A new experiment studying radiative pion decay, ${\pi }^{+}\to e^{+}{\nu }_e\gamma$ has been performed with the Crystal Box detector at the Clinton P. Anderson Meson Physics Facility (LAMPF). From a maximum-likelihood analysis of the Dalitz-plot distribution of 2364 coincident and random events, we obtain a unique value for $\gamma$, the ratio of the axial-vector to vector weak pion form factors, of 0.25 ± 0.12. The new world average is $\gamma =0.39\mathrm{}\pm 0.06$.

The possibility of $\mathrm{CP}$ asymmetries in decays of $W$ and $Z$ bosons is examined. The asymmetries studied are the differences in the branching ratios of charge-conjugate modes such as $W^{+}\to t\overline{b}$ vs $W^{-}\to \overline{t}b$ or $Z\to \overline{b}s$ vs $Z\to b\overline{s}$ In the standard model, with three generations, such effects are found to be vanishingly small. With four generations, flavor-changing transitions such as $Z\to b\overline{s}$ or $b^{\prime }\overline{b}$ may be observable, but the prospects for observing $\mathrm{CP}$ asymmetries appear discouraging. We also discuss the situation in some other models of $\mathrm{CP}$ nonconservation.

Polarized and unpolarized $d+d\to n+{}_{}{}^3{}_{}{}^{}\mathrm{He}$ fusion reaction cross sections in the center-of-mass energy region of 20-150 keV are calculated in a distorted-wave Born approximation. The calculated unpolarized cross sections and the anisotropy of the angular distributions are within 20% of the experimental data. The polarized cross sections are found to be Å7.7% of the unpolarized ones despite the inclusion of the $D$-state component in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. This shows that the idea of a "neutronlean" reactor may still be feasible.

The simultaneous single-electron capture and production of doubly charged He ions in single collisions between fast protons and He atoms has been studied experimentally as a function of scattering angle (0.3-3.2 mrad) and impact energy (200-500 keV). The angular dependences reveal a critical angle at which the probability for production of ${\mathrm{He}}^{++}$ simultaneously with electron capture peaks sharply. The peak itself as well as the energy dependence of the size and shape of it may be interpreted qualitatively in terms of a capture mechanism proposed already in 1927 by L. H. Thomas on the basis of classical mechanics.

Poisson photons may be converted into sub-Poisson (amplitude-squeezed) photons by the action of an electron current configured in an external feedback loop. The generation mechanism involves single-photon transitions so that the source can be made arbitrarily sub-Poissonian. Non-linear optics is not invoked. A useful configuration involves a photon emitter illuminating a detector-source combination in a closed-loop system. Two solid-state implementations of the detector-source combination are suggested.

We introduce and analyze the concept of a wakeless triple-soliton accelerator in a plasma fiber. Under appropriate conditions the triple soliton with two electromagnetic and one electrostatic waves in the beat-wave resonance propagates with velocity $c$ leaving no plasma wake behind, while the phase velocity of the electrostatic wave is made also $c$ in the fiber.

Laser-induced electronic excitation of adsorbed Mo${\mathrm{}\left(\mathrm{CO}\right)\mathrm{}}_6$ followed by CO desorption is observed to occur on Si(111) (7 × 7) irradiated with 257-nm (4.8 eV) laser radiation. The process is shown to be completely nonthermal in nature. In contrast, irradiation with 514-nm radiation is found to be identical to resistive heating of the crystal. Adsorption does not open up new excitation mechanisms that allow decomposition to occur in the visible region of the spectrum.

Isotopic binary liquid mixtures are universally characterized by an upper critical solution temperature as a consequence of zero-point motion in conjunction with the anharmonicity of the interatomic potential. This quantum effect is predicted, and demonstrated by small-angle neutron-scattering measurements, to be manifest at ambient temperatures for mixtures of deuterated and protonated polymers. Prior evidence of such phase behavior has been restricted to liquid mixtures of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.

Measurements are reported of the growth of breath figures, the patterns that form when a vapor is condensed onto a cold surface. The pattern for water on glass was studied by direct observation and light scattering as a function of the contact angle $\theta$, flux $F$, degree of supersaturation $\Delta T$, and time $t$. When $\theta =0\mathrm{}°$, a uniform layer forms whose thickness grows as $t$ at constant $F$ and $\Delta T$. For $\theta =90\mathrm{}°$ droplets are formed; at constant $F$ and $\Delta T$ the radius of an isolated droplet grows as $t^{0.23}$, but as a result of coalescences the average droplet radius grows as $t^{0.75}$. The growth process is self-similar—coalescences simply rescale the distances and leave the basic droplet pattern unaltered.

We report the first observation of the fourth vibrational overtone ($\Delta v=5\mathrm{}$) transition of molecular hydrogen in solid parahydrogen at 19 062 ${\mathrm{cm}}^{-1\mathrm{}}$. We also report the first measurements of an almost complete set of the third vibrational overtone ($\Delta v=4\mathrm{}$) transitions.

In this paper, we discuss how quenched strains in phonon and phason variables and/or quenched dislocations can lead to peak broadening and distortion in quasicrystal diffraction patterns. We argue that high-resolution electron micrographs and observations of distortions in electron diffraction patterns indicate the presence of anisotropic strains in the phason variable in the icosahedral phase of Al-Mn and related alloys. Such strains also contribute to the x-ray peak widths and line shapes.

We report new x-ray scattering data on AlMn alloys. None of our samples are perfect quasicrystals; all have finite-width x-ray peaks. Specific preparation techniques lead to samples with peak widths which correlate strongly with $G_{\perp }$ (phason momentum). We show that the data are consistent with either frozen-in phason strain or random icosahedral packing. These two models predict surprisingly similar but potentially distinguishable scattering widths, differing in their detailed dependence on $G_{\perp }$.

The problem of dissipative superflow in the absence of normal-fluid friction is considered within the hydrodynamic approximation. It is found that microscopic surface roughness gives rise to new mechanisms of dissipative vortex motion and vortex regeneration, accounting not only for the critical velocities at which self-sustaining dissipation becomes possible, but also for the occurrence of discrete dissipative events in oscillating flow.

We present Monte Carlo simulations on systems of parallel spherocylinders with length-to-width ratios $\frac{L}{D}$ between 0.25 and 5. We have determined the solid-liquid coexistence points in these systems by computing the absolute free energy of the solid and fluid phases. In addition we find that for $\frac{L}{D}>0.5\mathrm{}$ a thermodynamically stable smectic phase is formed at densities well below the thermodynamic melting point. The transition from nematic to smectic appears to be continuous. We find no evidence for long-range bond-orientational order in the smectic phase.

Results of an ac heat-capacity study of layering, melting, and layer-critical-point transitions of multilayer ${\mathrm{C}}_2$${\mathrm{D}}_4$ on graphite are reported. The layer critical temperature is found to decrease towards the bulk triple melting point with increasing film thickness. The wetting transition also appeared to be pinned at the triple point. The layer-critical-point transition of a film between one and two layers is found to be 2D Ising type.

The magnetic and the spin-orbit scattering by magnetic Fe impurities in Mg films are measured by means of weak localization and a Kondo-type behavior is found. The measurements suggest a magnetic screening of the Fe which depends on the distance between the Fe atoms but not on the film thickness of Mg. The spin-orbit scattering rate is an order of magnitude larger than the effective magnetic scattering rate.

The carrier localization and the inhibition of carrier transport along the growth axis (vertical transport) are studied by means of photoluminescence experiments performed at 1.7 K in purposely disordered GaAs/GaAlAs superlattices. When the well widths are randomly varied, minibands of extended states shrink and localized states are created in their tails. Consequently the vertical transport efficiency decreases and sharply vanishes when disorder induces fluctuations of the eigenenergies which are comparable to half of the unperturbed superlattice bandwidth. By an increase of temperature the vertical transport becomes thermally activated.

The electronic structure of semi-infinite semiconductors with energy-optimized reconstructed surfaces is calculated self-consistently for the first time. Using local-density-functional formalism and scattering theory, the calculations yield the potential, charge density, surface band structure, and wave-vector-resolved layer densities of states with extreme spectral resolution. The results quantitatively explain recent surface spectroscopy data on occupied and empty states for Ge(001) (2×1) and Si(001) (2×1).

Resolved fine structure on the broad $D5\mathrm{}$ photoluminescence band in silicon—recently associated with straight dislocations—gives evidence that the band is a superposition of at least eleven "line" spectra with systematic energetic positions and reaction-kinetic features. We suggest that the lines originate from the radiative recombination of electrons and holes at Shockley partial dislocations split apart by different widths.

We have studied the static-mode ion-beam sputtering of ${\mathrm{Si}}^{+}$ from a Si (100) surface during oxidation and nitridation. The data are consistent with the ionization of sputtered atoms resulting from the breaking of the local chemical bond during sputtering. A model is proposed to explain the dependences of the ionization probability of the ionization potential, emission energy, and isotopic mass.