The elastic scattering of ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$+${}_{}{}^{58,64}{}_{}{}^{}\mathrm{Ni}$ was measured at sulfur beam energies of 82, 88, 91, 93, 98, 102.5, 108, and 150 MeV. Evidence is found for a marked energy dependence of the optical potential at the strong-absorption radii around the Coulomb barrier, in good agreement with parallel information extracted from the fusion cross sections for the same systems.

Cross sections for the two-body channels populated in the ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$+${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ reaction at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=60.8\mathrm{}$ MeV have been measured by use of a coincidence technique which allows correction for secondary light-particle evaporation. The data show reaction yields with full equilibration of energy and mass-asymmetry coordinates. These results suggest an asymmetric fission mechanism and are contrary to what is expected from the previously proposed "orbiting" mechanism in light systems.

The giant dipole resonance in ${}_{}{}^{224}{}_{}{}^{}\mathrm{Th}$ excited to 44, 64, and 82 MeV was observed following the fusion of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$+${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. The total $\gamma$ spectrum is analyzed in terms of prefission and postfission $\gamma$ decay. The data can only be fitted by our requiring an enhancement of the giant-dipole-resonance $\gamma$ rays from the prefission nuclei which is explained by a large fission hindrance in the early decay steps from the compound system. A measurement of the $\gamma$-fission angular correlation supports this conclusion and establishes a deformed shape of the compound nucleus.

X-ray yields of the muonic helium (2-1) transition from $\mu$-catalyzed fusion have been measured for the first time in various mixtures of hydrogen isotopes. They were determined to be 0.032(4), 0.016(2), and 0.0019(5) for $\mathrm{pd}$, $\mathrm{dd}$, and $\mathrm{dt}$ fusion, respectively.

We show that pump-induced modifications to the gain and dispersion profiles can induce a weak side-band modulation which acts as a driving term to sustain chaotic relaxation oscillations in a single-mode, resonant, optically pumped molecular laser. Our study suggests that a careful two- or more-parameter unfolding of higher-codimension bifurcations should lead to a wealth of nonlinear dynamical behavior within easy access of current experiments.

We analyzed for the first time systematically the half widths of a great variety of neutron spectra of simple dense classical fluids as functions of wave number and density. We find that the observed behavior of the half width near its minimum value can be understood quantitatively on the basis of a generalization of a diffusion model, proposed before.

The metastable state of the $\mathrm{EL}2\mathrm{}$ defect in GaAs is characterized by the absence of any experimentally observed defect level. We present a model of its atomic configuration, which allows a simple interpretation of this nonobservation as well as the direct and reverse transitions from the stable to this metastable state. The model consists of a split interstitial configuration of the initial stable ${\mathrm{As}}_{\mathrm{Ga}}-{\mathrm{As}}_i$ pair. We first use simple physical arguments to show that this configuration has the required electronic properties. We then substantiate these arguments by a full calculation of the local atomic configuration and of the corresponding electronic structure.

This work presents the first real-space images which resolve individual close-packed metal atoms on the surface of a Au(111) thin film. Measurements made in both air and ultrahigh-vacuum environments reveal the same highly ordered, laterally extensive close-packed structure with a corrugation amplitude of ≃0.3 Å at a tip bias of +30 mV and a tunneling current of 3 nA. The possible role of surface electronic states on Au(111) in producing the corrugations imaged by the scanning tunneling microscope is discussed.

Structural change of $\mathrm{GaP}\left(\overline{1}\overline{1}\overline{1}\right)$ surfaces induced by irradiation with laser pulses in the photon energy region between the indirect and direct band gaps has been investigated. A dip was found near the indirect band-gap energy in the relation between the photon energy and the threshold laser fluence above which the change from the (17×17) reconstructed structure to the (1×1) structure occurs. It is concluded that the structural change induced by laser pulses near the indirect band-gap photon energy is due to surface electronic excitation.

High-resolution electron microscopy shows the coexistence of several distinct core structures for coincident-site lattice boundaries in NiO bicrystals near $\Sigma =5, \mathrm{}\left(310\right)\mathrm{}$, and $\Sigma =13, \mathrm{}\left(510\right)\mathrm{}$, where $\Sigma$ is the reciprocal density of coincident lattice sites. Lattice expansion perpendicular to the $\Sigma =5\mathrm{}$ grain boundaries is considerably smaller than calculated theoretically. Structural units in boundaries near $\Sigma =13, \mathrm{}\left(510\right)\mathrm{}$ exhibit translations normal to the grain-boundary plane, resulting in a rumpling of the interface on an atomic scale. Our observations suggest the presence of a high concentration of Schottky pairs in the grain boundaries.

A long-lived transient form of chaos was observed by rf perpendicular pumping of spheres of yttrium iron garnet in the region of the first-order Suhl instability. The average temporal lengths of these transients as functions of pumping power were fitted by a theory involving capture of the chaotic trajectory by multiple periodic attractors.

The first experimental observation of a soliton-antisoliton annihilation process in solid state is reported. By precise control of the collision point of a fluxon and an antifluxon in a Josephson transmission line, the annihilation process is observed not only in time but also in space by the Josephson sampler which is coupled to the center of the transmission line. The observed line shapes agree well with the results of numerical calculations which show that a fluxon and an antifluxon fade off in the breather decay mode after collision, and that a fluxon behaves as a relativistic particle.

We report the first experimental evidence for the existence of free polyexcitons ($n>\mathrm{}2\mathrm{}$) in a semiconductor. Resolved polyexciton lines, together with the biexciton line, were observed in the green photo-luminescence spectrum of ultrahigh-purity silicon. We propose that the unresolved luminescence of these polyexcitons in the "ordinary" near-infrared photoluminescence process may provide an alternative explanation for a band interpreted by others to be due to a new condensed phase of the electron-hole plasma.

Intramolecular diffusion-controlled reactions between internal groups on an entangled polymer are considered in the "reptation" or "tube" model. The results motivate new direct microscopic experimental tests of reptation. For short times compact exploration is found in which the fraction $R\left(t\right)\mathrm{}$ that has reacted grows like the exploration volume of a monomer followed by a noncompact regime when $R\left(t\right)\mathrm{}$ scales like the distance moved along the tube. At long times compact behavior is recovered and a rate constant is found which is much smaller than the relaxation rate of the polymer.

We have for the first time directly observed the flow of angular momentum alignment in a heat-conducting gas of rotating molecules (Kagan polarization). The sign has been determined. It is found that the polarization is proportional to the square of the angular momentum.

Measurements of ${\Sigma }_c^0-{{\Lambda }_c}^{+}$ and ${{\Sigma }_c}^{++}-{{\Lambda }_c}^{+}$ mass differences have been made in a beam of incident neutrons. Our ${{\Sigma }_c}^{++}-{{\Lambda }_c}^{+}$ mass-difference measurement is 167.4±2.1 MeV/${\mathit{c}}^2$, in good agreement with previous results. We measure the ${\Sigma }_c^0-{{\Lambda }_c}^{+}$ mass difference to be 178.2±2.0 MeV/${\mathit{c}}^2$. We use these measurements to determine the ${{\Sigma }_c}^{++}-{\Sigma }_c^0$ mass splitting to be -10.8±2.0 MeV/${\mathit{c}}^2$.

The results of an experimental search with a new method for the spontaneous conversion of muonium to antimuonium are reported. The upper limit for $G_{\mathrm{M}\overline{\mathrm{M}}}$, the coupling constant characterizing the strength of the interaction leading to the conversion, is measured to be $G_{\mathrm{M}\overline{\mathrm{M}}}<7.5\mathrm{}{G}_{\mathrm{F}}$ (90% confidence level), where $G_{\mathrm{F}}$ is the Fermi coupling constant. This result is about a factor of 3 lower than the previous limit and begins to probe predictions of the left-right-symmetric theory with a doubly charged Higgs triplet.

The flow of light nuclei ($Z=1, 2$) and intermediate-mass nuclear fragments ($3\le Z<\mathrm{}10\mathrm{}$) is measured in collisions of 200-MeV/nucleon Au+Au over a large solid angle. An increase in the fragment-position and momentum-space alignment relative to the reaction plane is observed: The fragments exhibit stronger flow effects than light particles.

Changes in the nuclear density arising from shape fluctuations are calculated in the case of the closed-shell nucleus ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The main contributions come from self-energy processes which renormalize the single-particle potential. Those due to ground-state correlations are much smaller. Processes involving the coupling to isoscalar vibrations are the most important, the role of isovector modes being small. Changes in the radius and in the diffusivity, of the order of 5% and 20%, respectively, are found.

The intramolecular vibrational energy distribution of infrared-multiphoton-excited C${\mathrm{F}}_2$${\mathrm{Cl}}_2$ molecules is studied with time-resolved spontaneous Raman scattering. The time evolution of the signals from three vibrational modes is studied up to 600 ns after excitation, and in the presence of ${\mathrm{N}}_2$ buffer gas. Following collisionless infrared multiphoton excitation a nonthermal distribution of vibrational energy is observed. This nonequilibrium distribution persists up to high levels of vibrational excitation, with at least 10 000 ${\mathrm{cm}}^{-1\mathrm{}}$ in three out of the nine vibrational modes.