Absolute cross sections and mass-spectral cracking patterns are reported for ionization of the free radicals C${\mathrm{D}}_3$ and C${\mathrm{D}}_2$. Such data for free radicals have previously been unmeasured because of their high reactivity. In this work, a beam of pure free radicals is generated by charge-transfer neutralization of a fast ion beam and is ionized by a crossed electron beam. With this approach, complete collection of mass-selected parent and fragment ions is obtained, permitting measurement of quantitative cross sections.

We observe the Stark effect in hydrogen by photoionizing a relativistic hydrogen beam prepared by photodetachment of the 800-MeV ${\mathrm{H}}^{-}$ beam at the Clinton P. Anderson Meson Physics Facility. Photoexcitation of ${\mathrm{H}}^0$ is performed in a magnetic field (up to 6.5 kG) that appears in the atomic frame as an electric field up to 3 MV/cm. Resonances observed with an angle Doppler-tuned laser beam match calculated energies. At sufficiently high field, near the Stark-Coulomb potential-barrier peak, we observe shape resonances with an apparent elevation of the base line on the high-energy side.

A preliminary comparison of the fundamental characteristics of undulator and Čerenkov free-electron lasers is presented. It is assumed that both devices are operating in the Compton regime and that they are driven by a short-pulse relativistic electron beam.

In thermal-barrier experiments in the tandem mirror experiment upgrade, axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.

Using a light-beating, Rayleigh-scattering technique, we observe the thermally excited biaxiality fluctuations which appear in the uniaxial disklike phase of a potassium-laurate, 1-decanol, ${\mathrm{D}}_2$O mixture close to the biaxial phase. The two polarized- and depolarized-mode linewidths undergo a critical slowing down, in agreement with a Landau-type prediction. From the wave-vector dependence of the damping rate, one estimates the bare transverse correlation length to be comparable with 2 micelle diameters.

An anomalous increase is observed in the Fréedericksz critical field in a very thin, homeotropically aligned liquid-crystal cell near the second-order nematic-smectic-$A$ phase transition. The anomaly is analyzed in terms of smectic layering induced at the two surfaces of the cell, decreasing the effective thickness of the nematic region and thereby increasing the effective critical field.

A model of thin-film ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures is described. The coupling of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ density to the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ film thickness is such that there is an instability, leading to phase separation at low ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations.

The crossover from critical behavior to ideal Bose-gas behavior in a dilute, weakly interacting Bose fluid at low temperatures is shown to be described by a scaling function depending on the temperature, density, and dimensionality. The results are applied to the experimental measurements of Crooker et al. on the superfluid density of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in Vycor glass. The satisfactory agreement obtained lends support to the model of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in Vycor as a dilute, weakly interacting, three-dimensional Bose fluid.

With use of a differential adsorption technique combined with acoustics, we have investigated the layered growth of solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on Grafoil from the pressurized liquid phase. At low pressures, we found a first-order phase transition each time a layer was completed, whereas at high pressures the transitions occurred near midlayer, at a coverage characteristic of an epitaxial phase of He on graphite. Our results support the predictions of recent theories of multilayer growth.

A lattice-gas model with particle-conserving hopping dynamics on a periodic lattice is exposed to a strong external field along one of the principal axes. The resulting stationary state is determined exactly in the limit of infinite ratio of jump rates in and perpendicular to the field direction. In this state the gas-liquid phase transition is of mean-field type. For not too strong coupling in the direction orthogonal to the field, several other phase transitions occur. All of these have mean-field character as well.

Precision ac heat-capacity scans of submonolayer Ar on Graphite Foam exhibit a liquidvapor anomaly near 55 K, a small, sharp, coverage-independent peak at 47.2 ± 0.2 K, and a broad anomaly near 49.5 K. The 47.2-K peak suggests that the Ar overlayer melts via a "weak" first-order (rather than continuous) transition. The 49.5-K anomaly appears to be related to the gradual decrease of correlation (possibly substrate-induced orientational order) that exists in the liquid phase.

Quasielastic and inelastic, incoherent scattering of neutrons has been used to probe the diffusional and vibrational motions of ethylene molecules on graphite basal-plane surfaces. The experimentally observed dynamical response is found to agree reasonably well with molecular-dynamics simulations of ethylene films under similar conditions of coverage and temperature.

The structural and dynamical behavior of overlayers of ethylene physisorbed on the graphite basal plane, has been investigated via computer simulation. The dynamics of various surface phases has been revealed by examination of the power spectra appropriate to molecular translational and rotational motion. Our results are discussed in the light of recent experimental work on the ethylene-graphite system.

The critical temperatures of the first three layers of Xe on a stepped surface of Pd, as well as the roughening temperature, have been recently measured by Miranda et al. These temperatures are about half those expected from theory for homogeneous infinite surfaces. Here we propose that this dramatic decrease is due to the finite size of the terraces in the stepped surface and to the existence of preferential adsorption sites along the steps.

We present the first realistic study of the surface-phonon spectrum of a reconstructed semiconductor surface. The extensive reconstruction of the Si(001)2 × 1 surface is found to localize a rich spectrum of surface modes including a band localized nearly completely on a rocking motion of surface dimers and an unanticipated optical mode localized in the first two subsurface layers. With use of a tight-binding theory for structural energies, the model incorporates the effects of bond rehybridization at the surface.

We show that a previous model for ${\left(\mathrm{GaAs}\right)}_{1-x}{\mathrm{Ge}}_{2x}$, when more accurately analyzed, does not fit the observed composition dependence of the direct energy gap. A simple model for the lattice disorder is proposed. This model fits the composition dependence of the gap with no adjustable parameters. Our results show that antisite disorder and the transition from zinc-blende to diamond symmetry have no significant influence on the energy gap of this material.

We report experimental results for a dynamic length scale ${\xi }_H$ as a function of polymer concentration $c$ for various molecular weight polystyrenes in the two solvents toluene and methyl ethyl ketone. We find that the quantity $\frac{{\xi }_H}{R_H}$ is a function only of the reduced concentration $\mathrm{kc}$, where $R_H$ is the hydrodynamic radius and $k$ is determined from a virial expansion of the diffusion coefficient. However, the dynamic and static length scales are not proportional.

Hybridization between the helium $1s$ orbital and the unoccupied metal states is corrugated in the opposite sense to the charge density. This explains the anomalous smoothness observed in helium diffraction for metal surfaces. With a simple model which includes hybridization for Ni, Cu, Pd, and Ag(110) surfaces, we obtain corrugations, softness, and well depths which are in good agreement with experiment, and also agree with self-consistent calculations of charge density.

Intrinsic paramagnetic defects have been detected by ESR measurements in tetrathiafulvalene -$p$- chloranil single crystals, associated with the neutral-to-ionic phase transition at $T_c=84\mathrm{}$ K. Two kinds of paramagnetic species are identified with delocalized spins propagating on either donor or acceptor sites. Total spin density and electric conductivity were found to increase discontinuously as temperature is lowered across $T_c$. These features are attributed to the formation of mobile solitonlike (and antisolitonlike) paramagnetic defects induced in the dimerized lattice at the phase transition.

We have measured the electric field dependence of the Young's modulus and internal friction of orthorhombic Ta${\mathrm{S}}_3$. The modulus is observed to decrease by a few percent when the field exceeds ${\epsilon }_T$, the threshold for non-Ohmic conduction. At the same field, the internal friction increases rapidly, saturating at ∼ 1% for fields $\epsilon >2\mathrm{}{\epsilon }_T$.