A measurement of the coherent regeneration amplitude in carbon in the energy range 30-130 GeV is presented. The results are consistent with the dominance of this process by $\omega$ exchange, and a precise value of the intercept of the $\omega$ trajectory is obtained: ${\alpha }_{\omega }\mathrm{}\left(0\right)=0.390\mathrm{}\pm 0.014$.

As the result of a recently reported experiment, a search has been made for deuterons from the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}+{}_{}{}^3{}_{}{}^{}\mathrm{He}\to \alpha +d+e^{+}+\nu$. Two independent experiments were done at $E_{\mathrm{He}}=15\mathrm{} \mathrm{and} 25$ MeV. In both cases, upper limits were obtained which are about a factor of 20 below the previously measured cross section (3.4 nb/MeV sr).

The neutron-proton spin-correlation parameter, $A_{\mathrm{yy}}$, has been measured at four angles using a 50-MeV polarized neutron beam and polarized proton target. The measured values have been included in new phase-shift analyses of the 50-63-MeV data, producing for the first time (at these energies) a definite and positive value 2.6°±1.0° for ${\overline{\epsilon }}_1$, the important $3S_1-3\mathrm{}{D}_1$ phase-shift mixing parameter of the $n-p$ system.

In the reactions 20-GeV Ar + Au and 8-GeV Ne + U, the angular and energy distributions of six nuclei with $Z>~3$ imply approximately isotropic emission from systems moving at ${\beta }_0\approx 0.04 \mathrm{to} 0.11$. The invariant cross sections fit either Maxwellians ($\tau \approx 52 \mathrm{to} 65$ MeV) or exponentials in momentum ($P_0\approx 240 \mathrm{to} 300$ MeV/c) in the moving frame. The low ${\beta }_0$ and high $\tau$ are inconsistent with emission of nuclei ($Z>~3$) from a recoiling fireball unless extreme assumptions are made; they thus imply nonthermal emission.

A search for particle-bound polyneutron systems (${}_{}{}^6{}_{}{}^{}n-{}_{}{}^{12}{}_{}{}^{}n$) produced in ∼ 700-MeV proton interactions with uranium has yielded negative results. A radiochemical technique was used. The limits on production cross section—${10}^{-3\mathrm{}}$ to ${10}^{-5\mathrm{}}$ μb—are many orders of magnitude lower than previous limits at comparable energies, are lower than predictions based on extrapolation of yields of other neutron-rich species, and are in contrast to the positive results reported recently from work with 24-GeV protons on tungsten.

A calorimetric experiment, which combines picosecond C${\mathrm{O}}_2$-laser pulses with acoustic detection techniques, proves that the collisionless multiphoton dissociation of S${\mathrm{F}}_6$ is a statistical thermodynamic process. The standard Rice-Ramsberger-Kassel-Marcus theory of unimolecular reactions provides a quantitative explanation of the dissociation effect. The reaction rate is primarily limited by transitions in the quasicontinuum rather than by the anharmonicity of the first few discrete levels.

We describe a new efficient absorption mechanism for intense laser light in plasma resulting from an ion acoustic instability generated by a return current. Comparisons with experimental results are presented.

Low-energy-electron-diffraction studies of carefully cleaned and annealed surfaces of molybdenum (100) and tungsten (100) show that a phase transition can be induced by lowering the temperature below 300 K. The periodicity of the "reconstructed" surface is believed to be due to the formation of a displacement wave with a wavelength which is $2a$ ($a$ is the lattice parameter) for W(100) and $\sim 2.2a$ for Mo(100). The phase transition is reversible and seemingly second order. It appears possible that displacements of this type also occur in chemisorption on these surfaces.

Magnetic susceptibility and Mössbauer-effect measurements on a series of dilute alloys of Eu in Mg have been performed. In strain-free samples the Eu ions are divalent, but in cold-worked samples a large fraction are converted to the trivalent state. Isochronal annealing studies suggest that this is related to the trapping of dislocations on impurity clusters with the valence change occurring because of the strain field of the dislocation.

The stopping power in {111} Au for channeled ions which have not changed charge has been measured for bare nuclei for $Z=1-5\mathrm{}$ at 3.5 MeV/amu, and for ions bearing 0, 1, and 2 bound electrons for $Z_1=1-9\mathrm{}$ at 2 MeV/amu. An effective screening of $\sim 0.9e$ per electron is obtained. For the bare nuclei, deviations from simple ${Z_1}^2$ scaling are reported. At 2 MeV/amu an increase of 3.5% in $\frac{S}{{Z_1}^2}$ is seen from H to He, 3% from He to Li; but no differences are seen from $Z_1=5-9\mathrm{}$. However, at 3.5-MeV/amu deviations of as much as 20% per $Z_1$ are observed.

Order-disorder transitions on square, triangular, and honeycomb arrays are classified according to whether they can be continuous or must be first order by applying a criterion of Landau. In particular, the transitions to 2 × 2 arrays on triangular and honeybomb lattices are predicted to be in the universality class of the four-state Potts model. A physical realization of this model, ${\mathrm{N}}_2$ on Kr-plated graphite, is proposed.

The temperature-dependent susceptiblity of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ has been measured both statically and via a pulse technique in a field of 309 G using an rf-biased superconducting quantum-interference device (SQUID). In the pressure range 26.5 to 18 bar, the dynamic NMR susceptiblity agrees qualitatively with the theoretical weak-coupling predictions for the Balian-Werthamer state. However, the static susceptiblity, measured using the same rf-biased SQUID and detection system, is significantly smaller thatn the dynamic susceptibility.

I show that dynamic screening of phonons by spin-polarized conduction electrons is an important mechanism for anomalous field-dependent acoustical absorption in heavy rare-earth metals. The absorption coefficient is larger by a factor ${\left\{\frac{\partial {\left[N\right(E_{\mathrm{F}}\left)\right]}^{-1\mathrm{}}}{\partial E}\right\}}^2$ than that arising from the presenlty accepted mechanism of exchange modulation. Here $N\left(E_{\mathrm{F}}\right)$ is the density of states at the Fermi energy. The expected behavior in degenerate magnetic semiconductors is also discussed and interesting predictions for experiment are made.

We present a new NMR method for studying slow diffusion in multispin systems. With this method we can vary the effect of a particular spin species's motion relative to the others, thus enabling us to separate and identify the component motions. We present a new pulse sequence for measuring ${T_{1D}}^{\prime }$. a generalized dipolar relaxation time, and apply it to a study of potassium diffusion in KF.

It is shown that the strong far-infrared absorption in small Al particles, recently observed by Granqvist et al., may be explained as arising from the dielectric losses in the amorphous oxide layers covering the particles. Aggregation of particles into clusters is found essential in order to produce the observed dependence of absorptivity on particle size. The propsed mechanism provides a simple understanding of the absence of any superconducting ordering effects in the absorption.

Luminescence spectra of high-purity GaP at high pump intensity demonstrate the existence of an electron-hole-liquid phase. Line-shape analysis yields $n_0=6\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ for the liquid density and ${\varphi }_0=14\mathrm{}$ meV for the liquid binding energy in agreement with theoretical values. The liquid critical temperature is estimated to be $T_c\simeq 40°$K.

A method for studying nonequilibrium effects and their $\overrightarrow{\mathrm{k}}$-space anisotropy in bulk superconductors near $T_c$ is reported. The relaxation time in single-crystal In determined directly by this method, $\tau \mathrm{}\left(t\right)=0.77\mathrm{}\times {10}^{-10\mathrm{}}{\mathrm{}(1-t)\mathrm{}}^{-\frac{1}{2}}$, represents confirmation, both in form and magnitude, of the Schmid-Schön predictions for the relaxation of the magnitude of the order parameter near $T_c$. The associated electron-phonon inelastic scattering time is in excellent agreement with that determined for the comparable portion of $\overrightarrow{\mathrm{k}}$ space by cyclotron resonance.

By use of synchrotron radiation in the range 12 to 40 eV, angle-resolved photoemission spectra of the (100) Mo surface have been observed which exhibit a sharp peak near the Fermi Level. This structure points strongly in the normal direction, it is very sensitive to surface contamination, and its position does not change with photon energy. A similar peak has been attributed in the past to a surface state. We show that it is most likely due to a combination of a surface resonance and a $d$-band edge on both experimental and theoretical grounds.

We employed repeated-ring kinetic theory incorporating the equilibrium correlations in a dense fluid to study Brownian motion. It is demonstrated that the theory predicts the Stokes-Einstein law relating the diffusion coefficient of a Brownian particle to its radius and the fluid shear viscosity. In addition, the correct long-time behavior ($t^{-\frac{3}{2}}$) of the velocity autocorrelation function is obtained.

Although the time-dependent Foldy-Wouthuysen (FW) transformation of the "Schrödinger" equation $\frac{i\partial \psi }{\partial t}=H\psi$ yields the transformed equation $\frac{i\partial {\psi }^{\prime }}{\partial t}=H^{\prime }{\psi }^{\prime }$, where ${\psi }^{\prime }=U\psi$ and $H^{\prime }=UH{U}^{-1\mathrm{}}-\frac{\mathrm{iU}\partial \left(U^{-1\mathrm{}}\right)}{\partial t}$, the expectation values (${\psi }_1, H{\psi }_j$) of $H$ are not equal to (${{\psi }_i}^{\prime }, H^{\prime }{{\psi }_j}^{\prime }$), but rather, to (${{\psi }_i}^{\prime }, \mathrm{UH}{U}^{-1\mathrm{}}{{\psi }_j}^{\prime }$). But one still has $({{\psi }_i}^{\prime }, {{\psi }_j}^{\prime })=({\psi }_i, {\psi }_j)$. I discuss implications for perturbation calculations, scattering amplitudes, electrodynamics, and the external-field $\pi N$ ambiguity, and look at exact special cases.