The first experimental-theoretical study of Rydberg autoionizing resonances along an isoelectronic sequence is presented. This analysis demonstrates the intimate connection between electron-electron correlation, term dependence, and autoionization and underscores the power of multichannel quantum-defect theory in analyzing complex spectra.

The response to a monochromatic field of a three-level system, in which the (1,2) interval is relatively close to the (2,3) interval and the associated dipole matrix elements are arbitrary, is examined. It is found that, for weak pumping, proper detuning from exact two-photon resonance toward the more strongly coupled frequency produces a spin-½-type Rabi oscillation between levels 1 and 3, while proper detuning at strong pumping in the opposite direction produces a spin-1-type oscillation.

Numerical studies of the ablation-driven Rayleigh-Taylor instability in laser-accelerated targets show growth rates typically within a factor of 2 of the classical growth rate. The appearance of the "Kelvin-Helmholtz" instability depends on the form of the initial perturbation and also on the laser irradiance. Perturbations of the target surface and laser irradiation are simulated and compared.

The responses to shear of monomolecular films of stearyl alcohol (${\mathrm{C}}_{18}$${\mathrm{H}}_{37}$OH) and nonadecanoic acid (${\mathrm{C}}_{18}$${\mathrm{H}}_{37}$COOH) spread on water were studied as a function of density. A static shear modulus appears at a phase transition, previously assigned as the liquid-solid transition. The value of the modulus, however, is several orders of magnitude lower than the Kosterlitz-Thouless prediction.

A high-resolution acoustic impedance technique has been used to investigate the real squashing, $J=2\mathrm{}$, mode in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$. For a magnetic field perpendicular to the sound propagation direction, six components have been observed. The two center states appear to be a splitting of the $J_z=0\mathrm{}$ mode. This new splitting is observed to be dependent on $\frac{T}{T_c}$, pressure, and/or frequency, but is independent of magnetic field. In $H\cong 0$, additional features have been observed which can be associated with dispersion-induced mode splitting.

An entirely new, very general, atomically smooth, and optically symmetry-adapted reconstruction model for the Si(111)-7×7 surface is proposed. It consists of parallel rows of nontouching, $\pi$-bonded, hexagonal rings with center atoms and seems to be a realization of a static commensurate phase of a nonlinear lattice. An overwhelming number of experimental findings on the Si(111) surface finds an intuitive, simple, and natural explanation by the model.

The specific heat of potassium has been measured between 0.33 and 20 K, on an accurate and well-documented temperature scale. The results show no evidence of the previously reported phason contribution.

Photoinduced dichroism caused by bleaching of the interband transition in trans-${\left(\mathrm{CH}\right)}_x$ was observed in the picosecond time domain. The bleaching is induced instantaneously and decays with time as $t^{-\frac{1}{2}}$, changing to a lower power after ∼50 psec. Although the photoinduced bleaching persists to long times, polarization memory is lost after ∼${10}^{-9\mathrm{}}$ sec. The results provide a measurement of the diffusion constant of the charged excitations; $D\simeq 2\times {10}^{-2\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ at 300 and 80 K.

The time dependence of photoinduced optical absorption in the prototype one-dimensional semiconductor, polyacetylene, is investigated with femtosecond resolution. It is found that the induced absorption appears in less than 150 fsec. The dynamics of the absorption decay are measured and compared with a model based on geminate recombination on a one-dimensional chain.

High-resolution Si $2p$ core-level spectra reveal a homogeneous broadening for amorphous compared to crystalline silicon. This is due to static fluctuations in valence charges brought about by bond-length and bond-angle variations in the amorphous network. The observed Gaussian broadening of 256 meV corresponds to mean deviation from charge neutrality of approximately 0.11 elementary charges. The addition of hydrogen reduces the charge fluctuations by ∼25%.

Experimental evidence for thermal excitation of plasmons in two-dimensional electron systems is reported. The spectral intensity and temperature dependence of the thermal excitation are calculated by means of Bose-Einstein statistics, and give quantitative agreement with the results of far-infrared emission experiments. It is found that the plasmons may contribute significantly to the specfic heat of the two-dimensional electron system at low electron densities and high temperatures.

The superposition of the wake potentials of Coulomb-exploding fragments of diatomic molecular projectiles penetrating a solid cause potential oscillations at the surface. The total electron yield per projectile serves as a signal to detect these oscillations. The plasma frequency of the solid and the wake-potential wavelength can be deduced from the data.

The localization and electron-electron interaction parts of the small conductivity variation with temperature have been extracted from data obtained on narrow field-effect transistors. One-dimensional behavior is observed and is compared with measurements on the two-dimensional region of the test samples. Magnetoconductance which selectively removes the localization part of the resistance has allowed a theoretical interpretation of the total temperature dependence.

The discovery of new integrable two-dimensional Hamiltonian systems is reported. The analytic structure of the solutions makes necessary the generalization of the Painlevé conjecture, a widely used integrability criterion. Such a generalization is presented, which the authors believe should replace the usual conjecture for two-dimensional Hamiltonian systems. It is indeed compatible with all the systems already found and, in addition, leads to still new integrable cases.

The relativistic corrections to the Newtonian tidal accelerations generated by a rotating system are studied. The possibility of testing the relativistic theory of gravitation by measuring such effects in a laboratory in orbit around the Earth is considered. A recent proposal to measure a rotation-dependent tidal acceleration as an alternative to the Stanford gyroscope experiment is critically examined and it is shown that such an experiment does not circumvent the basic difficulties associated with the gyroscope experiment.

A quantum mechanical particle which moves in a symmetric double-well potential, and whose interaction with the environment is described in the classical regime by a phenomenological friction coefficient $\eta$, is considered. It is shown that, provided $\frac{\eta {q_0}^2}{\hslash }$ exceeds a critical value of order unity ($\pm q_0$ are the locations of the potential minima), the mean rate of tunneling between the degenerate minima decreases with temperature, leading at $T=0\mathrm{}$ to spontaneous symmetry breaking.

Possible advantages of replacing the Peccei-Quinn U(1) quasisymmetry by a group of genuine flavor symmetries are pointed out. Characteristic neutral Nambu-Goldstone bosons will arise, which might be observed in rare $K$ or $\mu$ decays. The formulation of Lagrangians embodying these ideas is discussed schematically.

Measurements of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$+ ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ capture in which high-energy $\gamma$ rays were detected in coincidence with recoiling ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ nuclei are reported. The $\gamma$-ray spectra are dominated by strong transitions to the excited $K=0\mathrm{}$ prolate band of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$. The excitation functions for the decays to the prolate states at 6.7 MeV ($0^{+}$) and 7.4 MeV ($2^{+}$) are consistent with giant dipole resonances. The width of these resonances is narrower than that of the oblate ground state, in surprising contrast to what is expected from energy and deformation considerations.

Experimental observations of coherent, millimeter-wavelength, higher-cyclotron-harmonic oscillations are reported for a system of an electron beam in a magnetic field traversing a Fabry-Perot resonator. The parameters of the experiment tend to support the interpretation that the strong multiple-harmonic interaction results from mode conversion of short-wavelength electrostatic waves (Bernstein modes) to long-wavelength electromagnetic resonator modes at the beam boundary.

Electromagnetic sideband spectra induced by a strong monochromatic high-frequency radio wave in an overdense ionosphere have been observed for the first time by means of a new direct observational technique. The observed asymmetry of the spectra, a characteristic feature of stimulated scattering, is tentatively attributed to parametric and linear-mode-conversion processes occurring in the irradiated ionospheric-plasma volume.