A direct measurement is reported of the velocity of positronium (Ps) ejected into a vacuum when 0- to 100-eV positrons ($e^{+}$) strike a negatively biased Cu(111) surface. At 30°C, about half the $e^{+}$ form Ps with normal energy component $\overline{E}=3.4\mathrm{}\left(3\right)\mathrm{}$ eV. At 790°C, most of the remaining $e^{+}$ form Ps but with $\overline{E}=0.14\mathrm{}\left(1\right)\mathrm{}$ eV, and a non-Maxwellian thermal distribution. We infer that surface-bound $e^{+}$ are thermally desorbed to form the extra Ps. These low Ps velocities suggest exciting possibilities for experiments on free Ps.

Measurements of the net desorption rate for hydrogen chemisorbed onto nickel films in the vicinity of the Curie temperature are presented. Anomalies are interpreted in terms of anomalous variations in the absolute rate constants for the chemisorption reaction. The results are in qualitative agreement with Suhl's theory for the effect of second-order phase transitions on absolute rate constants.

We show that the weak $\Delta I=\frac{1}{2}$ rule, $\frac{{m_W}^2}{{m_Z}^2}={cos}^2\theta$, in an $\mathrm{S}{\mathrm{U}}_L\mathrm{}\left(2\right)\mathrm{}\otimes \mathrm{U}\mathrm{}\left(1\right)\mathrm{}$ gauge model can be obtained from a dynamical Higgs mechanism. This is independent of weak isospin breaking in the fermion mass spectrum. A consequence of this mechanism is a sum rule which implies the existence of heavy quarks or leptons with ultraviolet masses comparable to the $W$ and $Z$ weak boson masses, ∼30-100 GeV.

It is argued that the parton distribution function ${\mathcal{P}}_{\frac{a}{A}}(x,{\overrightarrow{\mathrm{k}}}_T;\zeta )$ is well defined and useful in quantum chromodynamics. Here $\zeta =\frac{{\left(2\mathrm{}{P}_A·n\right)}^2}{(-n^2)}$ arises as a variable because of the use of the axial gauge $n·A=0\mathrm{}$. An approximation for ${\mathcal{P}}_{\frac{a}{A}}(x,k_T;\zeta )$ is given which is obtained from an approximate solution of the Bethe-Salpeter equation in the axial-gauge ladder model.

It is shown that the special leading-logarithm structure of the solution to the renormalization-group equation enables one to obtain a reasonable estimate of the still unknown terms of order ${\left(\mathrm{In}{q}^2\right)}^{-\gamma -2\mathrm{}}$. Implications of this result on future tests of quantum chromodynamics at short distances are discussed.

The optical isomer shift in the ${}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}\to {}_{}{}^{10}{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ atomic transition in neutral americium has been measured for the ∼1-ms spontaneously fissioning isomer, ${}_{}{}^{240}{}_{}{}^{}\mathrm{Am}_{}^m{}_{}{}^{}\left(\mathrm{S}\mathrm{F}\right)$. With use of a laser-excited optical pumping technique, this transition is shifted by +2.6 ± 0.2 Å to 6407.2±0.2 Å implying a difference in nuclear mean square radii, $\delta 〈r^2〉$, of 5.1±0.2 ${\mathrm{fm}}^2$ between the fission isomer and the normal ground state. These experiments provide the first direct experimental proof for the large deformations expected for fission isomers.

Yields of neutron-rich projectile fragments have been measured at 0° for the reaction of 212-MeV/amu ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ ions on an 890-mg-${\mathrm{cm}}^{-2\mathrm{}}$ beryllium target. Fourteen nuclides have been observed for the first time. The systematics of production cross sections are discussed.

Coincidence measurements with 35-MeV/amu $\alpha$ particles show that at least three projectile-fragmentation processes occur. The dominant process is "absorptive" breakup, where one component of the projectile interacts strongly with the target resulting in the emission of evaporation or nonstatistical particles while the other component behaves as a spectator. The other fragmentation processes which are observed account for only a few percent of the breakup cross section.

A set of approximate, analytical, adiabatic solutions is constructed for the motion of a point electron in a spatially sinusoidal wave of slowly varying amplitude. The set can describe electron transitions from trapped to untrapped conditions, or vice versa, and is formally symmetric with respect to both kinds of motion. The physical consequences of electron transitions on the damping of an electron plasma wave of large amplitude are discussed.

Spin decoupling and line narrowing are observed for the first time in an optical transition, ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\leftrightarrow {}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{F}}_3$ at 2°K, with use of optical free-induction decay. The ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei, when irradiated by an appropriate rf field, undergo forced precession about an effective field at the magic angle in the rotating frame. The fluctuating ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$-${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ dipolar interaction is thereby quenched and the optical linewidth drops from ∼10 to ∼2 kHz, as predicted in a theory of spin diffusion.

The flow between concentric cylinders with the inner cylinder rotating has been studied by visualization techniques. Velocity power spectra have shown that the flow is described by two characteristic frequencies before it becomes chaotic; the first frequency corresponds to traveling azimuthal waves. The second frequency is identified as a modulation of the waves and multiple values have been observed. These are believed to be the first such observations for any nonlinear nonequilibrium system.

The upper-hybrid resonance absorption of C${\mathrm{O}}_2$-laser light was observed in a plasma focus under a self-generated magnetic field. Near the peak compression of the plasma focus, backscattered energy of the extraordinary wave was reduced below half the value of the ordinary wave. The reduction is interpreted as evidence for upper-hybrid resonance absorption, whose coefficient is consistently estimated from the experimental data.

Elastic neutron diffraction has been used to study the structure and melting of submonolayer ethane films physisorbed on exfoliated graphite. Analysis of the relative intensity of seven Bragg reflections of the film yields the molecular orientation in the close-packed solid structure observed below 63 K. At higher temperature a novel melting process occurs which we tentatively interpret as a first-order transition to an intermediate phase followed by continuous transition to the disordered state.

The initial ordered chemisorption stage of oxygen on Al(111) has been investigated using polarization-dependent surface extended x-ray absorption fine-structure measurements above the O $K$ absorption edge. The O-Al bond length in the threefold hollow adsorption site has been determined to be 1.79±0.05 Å. A strong polarization dependence of the extended x-ray absorption fine-structure signal allows the determination of the O-O distance (2.90±0.05 Å) between the chemisorbed oxygen atoms.

The calculated surface generalized susceptibility, determined from ab initio self-consistent thin-film energy bands for the unreconstructed phase of W(001), displays a prominent peak at $\overline{M}$ when matrix elements and local-field corrections are included and supports proposed charge-density-wave interpretations of the observed reconstruction to the $c(2\mathrm{}\times 2)$ phase. These results are consistent with the Debe and King parallel-shift model and the suggestion that $\mathrm{W}\mathrm{}\left(001\right)c(2\mathrm{}\times 2)$-H at room temperature represents an impurity-stabilized substrate reconstruction.

The electrical conductivities and transition temperatures of films consisting of random mixtures of Hg and Xe condensed onto substrates held at 4.2 K have been measured. A metal-nonmetal transition in which the temperature coefficient of resistance changes from positive to negative has been found near a volume fraction close to that expected for random percolation. This transition is accompanied by a substantial drop in the superconducting transition temperature which is measured resistively.

A comparison was made between four low-temperature properties of La${\mathrm{Cu}}_2$${\mathrm{Si}}_2$ and Ce${\mathrm{Cu}}_2$${\mathrm{Si}}_2$. Whereas La${\mathrm{Cu}}_2$${\mathrm{Si}}_2$ behaves like a normal metal, Ce${\mathrm{Cu}}_2$${\mathrm{Si}}_2$ shows (i) low-temperature anomalies typical of "unstable $4f$ shell" behavior and (ii) a transition into a superconducting state at $T_c\simeq 0.5$ K. Our experiments demonstrate for the first time that superconductivity can exist in a metal in which many-body interactions, probably magnetic in origin, have strongly renormalized the properties of the conduction-electron gas.

It is shown that the C $1s$ core-electron line shape of graphite in photoemission is determined by an excitonic state near the Fermi energy in the hole-state density of states.

A search has been performed for new meson resonances produced in a 208-event/μb ${\pi }^{+}p$ experiment at 11.46 GeV/c utilizing the Stanford Linear Accelerator Center hybrid facility triggered on a fast forward proton. In the final states $p3\mathrm{}\pi$, $\mathrm{pK}\overline{K}\pi$, and $\mathrm{pp}\overline{p}\pi$ no significant new structures are observed in either the charged- or neutral-meson system. Upper limits for the production and decay of meson resonances with widths less than 30 MeV and masses less than 3 GeV/${\mathit{c}}^2$ are in the range 75-200 nb.

In a study of interactions of 400-GeV protons in an iron calorimeter, we have observed ${\mu }^{+}{\mu }^{-}$ pairs associated with a significant amount of missing energy indicative of final-state neutrinos. This missing energy is not due to any instrumental effects, nor to any trivial sources like double $K\overline{K}$ (or $\pi \pi$) decay. Interpretation of these data as production of a $D\overline{D}$ pair followed by a double muonic decay leads to a model-dependent estimate of a total production cross section of the order of 7-20 μb.