Using a molecular-dynamics technique for the classical sine-Gordon chain, we have found that nonlinear breather modes give rise to two excitation branches. A low-frequency resonance is associated with the propagating envelope and a high-frequency peak with the internal oscillations.

For massive-lepton pair production in meson-induced reactions, we use quantum chromodynamics perturbation theory to predict that the decay angular distribution in the pair rest frame will change from predominantly $1+{cos}^2\theta$ to ${sin}^2\theta$ as the longitudinal-momentum fraction of the pair $x_{\mathrm{F}}\to +1\mathrm{}$. The two angular distributions are associated respectively with ${\mathrm{}(1-x)\mathrm{}}^2$ and $Q^{-2\mathrm{}}{\mathrm{}(1-x)\mathrm{}}^0$ components of the valence-quark structure function of the meson.

Results are presented from a large-acceptance experiment in which muon-pair production was observed in the mass range 2 to 11 GeV/${\mathit{c}}^2$. Data were taken with ${\pi }^{\pm }$, $K^{+}$, and $p^{\pm }$ beams at 225 GeV/c on carbon, copper, and tungsten targets. Differential cross sections and the production dependence on pair mass, $x_{\mathrm{F}}$, $p_T$, incident-particle type, and target nucleus are discussed.

New data on muon-pair production at 225 GeV/c by ${\pi }^{-}$, ${\pi }^{+}$, and proton beams are analyzed with regard to the production mechanism. The observed spin alignment of the pair and the dependence of the cross section on beam-particle type are strong indications that the production is through electromagnetic quark-antiquark annihilation.

Data on muon-pair production by pions are used to determine the momentum distribution for valence quarks in the pion. The shape of a nucleon structure function is also obtained and is compared with a calculation based on existing data.

A model is presented which accounts for the shifts in the energy levels of a transferred nucleon due to the proximity of the colliding ions during the reaction process. Including these shifts through an effective $Q$ value in a distored-wave Born-approximation formalism leads to dramatic improvement of the calculated angular distributions for the reactions induced by 56-MeV ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ on ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$.

Measured yields and energy distributions of ${\mathrm{H}}_2^{+}$ emerging at 0° from the breakup of 2.2-MeV ${\mathrm{H}}_3^{+}$ in 1.5-7-μg/${\mathrm{cm}}^2$ carbon foils suggest that, in traversing a solid at the same velocity, the lifetime for survival with the original electron is the same for H and ${\mathrm{H}}_2^{+}$. Seen for the first time is a reversal in the 0° intensity asymmetry between the leading and trailing ${\mathrm{H}}_2^{+}$ breakup fragments with changing target thickness which we interpret as the result of competing wake alignment and enhanced destruction mechanisms.

Calculation of the motional Stark effect for $B\sim 50$ kG shows it to become dominant along Rydberg series of light elements at $T\sim 1000°$C before the diamagnetic shift changes the spectrum from Rydberg to Landau type, thus accounting for the $\frac{1}{2}\hslash {\omega }_c$ spacing observed in Li. This spacing is preserved when both of the competing effects—motional Stark and Landau modulation—are strong. The spectrum appears to depend critically on the relative position of the thresholds for the onset of the two effects.

Using results of the Weinberg-Salam model, we have calculated in lowest order the magnitude of the optical rotation parameter $\mathrm{Im}\left(\frac{E1\mathrm{}}{M1\mathrm{}}\right)$ in atomic bismuth for the four allowed $M1\mathrm{}$ transitions from the ground state within the configuration $6s^{2}6p^3$. Results based on Dirac-Hartree-Fock orbitals are found to vary significantly according to the form used for the electric dipole operator in the $E1\mathrm{}$ matrix element. This is interpreted as evidence for large correlation effects.

We report measurements of cross sections greater than ${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^2$ for laser-induced inelastic collisions between Sr and Ca. We describe an experimental system which produces such large cross sections and which greatly facilitates the study of laser-induced collisions through its ability to reduce noise from other collisional processes.

The possible three-pulse echoes in two-level systems of solids and gases are discussed with particular emphasis on backward-wave echoes. One example of the backward echoes was realized with the atomic sodium $D$ line. The detection of this echo requires neither optical shutters nor magnetic field. The decay rate of the echo due to Na-Ar collisions was measured. It is shown that the effect of velocity changing collisions is negligible compared with that of phase interrupting ones.

Using beams of low-energy electrons ($E<\mathrm{}5\mathrm{}$ MeV), a free-electron laser can be operated as a continuously tunable ($1000 \AA <\lambda <50\mathrm{} \mu$m), high-power ($P>\mathrm{}10\mathrm{}$ kW, cw) source of laser radiation. With electrostatic accelerators the electron beam can be recycled to increase the overall efficiency of the laser. Wall power to laser power efficiencies greater than 10% are possible.

The high-frequency "convective" loss-cone mode in a mirror-confined plasma is shown to be absolutely unstable as a result of wave reflection from the mirror throats, provided that the plasma length is greater than an axial wavelength. Critical lengths for stability are only a few ion gyroradii for fusion parameters, much smaller than previous estimates. This result places serious limitations on the design of mirror fusion reactors, and precludes the possibility of a linearly stable reactor with empty loss cone.

Using the Yvon-Born-Green equation of state for a square-well-potential fluid, we have determined the critical exponent $\gamma$, where $K_T\propto {|T-T_c|}^{-\gamma }$ along the critical isochore, to be 1.24±0.04. This nonclassical value is consistent with the best experimental and theoretical estimates of $\gamma$ and suggests strongly that a distribution-function theory with superposition approximation can correctly describe the equation-of-state behavior of a classical fluid in the vicinity of the single-component critical point.

A simple graphical approach is proposed which correctly predicts the occurrence or absence of surface segregation for virtually all binary alloys that have been studied experimentally. The rule is based on elemental variables related to the work functions and electron densities of the elements. These variables have been proposed by Miedema and used by him to predict binary-alloy heats of solution and elemental surface energies. The graphical condition for segregation is derived analytically and is valid for both substitutional and interstitial alloys.

An ionic crystal is viewed as a collection of compressible ions in polyhedral, spacefilling cells. Repulsion arises solely from the increased compression energy at the cell faces. Eighteen parameters (two per ion) are determined from the lattice spacings and compressibilities of the twenty alkali halides. These explain for the first time the observed structures of all these crystals as well as their thermal and pressure transitions—a significant advance over previous semiempirical theories.

Magnetic resonance measurements on a spinning sample of deuterated $n$-heptyloxyazozybenzene (HOAB-$d_{30}$) are presented. The resulting spectral patterns for the deuterated terminal methyl groups display the biaxial character of the smectic-$C$ phase. Values for those biaxial order parameters which survive under free molecular rotation are directly determined from the shape of the spectral patterns. Parameters expressing biased rotation are also present and contribute significantly to the biaxiality.

A simplified method of applying a renormalization-group analysis to Monte Carlo simulations of general systems is presented and illustrated with applications to the Ising model and the three-state Potts model.

The kinetic theory of the fluctuations of a gas not in equilibrium is given and applied to light scattering. Unequal heights of the Brillouin lines are found in the presence of a temperature gradient.

The nucleon structure function, $F_2$, has been measured up to $q^2=120\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ and for $40<\mathrm{}{W}^2<300\mathrm{}$ Ge${\mathrm{V}}^2$. The data exhibit a significant pattern of scaling violation. Compared to lower-energy data, $F_2$ shows an observable increase of ∼15% at high $q^2$ for $x<\mathrm{}0.4\mathrm{}$. The pattern of the increase may accommodate a threshold in $W$ or new theoretical parametrizations.