Production of nuclear-spin-polarized atoms by "collisional pumping" is proposed. Collisional pumping utilizes a succession of charge-transfer collisions between a fast ion or atom beam and a thick electron-spin-polarized target to polarize the beam. Calculational results are presented for 20-keV/u H or D ions or atoms incident on a polarized target. The process appears to have unique capabilities for polarizing fast H, D, T, and perhaps other nuclei, with the possibility of producing ampere currents.

A nonperturbative theory is developed for free-free transitions of electrons colliding with atoms in intense high-frequency laser fields. The energy of the incoming electrons is assumed to be smaller than that of the photons. Rather simple expressions are derived for the multiphoton absorption amplitudes. Large deviations from the laser-free case are shown to appear in elastic scattering. The theory applies at already existing laser frequencies but extends beyond, to the extreme ultraviolet range.

A novel technique for studying the dielectronic recombination process is presented. The interaction of very highly stripped trapped ions with a tunable energy electron beam is studied using an electron-beam ion source. Dielectric recombination resonances have been observed in ${\mathrm{Ar}}^{12+,13+,14+,15+}$ for single configurations ($\mathrm{KLL}$ resonances). The generality of this method which makes possible the selective study of electron-ion interactions of plasma diagnostic interest is discussed.

Detailed double-differential distributions in energy and angle of emission of electrons ejected into the continuum by collisional ionization of ${\mathrm{H}}^0$ projectiles with He are measured. Anisotropies of the resulting three-dimensional cusps are discussed in terms of contour lines taken at different levels. They show a structure that calls for an inclusion of higher terms in the expansion of the cross section, not contained in first Born calculations of Day and of Briggs and Day.

Explicit finite-difference numerical solutions of the time-dependent Navier-Stokes equations for axisymmetric flow between concentric cylinders are presented for propagating Taylor vortex fronts. They are a prototype example for the expansion of a periodic pattern into regions occupied by an unstable homogeneous basic state—here circular Couette flow. The propagation speed and the evolution of the periodic state behind the front are determined quantitatively as functions of Reynolds number for systems with three different radius ratios.

The evolution of domain ordering is studied with use of real-time x-ray scattering in stage-2, -4, and -6 Sb${\mathrm{Cl}}_5$-intercalated graphite, an experimental realization of a fourteen-state quasi two-dimensional superlattice. Quenching from the disordered phase reveals scaling behavior $L\sim t^{\alpha }$, with $\alpha =0.52\mathrm{}\pm 0.05$. Evidence for defect pinning is observed, leading to very sluggish long-time kinetics. The experimental findings are discussed in terms of recent theoretical predictions and Monte Carlo simulations of highly degenerate systems.

A possible explanation of the staging transition in graphite-intercalation compounds is presented.

The stability of multilayer formation versus clustering of C${\mathrm{F}}_4$ films adsorbed on graphite has been studied by reflection high-energy electron diffraction. It is found that an incomplete-to-complete wetting transition occurs at $T_w=37\mathrm{}$ K, a temperature far below the bulk triple point (89.5 K). The presence of a hysteresis indicates that the transition is first order.

Wetting characteristics of Ar, Kr, Xe, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, methane, and ethane adsorbed on Au(111) have been determined with a high-frequency microbalance. All gases form incompletely wet films below their bulk triple points and completely wet films above. The transitions to complete wetting are continuous, the maximum thicknesses of the layered films below $T_t$ varying as ${(T_t-T)}^{-\frac{1}{3}}$.

Vibrational spectra for $a$-Si and $a$-Si${\mathrm{H}}_{0.12}$ have been measured by inelastic neutron scattering, directly yielding the phonon density of states for the former material, and the partial phonon density of states for H vibrations for the latter. The relevance of these results to the interpretation of infrared and Raman spectra is discussed.

A novel infrared-emission technique has been used to make the first measurement of the linewidth of a molecule-substrate vibrational mode on a well characterized single-crystal surface. At saturation coverage, the observed linewidth of the C-Ni mode of CO on Ni(100) is 15 ${\mathrm{cm}}^{-1\mathrm{}}$. This result is in agreement with predictions for broadening due to deexcitation by two-phonon emission.

The existance of phase excitations (phasons) in a solid with charge-density waves is clearly demonstrated in a diffraction experiment. It is found that a phason temperature factor, analogous to the usual Debye-Waller factor for phonons, must be introduced in interpreting diffraction data. At 90 °C, this factor attenuates all first-order satellites by a factor of 4.

Three forms of elastic and plastic deformations at Au(111) surfaces have been observed by high-resolution electron microscopy. These are normal expansion of the first surface interplanar spacing, lateral expansions through surface dislocations, and the development of hill-and-valley structure or elastic buckling depending upon the sample morphology. These effects are interpreted as evidence for a positive, tangential surface pressure (surface stress) consistent with effects in gold alloys, and small-gold-particle internal and electronic structures.

A detailed investigation of vortex nucleation in He II containing traces of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ isotopic impurity is reported. Systematic measurements of the nucleation rate due to negative ions were made at 23 bars. A model is proposed which, when fitted to the data, implies that a single ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atom trapped on the surface of the ion has the dual effect of reducing the critical velocity for vortex nucleation by circa 4 m ${\mathrm{s}}^{-1\mathrm{}}$, while simultaneously increasing the corresponding rate constant by a factor of circa ${10}^3$.

It is proposed that ac voltage ($v_{\mathrm{a}\mathrm{}\mathrm{c}}$) oscillations observed in sliding charge-density-wave systems are caused by phase vortices at the sample ends. The model's prediction that $v_{\mathrm{a}\mathrm{}\mathrm{c}}$ is length independent is verified experimentally. The long-range phase coherence implied by thermal-gradient experiments is consistent with the vortex model but not with models based on impurities.

Measurement of transient photoconductivity in $a$-${\mathrm{As}}_2$${\mathrm{Se}}_3$ through the glass transition temperature $T_g$ gives the temperature dependence of the density of recombination centers. The activation energy of this density increases suddenly at $T_g$, leading to identification of the centers with negative-$U$ defects.

The dynamics of trans-polyacetylene is studied within the adiabatic model of Su, Schrieffer, and Heeger. A single soliton has a maximum velocity, $v_m$, which can be higher than the sound velocity. A long-time simulation shows that, from an initial electron-hole pair, a soliton and antisoliton emerge with velocities $\pm v_m$ and that a persistent breather is left behind. This breather may account for the high-energy intragap structure observed in photoinduced absorption, including isotope dependence.

The charge density of GaAs was studied at various $\mathcal{k}$ points and for various bands by the augmented-spherical-wave method, using muffin-tin spheres at the Ga, the As, and, additionally, the interstitial sites. The lowest $X_c$ conduction-band points were found to be unique in having a high "charge" density in the interstitial spheres. It has therefore been predicted and verified that the $X_c$ points move up in energy relative to the ${\Gamma }_c$ point when closed-shell atoms (like He) are substituted at the interstitial sites. The calculations also indicate indirect-direct conversion for SiHe.

Convincing evidence has been discovered for bulk superconductivity in U${\mathrm{Pt}}_3$ at 0.54 K based on specific-heat, resistance, and ac susceptibility measurements. In addition, new evidence is presented that indicates that U${\mathrm{Pt}}_3$ is a spin-fluctuation system. If true, this is the first coexistent superconductor—spin-fluctuation system.

The crystal-momentum dependence of the line shapes appearing in photoexcitation from the valence levels of ordered S on Fe(100) has been measured with angle-resolved photoelectron spectroscopy. The variation in line shape with $\overrightarrow{\mathrm{k}}$ is a consequence of the hybridization of the energy bands of the two-dimensional overlayer with those of the three-dimensional solid. This observation, predicted theoretically by Liebsch, yields detailed information about the bonding interactions of ordered chemisorption systems.