A new representation for quantum general relativity is described, which is defined in terms of functionals of sets of loops in three-space. In this representation exact solutions of the quantum constraints may be obtained. This result is related to the simplification of the constraints in Ashtekar's new formalism. We give in closed form the general solution of the diffeomorphism constraints and a large class of solutions of the full set of constraints. These are classified by the knot and link classes of the spatial three-manifold.

New model-independent constraints on possible modifications of Newtonian gravity over solar-system distance scales are presented and their implications discussed. The constraints arise from the analysis of various planetary astrometric data sets. The results of the model-independent analysis are then applied to set limits on a variation in the $\frac{1}{r^2}$ behavior of gravity, on possible Yukawa-type interactions with ranges of the order of planetary distance scales, and on a deviation from Newtonian gravity of the type discussed by Milgrom.

We study a wide class of complex spaces each of which may be spanned by "internal" degrees of freedom of certain physics models with simple supergravity in (3+1)-dimensional Minkowski space-time. We show that there are connections among these complex spaces suggesting the possibility of phase transitions which would cause drastic changes in the physical observables of such models. The web of superstring models so connected suggests the existence of a "unified" superstring model of which the models studied here are special cases.

Regge phenomenology is used to infer the positivity of certain helicity-dependent quark distribution functions with their low-$x$ behavior subtracted out. These forms adequately describe the data for muon scattering from polarized protons but violate the Ellis-Jaffe sum rule, casting doubt on the reliability of perturbative QCD. Predictions made earlier with the theory of quark geometrodynamics are in agreement with the data.

Tagged-photon measurements of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\gamma ,pn)$ reaction in the photon energy range 83-133 MeV are reported. The measurements have achieved a good angular resolution, ≃4°, and a better missing-energy resolution, ≃8 MeV, than any previous measurement. This has allowed events to be selected in which both the neutron and proton are ejected from the $1p$ shell of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ nucleus. The correlation of the momenta of the outgoing nucleon pairs is quantitatively explained by the quasideuteron mechanism.

Protons scattered inelastically from ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ were detected at laboratory angles between 6° and 13° in coincidence with $\gamma$ rays from deexcitation of the $1^{+} T=1\mathrm{}$ state in the recoil nucleus. Coincident cross sections for $\gamma$ rays emitted normal to the reaction plane and angular correlations for $\gamma$ rays emitted in the proton scattering plane at angles between 90° and 270° are presented and compared with predictions from both nonrelativistic and relativistic models of proton-nucleus scattering calculated in the impulse approximation. These data display a clear preference for the relativistic-model prediction.

Evidence is presented for the occurrence of superdeformation in $\mathrm{sd}$-shell nuclei. Neutron-proton coincidence spectra from the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reaction at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=24\mathrm{}$ MeV were measured. The neutron evaporation spectrum at low energies directly reflects the large deformation of the emitting system, extending to states above the yrast line. The strong energy dependence of the effect implies that the compound nucleus undergoes a rapid change from nearly spherical to superdeformed shapes over a narrow range of excitation energy. The transition to a fissioning system should be observable.

Low-energy (< 20 eV) free electrons produced in above-threshold ionization have been scattered by an intense optical standing wave (the Kapitza-Dirac effect). At intensities of ${10}^{13}$ to ${10}^{14}$ W/${\mathrm{cm}}^2$ there is very high momentum transfer between the standing-wave "lattice" and the electrons ($\Delta p\approx 1000\hslash k$). In this regime, the scattering rate approaches the optical frequency, and the electron motion is most easily analyzed by classical mechanics.

We have observed in high-$Z$ ions, electric-dipole-allowed, two-electron-jump core-changing transitions in fluorinelike and oxygenlike lanthanum. X-ray transitions between the upper $2s^{2}2p^{n}3p$ excited levels and the lower $2s2p^{n+2\mathrm{}}$ hole states of oxygenlike and fluorinelike lanthanum are reported. These transitions are particularly interesting as a test of the theoreticaltomic physics calculations as well as a possible plasma density diagnostic.

Larger argon atomic clusters undergo a size-specific transition from rigid to fluid forms as the cluster temperature is varied. The observed nature of this phase change, detected by measurement of the spectral line shape of a benzene dopant molecule, favors the phase-coexistence model of the cluster melting transition.

Thermal S${\mathrm{F}}_6$ molecules have been observed to decompose and desorb as various atomic and molecular negative ions upon impact with room-temperature alkali-metal surfaces. Many of the negative ions observed have electron affinities which are less than the alkali-metal work function; hence the mechanism for ion formation and desorption is complex. A simple model is proposed to account for some of the observations.

The transition between hexagons and rolls in convective patterns has been studied in pure water under non-Boussinesq conditions. The transition is characterized qualitatively by shadowgraph images and quantitatively by heat-flow measurements and a local technique based on the deflections of a laser beam. We also show that with this local technique one can recover global quantities such as the convective heat flow.

We have used x-ray diffraction to study oriented strands of hexa-hexylthiotriphenylene discotic liquid crystals. The low-temperature $D_{\mathrm{ho}}$ phase consists of a triangular array of columns with periodic positional and incommensurate helical intracolumn order. A three-column superlattice results from the frustration imposed by molecular interdigitation in triangular symmetry. We suggest that the $D_{\mathrm{hd}}\to D_{\mathrm{ho}}$ transition is driven by the increase in hydrocarbon tail stiffness and length with decreasing temperature.

We report on the first high-resolution x-ray measurements of a second-order nematic to (bilayer) smectic-$A$ phase transition in a polymeric liquid crystal, $P4.1\mathrm{}$ polysiloxane. The critical exponents of the parallel and perpendicular correlation lengths and susceptibility were ${\nu }_{\parallel }=0.77\mathrm{}\pm 0.05$, ${\nu }_{\perp }=0.57\mathrm{}\pm 0.08$, and $\gamma =1.33\mathrm{}\pm 0.10$, respectively. The bare correlation lengths, ${\xi }_{\parallel }^0{q}_0$ and ${\xi }_{\perp }^0{q}_0$, were 3.27 ± 0.08 and 1.09 ± 0.14, respectively, and are unusually large in comparison to monomeric liquid crystals.

A diffraction study of lattice constants in the $P_{{\beta }^{\prime }}$ ("rippled") phase of hydrated lecithins suggests that the lamellar modulation arises from a delicate interplay between the energies associated with membrane curvature and with hydration, as modified by interactions between membranes. Our results imply the existence of a Lifshitz point in the global phase diagram of the lecithin-water system.

We present the first experiments on coherent backscattering of light in a quasi-two-dimensional system, and find reasonably good agreement with a theory that accounts for small departures from strictly two dimensions. The photon density near the sample boundaries is obtained from the angular dependence of the diffusely transmitted and reflected light, and is found to be in substantial agreement with the two-dimensional photon diffusion equation. Prospects for observing the logarithmic correction to the optical transmission of a two-dimensional system appear encouraging.

Charge exchange of ${\mathrm{Ar}}^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{N}}_2^{+}$ ions with a Pt(100) surface has been studied over incident energies ranging from 10 to 400 eV. Below 100 eV, the ion survival probably increases with decreasing energy. This increase represents the changing penetration depth of incident ions into the ion-surface interaction potential. Comparison of the result for ${\mathrm{Ar}}^{+}$ incidence with that for nitrogen ions shows that resonance neutralization is of minor importance for the ion survival probability and shows the preferential neutralization of nitrogen ions at high energies.

Spin-selective excitations (Stoner excitations) have been unambiguously identified by comparisons between spin-polarized-electron energy-loss spectra and calculated transition density of states. Analysis of the specular beam for polarized electrons incident on thin films of bcc Co shows three sharp, highly polarized loss features (background corrected asymmetry near 100%). Comparison with the theoretical Stoner and spin-nonflip transition density of states shows excellent agreement, permitting identification of the associated loss mechanisms.

Pulsed luminescence is observed in time- and frequency-resolved measurements of the $Z_3$ exciton luminescence of CuCl single crystals under excitation at the highly absorptive region of the $Z_{12}$ exciton resonance. A high-repetition-rate tunable picosecond uv light source was used. The pulse traveling time coincides well with the time calculated from the corresponding polariton group velocity. It is found that the redistribution of polaritons due to intraband relaxation is not important to the temporal behavior of the free-exciton luminescence, but the polariton propagation effect is essential, in contrast to the discussions found in previous work.

Available theories find pure (electronic) interband Auger lifetimes in $n$-type Si that are significantly larger than experimental values. Theories that include phonon-assisted processes find lifetimes that are still too large. Close examination of these theories, however, reveals that potentially compromising approximations were invoked. Here we report calculations whose accuracy is tested with respect to all relevant parameters, and we find pure Auger lifetimes in very good agreement with experiment over a wide range of temperatures. Thus the experimental results can be accounted for without the invoking of phonon-assisted recombination.