Coincidence measurements were performed to separate the charge-transfer and directionization channels for 15-100-keV ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{+}$ collisions with Ne,Ar,Kr. Absolute cross sections are given for the production of single and multiple ionization of the target by these two ionization mechanisms. It is demonstrated that as the collision system becomes heavier, the multiple ionization components of these two channels become increasingly more important, and the production of free electrons results primarily from higher-order charge-transfer channels—not from direct single target ionization.

With use of a method of Arnol'd, we derive the necessary and sufficient conditions for the formal stability of a parallel shear flow in a three-dimensional stratified fluid. When the local Richardson number defined with respect to density variations is everywhere greater than unity, the equilibrium is formally stable under nonlinear pertrubations. The essential physical content of the nonlinear stability result is that the total energy acts as a "potential well" for deformations of the fluid across constant density surfaces; this well is required to have definite curvature to assure stability under these deformations.

The propagation of ion-acoustic solitons in a plasma with negative ions has been investigated experimentally. At a critical concentration of negative ions, both compressive and rarefactive solitons are observed. The velocity and the width of the solitons are measured and compared with the soliton solution of the modified Korteweg-de Vries equation.

Measurements during pulsed laser irradiation indicate that amorphous Si melts at a temperature 200 ± 50 K below the crystalline value. Below energy densities required to melt the amorphous layer fully, the data are interpreted in terms of an explosive crystallization. The initial liquid layer solidifies to form coarse-grained polycrystalline Si. A thin, self-propagating liquid layer travels through the remaining amorphous Si at a velocity of 10-20 m/s, producing fine-grained polycrystalline Si.

The technique of picosecond electron diffraction is used to resolve in time the laser-induced melting of thin aluminum films. It is observed that under rapid heating conditions, the long-range order of the lattice subsists for lattice temperatures well above the equilibrium point, indicative of superheating. The melting time is found to vary according to the degree of superheating. The initial density of nuclei is determined under the assumption of a two-dimensional expansion model. These results show for the first time the relationship between superheating and the rate of transformation.

Simple conditions are presented under which the fractal dimension of a random walk on an aggregate, $d_w$, is given by $d_w=D+1\mathrm{}$, where $D$ is the aggregate's fractal dimension. These conditions are argued (with one simple speculative assumption) to apply for $D<\mathrm{}2\mathrm{}$, implying a breakdown of the Alexander-Orbach rule $d_w=\frac{3D}{2}$. Existing results for percolation clusters, lattice animals, and diffusion-limited aggregates seem to favor our new rule.

Measurement of the fractal dimension, $D$, of colloidal aggregates of small silica particles is reported. We observe power-law decay of the structure factor $\mathrm{}\left[S\right(k\left)\mathrm{}\sim k^{-D}\right]$ by both light and x-ray scattering showing that the aggregates are fractal. $D$ is found to be 2.12±0.05, which is intermediate between recent numerical results for the kinetic models of diffusion-limited aggregation ($D=2.5\mathrm{}$) and cluster aggregation ($D=1.75\mathrm{}$), but is rather close to the value for lattice animals ($D=2.0\mathrm{}$), which are equilibrium structures.

Measurements of multilayer adsorption of solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on graphite have been made for $0.965 \mathrm{K}<~T<~1.24 \mathrm{K}$ at intervals of ∼ 15 mK using fourth sound in He II as a probe. The first observation of the increase of the two-dimensional critical temperature $T_{\mathrm{cj}}$ with increasing layer number $j$ is reported. We extrapolate our values of $T_{\mathrm{cj}}$ for $j\to \infty$ and obtain the bulk-roughening transition temperature, confirming a prediction of current theories. The modeling of the acoustics including effects of the solid-liquid interface is also presented.

Brillouin scattering from $a$-Si${\mathrm{O}}_2$ up to 17 GPa shows that an irreversible change in the longitudinal sound velocity takes place between 10 and 17 GPa and that this change is stable at atmospheric pressure. Raman spectra of the retrieved sample indicate that it is also amorphous but differ noticeably from that of the original sample.

Low-frequency conductivity measurements (100 Hz to 100 MHz) are reported in the charge-density-wave state of orthorhombic Ta${\mathrm{S}}_3$. The observed frequency-dependent response is in clear disagreement with recent predictions which are based on the Fukuyama-Lee model. The results can be described by $\sigma \left(\omega \right)=A{\left(i\omega \right)}^{\alpha }$, with $\alpha <1\mathrm{}$, in a wide frequency range, suggesting relaxational dynamics associated with the collective mode.

We present the results of the first ab initio calculation of the optical spectra of Li${\mathrm{C}}_6$ up to 40 eV. The results are in excellent agreement with experiment and are used to identify the structures seen in the optical spectra of this compound, and are also used to clarify the origins of the observed and calculated plasmons. Some of the results appear to be common to all alkali-graphite intercalation compounds.

A quantum system which can tunnel out of a metastable state, and which interacts with an environment at temperature $T$, is considered. It is found that heat enhances the tunneling probability at $T=0\mathrm{}$ by a factor $\exp \left[A\mathrm{}\right(T\left)M{\omega }_0\frac{q_0^2}{\hslash }\right]$, where $M$ is the mass of the system, ${\omega }_0$ is the frequency of small oscillations about the metastable state, and $q_0$ is the tunneling distance. For an undamped system $A\mathrm{}\left(T\right)\mathrm{}$ is exponentially small, $A\mathrm{}\left(T\right)\mathrm{}\propto \exp \left(\frac{-\hslash {\omega }_0}{k_{B}T}\right)$, whereas for a dissipative system $A\mathrm{}\left(T\right)\mathrm{}$ grows algebraically with temperature.

Invasion-percolation observables which should scale with exponents interrelated through scaling and hyperscaling hypotheses are investigated. Evidence is presented that the hyperscaling relation between $\tau$, the spatial dimension, and the fractal dimension breaks down at the defender threshold. Thus an independent exponent must be introduced to describe the scaling of the finite-defender-cluster distribution.

Production of $\phi$ mesons in $e^{+}{e}^{-}$ annihilation at a center-of-mass energy of 29 GeV has been observed with the time-projection chamber detector at the PEP storage ring. The $\phi$ production rate has been measured in the energy range $0.075<x<\mathrm{}0.55\mathrm{}$ ($x=\frac{2E_{\phi }}{\sqrt{s}}$), giving 0.077±0.012(stat)±0.016(syst) $\phi$'s per event. The average value of ${p_t}^2$ relative to the thrust axis is 1.0±0.4 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$.

Using Monte Carlo methods, we conclude that the SU(4) gauge theory has a first-order confinement transition at finite temperature. On a lattice with small timelike extent, the bulk transition of the Wilson action drives the finite-temperature transition. We used an action with plaquettes in the fundamental and adjoint representations in order to avoid the bulk transition while keeping the lattice size manageable.

A consistent interpretation is presented of spontaneous fission events previously observed in Hg sources separated from CERN W targets. The energy spectra are interpreted in terms of fission to four fragments with $E\left(\mathrm{total}\right)\simeq 320$ MeV. The value of $\overline{\nu }$ is estimated to be between 3 and 5. The measured masses of the fissioning nuclei are consistent with various molecules of element 112 with about 160 neutrons. The prospects of producing superheavy elements with use of targets around W and projectiles like Sr and Zr are discussed.

We have observed a diminution in the measured mean free path of secondary fragments of heavier charges $Z>~15$ at ∼3 standard deviations. For heavier charges $Z>~29$, there seems to be a further reduction in the anomalous mean free path. As many as thirteen secondary interactions of projectile fragments were observed within about 50 μm from their production point against at the most one expected event. The loss of charge observed in these thirteen projectile fragments with respect to the primary charge is very small.

A computer simulation has been applied to the calculation of Stark profiles of hydrogenic ions for the conditions of inertial confinement fusion. Drastic modifications of the Lymanline profiles are observed when ion dynamics is taken into account.

High-resolution threshold photodetachment of ${\mathrm{H}}_2$C-CH${\mathrm{O}}^{-}$ has resulted in the observation of ∼50 narrow resonances (some <20 MHz full width at half maximum). These resonances are due to autodetachment from a diffuse negative-ion complex consisting of an electron weakly bound (∼5 ${\mathrm{cm}}^{-1\mathrm{}}$) by the dipolar electric field of the neutral. Electric fields of < 70 V/cm are adequate to detach the electron from the lower (bound) levels where autodetachment is not possible. Such states should exist for all neutrals with dipole moments greater than about 2 D.

The complete density matrix for the $n=3\mathrm{}$ level of hydrogen coherently excited by electron capture, ${\mathrm{H}}^{+}+\mathrm{He}\to \mathrm{H}\mathrm{}(n=3\mathrm{})+{\mathrm{He}}^{+}$, at medium to high energies is calculated with the first Born approximation and for the first time with a multiple-scattering theory, namely the continuum distorted-wave approximation. Final-state interactions are also explicitly incorporated to account for the long-range coupling effects between the degenerate states of the projectile. Comparison is made with recent experimental data and a quasiclassical interpretation of coherences in terms of the orbital parameters of the captured electron is proposed.