Percolation critical exponents are derived, for the first time, for a two-dimensional system of randomly distributed conducting sticks, which provides a very convenient model for the study of continuum percolation. In the present computer study it was found that the corresponding conductivity exponent, $t$, has the value of 1.24±0.03 and that the cluster exponents $\beta$, $\gamma$, and $\tau$ have the values 0.14±0.02, 2.3±0.2, and 2.0±0.1, respectively. These results, which are in excellent agreement with values derived for lattices, show that the conductivities of continuum systems and of lattice systems belong to the same universality class.

Results are presented from the analysis of solar system astrometric data, notably the range data to the Viking landers on Mars. A least-squares fit of the parameters of the solar system model to these data limits a simple time variation in the effective Newtonian gravitational constant to (0.2 ± 0.4) × ${10}^{-11\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$ and a rate of drift of atomic clocks relative to the implicit clock of relativistic dynamics to (0.1 ± 0.8) × ${10}^{-11\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$. The error limits quoted are the result of uncertainties in the masses of the asteroids.

An exact, asymptotically flat, stationary solution of the field equations of O(2) extended supergravity theory is presented. This solution has a mass, central electric charge as well as a supercharge, and constitutes the first exact, supersymmetric generalization of the black-hole geometries. The solution generalizes the extreme Reissner-Nordström black holes.

Inclusive distributions are presented for final-state ${\pi }^0$ and ${\eta }^0$ mesons produced in $\gamma p$ interactions. For photon energies from 50 to 130 GeV, both cross sections scale for fractional momenta $x_F>~0.30$. The average transverse momentum for both mesons is approximately constant with energy and equal to 0.51±0.05 GeV/c at large $x_F$. The $\frac{{\eta }^0}{{\pi }^0}$ ratio similarly rises from 0.07±0.03 at $x_F=0.38\mathrm{} \mathrm{to} 0.30\pm 0.09$ at $x_F=0.90\mathrm{}$. The ${\pi }^0$ distributions are in good agreement with those seen in the scattering of highly virtual incident photons.

An experiment was performed to capture fractionally charged particles produced in heavy-ion collisions and to concentrate them in samples suitable for analysis by various techniques. Two of the samples so produced have been searched, with use of an automated version of Millikan's oil-drop apparatus. The beam was ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ at 1.9 GeV/nucleon, incident on a lead target. Less than one fractional charge per 1.0×${10}^4$ Fe-Pb collisions was found to be produced, and, with further assumptions, less than one per 2.0×${10}^6$ collisions.

The measured x-ray fluxes of old, nearby pulsars are used to constrain the monopole flux times the cross section for monopole-catalyzed nucleon decay. Observations of PSR 1929 + 10 provide the best limit: $F\times \frac{\sigma v}{(3\mathrm{}\times {10}^{-18\mathrm{}} {\mathrm{cm}}^3 {\mathrm{s}}^{-1\mathrm{}})}<~2\times {10}^{-22\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$. When the monopoles captured by the progenitor star while it was on the main sequence are taken into account, the limit becomes $F\times \frac{\sigma v}{(3\mathrm{}\times {10}^{-18\mathrm{}} {\mathrm{cm}}^3 {\mathrm{s}}^{-1\mathrm{}})}<~2\times {10}^{-28\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$.

New experimental data for the elastic scattering of 7-24-MeV neutrons from ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ are presented, and a microscopic folding-model study of neutron and proton elastic scattering data for ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ at energies below 60 MeV is made to examine the isospin dependence of an effective interaction derived from nuclear-matter calculations. The theoretically derived interaction yields reasonable agreement with the data if the Coulomb potential is treated properly.

The ground-state quadrupole moments of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$, ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$, ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$, ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$, ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$, ${}_{}{}^{163}{}_{}{}^{}\mathrm{Dy}$, ${}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{177}{}_{}{}^{}\mathrm{Hf}$, ${}_{}{}^{179}{}_{}{}^{}\mathrm{Hf}$, ${}_{}{}^{191}{}_{}{}^{}\mathrm{Ir}$, and ${}_{}{}^{193}{}_{}{}^{}\mathrm{Ir}$ were determined with an uncertainty of less than one percent by measuring the quadrupole hyperfine-splitting energies of muonic $M$ x rays. The results are used to determine experimentally Sternheimer shielding factors for the $4f$, $5d$, and $6p$ electronic states of the respective atoms. The deduced shielding factors for the $5d$ electronic states were found to vary considerably among these elements, presumably as a result of configuration mixing.

Multicharged neon recoil ions at electronvolt energies are produced inside a Penning ion trap by fast, stripped-heavy-ion impact on neon atoms. The stored recoil ions capture electrons in collisions with the parent neon gas. The measured storage times are used with mean energy determinations and neon gas densities to determine charge-transfer rate coefficients. A significant dependence on ion charge ($3<~q<~6$) is observed.

The results of a theoretical and experimental investigation of satellite line strengths in two-photon transitions to Rydberg autoionizing states in multielectron atoms are presented. In studies of $6snd-7s{n}^{\prime }d$ two-photon transitions in Ba I, the distribution of line strengths over various final $7s{n}^{\prime }d$ states for a given $6snd$ initial state is found to vary dramatically as a function of detuning from intermediate $6p{n}^{\prime \prime }d$ autoionizing resonances. These results indicate the influence of electron rearrangement during the multiphoton transition.

Absolute measurements of an integral of the differential cross section for the scattering of electrons by CsBr are reported in the 1-22.5-eV energy range. A precise prescription for comparison with theory is presented, thus setting a benchmark for testing calculations. Comparison with Born-approximation calculations is also presented.

Stimulated Rayleigh-Brillouin gain spectra of pure gases are reported. Stimulated Rayleigh scattering is now clearly distinguishable from stimulated Brillouin scattering without the aid of induced absorption. As examples, spectral line shapes of Ar and S${\mathrm{F}}_6$ at various pressures and the associated gain asymmetry are discussed. Potential applications of coherent Rayleigh-Brillouin spectroscopy are suggested.

A storage-ring free-electron laser oscillator has been operated above threshold at a visible wavelength $\lambda \simeq 6500$ Å.

The ion acoustic wave and harmonics driven by stimulated Brillouin scattering are probed by space- and time-resolved ruby-laser Thomson scattering. Corresponding to the 6% backscatter observed, the ion wave is seen to saturate at (5-10)% amplitude, a level quantitatively predictable from the nonlinear frequency shift caused by ion trapping.

Stimulated Brillouin scattering spectra observed from homogeneous and extended target plasma being irradiated with C${\mathrm{O}}_2$ laser pulses reveal the presence of ion trapping, the onset of which coincides with a pronounced deviation from the steep stimulated Brillouin scattering increase with laser power. The evaluation of spectra permits the direct measurement of the amplitude of the backscattering ion-acoustic wave and it also provides information on the deformation of the ion velocity distribution under the influence of this wave.

By comparison with fully kinetic Fokker-Planck calculations, a nonlocal macroscopic formula has been derived for the thermal heat flux. This formula leads in a physically relevant way to the saturation and the delocalization of the heat flux. Its introduction in a fluid code is straightforward and gives to some extent results comparable with classical flux-limited transport calculations with $f=0.1\mathrm{}$. However, it also describes a significant level of preheating.

The three-dimensional stability of magnetic equilibria with islands is investigated with use of reduced magnetohydrodynamics. It is found numerically that tokamak equilibria with large $\frac{m}{n}=2\mathrm{}$ islands can be ideally unstable. The instability is explained by the strongly modified $q$ profile in the helical equilibrium, where the $q=\frac{3}{2},\frac{5}{3},\dots$ resonant surfaces are all close to the inner separatrix of the islands. An analytic treatment of the three-dimensional coalescence instability is given to illustrate this effect.

This Letter presents direct evidence for large displacements parallel to the surface of the atoms in the outer double layer of the cleaved Si(111)-(2×1) surface. This result excludes the buckling model for this surface reconstruction. Comparison of surface blocking patterns, obtained by medium-energy ion channeling and blocking, with computer simulations for the $\pi$-bonded chain model shows good agreement between experiment and model. The $\pi$-bonded molecule is not in agreement with the data.

Time-of-flight analysis of direct atomic recoils produced by pulsed ${\mathrm{Ar}}^{+}$-ion irradiation of a polycrystalline magnesium surface enables the identification of hydrogen, as well as other surface atoms. The data demonstrate the direct detection of surface hydrogen and the first direct observation of oxygen-induced surface segregation of hydrogen in a metal at room temperature.

Brillouin light scattering has been observed from parametrically excited magnons by parallel pumping in a 4-μm yttrium-iron-garnet film at 9.4 GHz. Scattering from the spin waves could be observed continuously from the thermal level through the nonlinear region of parametrically excited magnons. Magnons with small wave vectors propagating perpendicular to the static field are seen to have the lowest critical field as predicted theoretically. The spectra of these parametric excitations show a pronounced structure.