the effective shear modulus ${\mathrm{\mu }}_{\mathrm{*}}$ in terms of ${\mathrm{\sigma }}_{\mathrm{*}}$. In some cases the bounds are attainable by certain microgeometries and thus optimal. Knowledge of the conductivity can yield sharp estimates of the elastic moduli (and vice versa) even for infinite phase contrast.

We have observed bulk solid superheating of the order of 650 K and persisting for Δt>500 ns in shocked single crystals of KBr at 22 to 33 GPa and CsBr at 30 to 42 GPa. These experiments use nanosecond-resolution, twelve-channel optical pyrometry to measure the temperature of the advancing shock front in the crystal, and the rarefaction-overtake technique to simultaneously measure sound velocity. This novel combination of measurements allows a virtually continuous determination of Hugoniot temperature versus pressure or density for transparent crystals.

The absorption and change in velocity of acoustic wave interacting with electrons in a superlattice irradiated by an infrared laser field are investigated. Near the roots of the equation ${\mathit{s}}_0$/Δd=‖${\mathit{J}}_0$(eFd/Ω)‖ (2Δ and d are the miniband width and period of the superlattice, F, and Ω are amplitude and frequency of the laser field, and ${\mathit{s}}_0$ is the sound velocity) a strong renormalization of sound velocity and intense absorption peaks are predicted. These phenomena have a straightforward relation with the dynamic localization of electrons and are called dynamic localization of acoustic waves.

A neutron powder diffraction study of the m=2 ferroelastic phase transition at 755 K in ${\mathrm{Na}}_2$${\mathrm{CO}}_3$ is presented, showing the first occurrence of ideal lattice melting. A detailed analysis of the profile of the diffraction peaks shows a striking broadening which develops close to ${\mathit{T}}_{\mathit{c}}$, consistent with recent theoretical predictions for this type of transition.

Using diamond anvil cell techniques and powerful synchrotron sources, we carried out an energy dispersive x-ray diffraction study on sulfur to 212 (GPa) and detected a transition to a new phase at 162±5 GPa. This new phase has been indexed as the β-Po structure based on agreement with the d spacings and the measured diffraction peaks. This is consistent with the published results on high pressure phases of Se and Te. The Raman spectra of the fully recovered sulfur indicate permanent changes in the molecular structure. This paper represents the first experimental results in which a structural determination has been made for the low-atomic-number materials above 100 GPa.

A novel strain-induced transition of the director orientation in a monodomain cross-linked side-chain liquid-crystal polymer system is reported which is analogous to the Fréedericksz transition in simple nematics. This new type of transition has a characteristic threshold strain which arises from an internal elastic barrier to rotation and is related to the level of anisotropy of the polymer chain trajectory.

Using NMR techniques we monitor the structure and number of vortices in a cylindrical container, rotating at constant angular velocity Ω, before and after the passage of the A-B interface. The initial A phase contains the equilibrium number of vortices. The final B phase is found to have a deficit of vortices which depends nonmonotonically on the measured time ${\mathit{t}}_{\mathit{A}\mathit{B}}$ of the A→B transmission. We argue that a vortex layer is formed in front of the moving A-B interface. The measured vortex deficit can be understood by analyzing the stability of the layer.