Below $T_c=122.9\mathrm{}$ °K, ${\mathrm{C}}_{24}$K shows a transition from the intralayer disordered state to an ordered state. For $T>\mathrm{}{T}_c$, present diffuse scattering data from the potassium intercalate in graphite indicate a two-dimensional (2-D) character of the short-range order. As this 2-D scattering increases with decreasing temperature, $\mathrm{hk}0\mathrm{}$ superlattice reflections appear at $T_c$ which have a true 3-D character that results from essentially simultaneous intralayer and interlayer ordering. This long-range order vanishes as ${|T-T_c|}^{\beta }$ with $\beta =0.180\mathrm{}\pm 0.01$.

We describe a model of H chemisorption on Ni, Pd, and Pt in which both extended $\mathrm{sp}$ bands and the more localized $d$ bands are appropriately incorporated. It is found that interaction between the H $1s$ level and the substrate $d$ levels plays an important role which increases in the order Ni> Pd> Pt. The results are in good accord with ultraviolet-photoelectron-spectroscopy data.

Neutron-scattering measurements have been performed on nickel to obtain more information about $\chi \mathrm{}(q,E)\mathrm{}$,the generalized susceptibility. Evidence of an optical mode that crosses the main [100] spin-wave branch was found in agreement with predictions based on the intinerant-electron band-theory approach. No optical mode was found for the [111] direction; however, the dispersion curve is found to deviate from a quadratic dispersion law.

The cross section for 180° elastic scattering of antiprotons by protons between 406 and 922 MeV/c has been measured. A single-arm spectrometer detected recoil protons corresponding to events with $〈{cos\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}〉=-0.994\mathrm{}$. The regions of the reported resonances at 1936 and 2020 MeV were scanned in 10-MeV/c steps with a typical statistical error of ≈ 7% and an rms mass resolution of ±3 MeV. No narrow enhancements ($\Gamma <10\mathrm{}$ MeV) were observed.

${\Lambda }^{\circ }$ polarization in inclusive production by protons on iridium has been measured at 28.5 GeV/c over the region $0.55 \mathrm{GeV}/c<~P_{\perp }<~1.6 \mathrm{GeV}/c$. The polarization rises from (-12.6±5.5)% at 0.55 GeV/c to (-33.4±4.6)% at 1.6 GeV/c. Significant differences from previous measurements at other energies with different targets are observed, particularly at low $P_{\perp }$.

The pointlike coupling of the photon to the quark current yields simple predictions for the leading fast mesons in high-$p_t$ $\gamma N\left(\mathrm{lN}\right)\mathrm{}$, $e^{+}{e}^{-}$, and $\gamma \gamma$ reactions, e.g., for $\gamma p\to \mathrm{hX}$ one gets $K^{+}:K^{-}:K^0:{\overline{K}}^0=8:0:1:0\mathrm{}$ and ${\pi }^{+}:{\pi }^{-}:{\pi }^0=8:1:\frac{9}{2}$. It is noted that in $\gamma N$ collisions the dominant jet cross section arises from the hard subprocess: $\gamma +\mathrm{gluon}\to q+\overline{q}$. This may yield the gluon distribution via $s s_0\left(\frac{d\sigma }{d{s}_0}\right)K^{-1\mathrm{}}=f_g\mathrm{}\left(x\right)\mathrm{}$ and lead to a sum rule: $\int {s_0}^2\left(\frac{d\sigma }{d{s}_0}\right)\frac{d{s}_0}{\mathrm{sK}}=\int x{f}_g\mathrm{}\left(x\right)\mathrm{dx}$, where $K=4\mathrm{}\pi \alpha {\alpha }_s\ln \left(\frac{s}{2{p_{tmin}}^2}\right)$.

It is shown that if dynamical symmetry breaking of weak interactions were due to an extra-strong $\mathrm{SU}\mathrm{}\left(n\right)\mathrm{}$ gauge interaction which is unified with quantum chromodynamics according to the simplest scheme it would contradict the behavior of coupling constants predicated by the renormalization group. This is proven first for any nonexotic irreducible representations of $\mathrm{SU}\mathrm{}\left(N\right)\mathrm{}\supset \mathrm{SU}\mathrm{}\left(3\right)\mathrm{}\otimes \mathrm{SU}\mathrm{}\left(2\right)\mathrm{}\otimes \mathrm{SU}\mathrm{}\left(1\right)\mathrm{}\otimes \mathrm{SU}\mathrm{}\left(n\right)\mathrm{}$; then it is argued that the theorem can be extended to any unifying simple Lie group.

Measurements of the reactions $e^{+}+e^{-}\to e^{+}+e^{-}$, ${\mu }^{+}+{\mu }^{-}$, and ${\tau }^{+}+{\tau }^{-}$ at PETRA energies ($s^{\frac{1}{2}}=13,17,27.4,30\mathrm{} \mathrm{and} 31.6$ GeV) are reported. The results show that these reactions agree well with the predictions of quantum electrodynamics thus determining that all the known charged leptons are pointlike particles to a distance < × ${10}^{-16\mathrm{}}$ cm.

The cross section of the pion-induced proton-knockout reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$→${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ has been measured with positive and negative pions. The ratio of the cross sections is compared with the ratio calculated in the plane-wave impulse approximation and agreement is found for those events that lead to particle-stable final states.

The structure and the degree of isospin mixing for the $1^{+}$ levels in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ at 12.71 MeV ($T=0\mathrm{}$) and 15.11 MeV ($T=1\mathrm{}$) are determined from measured 180°-electron-scattering form factors. The resulting charge-dependent isospin-mixing matrix element ranges from 130 to 165 keV depending on the theoretical model of the isoscalar form factor.

The resonant vibrational excitation of ${\mathrm{N}}_2$ by low-energy electrons is used to illustrate the newly developed $R$-matrix theory of vibrational excitation and dissociative attachment of Schneider, Le Dourneuf, and Burke. The ab initio calculations are performed which clearly illustrate the role of the internal states in describing these excitation processes and support the authors' contention that the essential physics is completely contained within a Born-Oppenheimer treatment of the total molecular Hamiltonian.

The instability theory for sinusoidal perturbations is extended by use of an appropriate energy-conservation criterion. This theory predicts that perturbations initially grow exponentially but can subsequently either stabilize or decay depending on their transverse size.

We present a general, self-consistent, nonlinear theory of the free-electron laser (FEL) process. The formulation of the temporal steady-state problem results in a set of coupled nonlinear FEL equations governing the spatial evolution of the amplitudes and wavelengths of the fields. We show that intrinsic FEL efficiencies can be greatly enhanced by spatially contouring the magnetic pump-field parameters. In the optical regime, the single-pass efficiencies are found to exceed 20%.

The dynamic behavior of the plasma produced from a thin foil target irradiated by a focused intense electron beam is observed optically by shadowgraphy and interferometry. Evidence of anomalous stopping of intense electron beam on the surface of the low-$z$ target is shown experimentally.

Electrostatic ion cyclotron waves observed in space at altitudes between 5000 and 8000 km often have a sinusoidal form. Occasionally, however, wave forms having a spiky or sawtooth form indicative of steepening are observed. The nonlinear fluid equations which characterize the electrostatic ion cyclotron wave have traveling-wave solutions with sinusoidal, spiky, and sawtooth forms. The wave form depends on the amplitude and phase velocity of the wave.

Alpha and proton reaction product spectra have been measured in exploding pusher experiments on the symmetrical illumination six-beam ZETA laser system (Nd-doped phosphate glass). DT ion temperatures and positive energy shifts for $\alpha$ particles and protons were obtained for experiments with incident on-target power between 1 and 2.5 TW. Time-averaged electrostatic potentials have been measured up to 330 kV.

Nuclear resonance scattering of photons from ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ has been used to study the orientation of the ${\mathrm{N}}_2$ molecular axis with respect to the adsorbing Grafoil plane. At 78 °K, and around 1 monolayer coverage, the ${\mathrm{N}}_2$ molecules were found to be aligned preferentially parallel to the plane of graphite. At lower coverage, of ∼0.8 monolayer, some parallel orientation is retained while at a coverage of 1.5 monolayers no preferred orientation was obtained.

Inelastic light scattering from photoproduced carriers is observed in stressed high-purity silicon at low temperatures. The two electronic Raman structures observed correspond to transitions between stress-split valence bands at the zone center. These transitions can be stress-tuned to the $\Gamma$-point phonon energy, thereby exhibiting Fano-type interference between competing scattering amplitudes.

The energy spectrum and wave functions for a particle in periodic potentials with incommensurate periods are obtained analytically and reduced to specific phase trajectories. The spectrum is of the devil's-stairs type. States may be extended and localized, separated by mobility edges. These results are applicable to incommensurate linear chain structures (such as those in ${\mathrm{Hg}}_{3-\delta }\mathrm{As}{\mathrm{F}}_6$) and to the fine structure of de Haas-van Alphen oscillations.

With use of the symmetry properties of the half-filled single-band Hubbard Hamiltonian, there is derived an effective Hamiltonian on a decimated lattice in which the spin and charge operators occur explicitly. Being a generalization of the Blume-Emery-Griffiths Hamiltonian for ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures, this new statistical mechanical model permits one to give a preliminary discussion of the phase diagram of the correlated electron gas by establishing analogies with their results.