A beam of monoenergetic positrons incident on clean metallic surfaces has been employed to measure positronium yield as a function of sample temperature and positron energy. These results show an increase in the positronium fraction as a function of temperature which is identified as thermally activated detrapping of positrons from a surface trap. Estimates are made of the depths of these traps. Positrons are observed to trap at vacancies and a monovacancy formation enthalpy is extracted from the data utilizing a simple model.

Time-resolved absorption measurements are reported which imply an upper limit of 1 psec on the localization time for optically excited carriers in amorphous and crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$. These measurements also imply a short recombination time in crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$ (${\tau }_c\approx 14$ psec), but a long recombination time in amorphous ${\mathrm{As}}_2$${\mathrm{S}}_3$ (${\tau }_a\gg 300$ psec). This transient induced absorption does not decrease as sharply with increasing wavelength as a previously reported metastable absorption.

We have resolved individual components of the broad vibrational band of liquid water by using polarization coherent anti-Stokes (active) Raman spectroscopy. None of the existing models of liquid water based on the unresolved spontaneous Raman spectrum describe in full the observed spectra.

The buildup of persistent photoconductivity, presently a controversial phenomenon, is observed by measuring densities and mobilities of photoinduced excess electrons in thin $n$-GaAs layers between successive illuminations. Evidence from this novel type of analysis supports a model assuming charge separation by macroscopic potential barriers. We explain quantitatively how the photon dose logarithmically increases the number, but not necessarily the density, of persisting carriers and ascribe mobility enhancements to screening of ionized impurities.

Previous work has used the primordial abundance of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to infer limits on the number of neutrinos with full-strength neutral-current weak interactions. By accounting for the quark-gluon constituents of hadrons, we extend the analysis to earlier times and higher temperatures and densities and, therefore, to considerably weaker interactions. The maximum number of new, superweakly interacting, light (≲ MeV) particles is between ∼1 and ∼20.

Within the context of a six-quark model combined with quantum chromodynamics we study the asymmetry in the decay of heavy charged mesons into a definite final state as compared with the charge-conjugated mode. We find that, in decays of mesons involving the $b$ quark, measurable asymmetries may arise. This would present the first evidence for $\mathrm{CP}$ noninvariance in charged systems.

We give the normalized leading asymptotic $Q^2$ dependence of the pion form factor in quantum chromodynamics: $F_{\pi }\left(Q^2\right)\underset{Q^2\to \pm \infty }{\to }-\frac{2{f_{\pi }}^2}{b}{Q}^2\ln \left|Q^2\right|$, where $f_{\pi }$ is the pion decay constant and $b=\frac{(11-\frac{2}{3}{N}_f)}{16{\pi }^2}$. Up to non-leading-logarithmic corrections, this is equivalent to $F_{\pi }\left(Q^2\right)\underset{Q^2\to \pm \infty }{\to }8\pi {\alpha }_s\left(Q^2\right)\frac{{f_{\pi }}^2}{(-Q^2)}$. These results are obtained by solving the light-cone pion Bethe-Salpeter equation in quantum chromodynamics to leading-logarithmic accuracy.

Properly selected pions peripherally produced in the reaction $K^{-}p\to K^{-}p{\pi }^{+}{\pi }^{-}$ at 14 GeV/c exhibit an asymmetry in a particular reference frame designed to check a conjecture by Weiner that such pions may be evaporated from an excited hadron before thermodynamical equilibrium is reached. This is the first observation of localization in hadronic interactions.

Data for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$+${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ elastic and inelastic scattering are presented for a range of bombarding energies from 80 to 150 MeV. The experimental results show strong evidence for resonancelike effects similar to those seen in light systems. The inelastic data suggest the dominance of multistep processes.

The energy dependence of the reation ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$,${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$)${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ has been studied in the range $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=22-42\mathrm{}$ MeV. A striking resonance is observed at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=30.75\mathrm{}$ MeV which correlates with a very pronounced structure in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ inelastic channels. The total resonance cross section observed in the ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$+${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ channel implies a reduced partial width comparable to those of the inelastic channels. These data suggest, for the first time, the possible importance of nucleon degrees of freedom in the structure of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ resonances.

A Lane-model analysis of the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$+nucleon system has provided a detailed description of both cross-section and polarization data for ($p$,$p$) elastic, ($n$,$n$) elastic, and ($p$,$n$) quasielastic scattering over a range of energies. Emphasis is placed on consistency in isospin conservation, data-set completeness, and multichannel constraints. Results favor a symmetry potential which is energy dependent with volume and surface form factors for the real and imaginary wells, respectively. A symmetry spin-orbit $\left(\overrightarrow{1}·\overrightarrow{\sigma }\right)\left(\overrightarrow{\mathrm{t}}·\overrightarrow{\mathrm{T}}\right)$ interaction was found to be unnecessary.

A nonlinear internal wave equation that describes stratified fluids with finite depth has been studied. $N$-soliton solutions were found through Hirota's method. Although the equation tends to either the Korteweg-de Vries equation or the Benjamin-Ono equation in the shallow- or deep-fluid limit, respectively, the $N$-soliton solutions obtained tend to the Korteweg-de Vries solitons in the shallow-fluid limit but do not tend to the Benjamin-Ono solitons in the deep-fluid limit. Therefore, there is no smooth transition from one kind of soliton to another with varying depth of the fluid.

An intense electromagnetic pulse can create a weak of plasma oscillations through the action of the nonlinear ponderomotive force. Electrons trapped in the wake can be accelerated to high energy. Existing glass lasers of power density ${10}^{18}$W/${\mathrm{cm}}^2$ shone on plasmas of densities ${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ can yield gigaelectronvolts of electron energy per centimeter of acceleration distance. This acceleration mechanism is demonstrated through computer simulation. Applications to accelerators and pulsers are examined.

Experimental results from high-power neutral-beam-injection experiments on the Princeton Large Torus tokamak are reported. At the highest beam powers (2.4 MW) and lowest plasma densities [$n_e\mathrm{}\left(0\right)=5\mathrm{}\times {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$], ion temperatures of 6.5 keV are achieved. The ion collisionality ${{\nu }_i}^{*}$ drops below 0.1 over much of the radial profile. Electron heating of $\frac{\Delta T_e}{T_e}\approx 50\%$ has also been observed, consistent with the gross energy-confinement time of the Ohmically heated plasma, but indicative of enhanced electron-energy confinement in the core of the plasma.

We report the results of injecting 90 kW of microwave power near the lower hybrid frequency into the Alcator-$A$ tokamak through a two-waveguide array. The observed plasma heating is in disagreement with that expected from linear waveguide-plasma coupling theory. From these results and auxiliary rf probe measurements we infer the nonlinear formation of a high-$k_{\parallel }$ wave power spectrum at the plasma edge.

Interferometric measurements have been made of the electron-density structures produced in plasmas generated by the interaction of 1.06-μm radiation with targets at irradiances $I$ of ${10}^{16}$ W ${\mathrm{cm}}^{-2\mathrm{}}$. Strong profile steepening near critical density with an $I^{0.16}$ scaling is observed together with supercritical density bumps and structures attributable to magnetic field effects.

The time evolution of the structure function and of the cluster (or grain) distribution following quenching in a model binary alloy with a small concentration of minority atoms is obtained from computer simulations. The structure function $\overline{S}\mathrm{}(k,t)\mathrm{}$ obeys a simple scaling relation, $\overline{S}\mathrm{}(k,t)=K^{-3\mathrm{}}F\left(\frac{k}{K}\right)$ with $K\left(t\right)\mathrm{}\propto t^{-a}$, $a\cong 0.25$, during the latter and larger part of the evolution. During the same period, the mean cluster size grows approximately linearly with time.

It is shown that thin layers of Pd or Pt enhance the uptake rate of hydrogen into bulk Nb with almost every hydrogen atom that hits the surface passing into the bulk. A model for the kinetics is formulated showing that in a composite-system large uptake rates are achieved by combining the surface properties of Pd with the bulk properties of Nb, thereby resulting in a smaller barrier for migration into the bulk.

We present as idealized solvable model for the low-temperature properties of strain defects dissolved in alkali halides. The predicted specific heat and thermal conductivity are very much like those for glasses. Our calculation suggests a strong analogy between the low-temperature properties of strain defects and those of glasses.

The crossover from first order to continuous transition induced by a symmetry-breaking field $g$ in an $n=2\mathrm{}$ cubic model, for which a stable fixed point is not accessible, is studied. It is shown that unlike previously studied cases, the ($g$,$T$) phase diagram is rather complicated, exhibiting critical end points, tricritical, and fourth-order critical points. It is suggested that this phase diagram be studied experimentally in ferroelectric ${\mathrm{Tb}}_2$(${\mathrm{Mo}{\mathrm{O}}_4)}_3$ by applying uniaxial and shear stresses.