Measurements of inelastically scattered electronvolt neutrons have been completed with a pulsed neutron source and neutron time-of-flight techniques in combination with a resonant-neutron-capture detector. Measurements are presented on liquid nitrogen and benzene for incident neutron energies in the range 1.5 to 15 eV and for $q$ values from 13 to 120 ${\mathrm{\AA }}^{-1\mathrm{}}$. These are the first measurements of inelastic neutron scattering in this energy range.

Ultraviolet photoelectron spectra of optically excited states in the band gap of trans-polyacetylene are reported. The data are discussed in the context of theoretical models of optically induced polaron and soliton formation in trans-${\left(\mathrm{CH}\right)}_x$, and appear complementary to previously reported pulsed optical absorption data.

The optical absorption spectrum of lightly doped polyacetylene exhibits structure below the interband threshold which has been attributed to solitons. Here it is shown that such structure could be due to local-field corrections, i.e., the electromagnetic interaction between chains. The peak occurs because the polarizability of a quasi one-dimensional system exhibits strong fluctuations below threshold.

A three-body problem, concerning two holes in a nondegenerate valence band and a single electron in a conduction band, with strong short-range interactions, is solved exactly in any number of dimensions. The binding depends nontrivially on the ratio of the valence to conduction bandwidths (i.e., on the inverse ratio of their effective masses) and always vanishes when this ratio exceeds 1. The effective mass of the "trion" bound state also depends sensitively on this ratio and on dimensionality.

The first observation of atomic line radiation due to the photon-stimulated desorption of excited neutral atoms is reported. Synchrotron radiation incident on alkali-halide surfaces is found to result in a surprisingly high yield of desorbed excited alkali atoms, more than three orders of magnitude larger than the yield of desorbed ions. Marked structure in the incident-photon-energy-dependent ${\mathrm{Li}}^{*}$ resonance line intensity suggests that the desorption is induced by secondary electronic processes.

New results for Cs excitation spectra from Na-Cs alloys are presented and the effect of environment on the Cs $5p$ edge is discussed.

The electrical properties of wires with cross-sectional areas of order 1 × ${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$ and lengths as small as 0.2 μm have been studied. At temperatures below about 10 K, the resistance of the wires increases with decreasing temperature, as found in previous studies of much longer wires. For wires shorter than about 5 μm, the resistance rise decreases as the length of the wire is decreased. From these results a characteristic length scale of approximately 0.2 μm at 1.5 K is found, in good agreement with the current theory.

The effective Hubbard $U$ for the bonding coordination defect in glassy Se is investigated. This is accomplished by applying local-density total-energy calculations directly to charged defects. The existence of a sizable negative contribution to $U$ from structural relaxation is confirmed. However, a still larger Coulomb repulsion gives rise to an overall positive $U$. This result is not inconsistent with a negative $U$ in the compound chalcogenide glasses.

The dimensionality of the spin dynamics in bis-tetramethylselenafulvalenium hexafluorophosphate ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$, is studied in the high-pressure metallic phase at low temperatures via the field dependence of proton relaxation rates. Two regimes are identified: a low-field regime (1-12 kOe) where the relaxation rate is independent of field and a highfield regime (12-120 kOe) where the relaxation rate depends logarithmically on field. The high-field behavior is interpreted as spin dynamics in a quasi two-dimensional metal with a crossover to three-dimensional spin dynamics at low field.

Neutron-polarization-analysis measurements on single-crystal Cu-Mn alloys show the presence of a long-period spin modulation with a concentration-dependent period. Coexistent with these modulated regions are smaller regions in which the spin correlations are determined by the atomic short-range order and which have ferromagnetic moments. The interactions between these ferromagnetic and modulated regions are an essential element in understanding the complicated magnetic behavior of this spin-glass system.

The exact eigenstates of a one-dimensional tight-binding model with a periodic diagonal potential that can be commensurate or incommensurate with the lattice are found. If the period is incommensurate, all the eigenstates are localized. The localization length and the density of states are identical to those of a related disordered system. The case of a commensurate potential, for which all the states are extended, and the approach to the incommensurate case are also discussed. The solution is achieved by mapping the model into a time-dependent quantum problem in which the potential is periodic in time.

The pressure dependence of the fundamental optical absorption edges in solid Xe has been investigated up to the maximum pressure of 55 GPa by using a diamond-anvil-type high-pressure cell at room temperature, and the band-gap energies as a function of pressure have been obtained from 44 to 55 GPa. The metallic transition pressure for solid Xe is estimated to be about 200 GPa or greater from the present pressure versus band-gap energy measurements, under the assumption that the solid Xe does not undergo any structural phase transition above 55 GPa.

Results of single-particle tunneling experiments performed on films of indium oxide are presented. The zero-bias anomalies change character with decreasing thickness from a three-dimensional behavior to a logarithmic energy dependence. Theoretical consideration suggest a crossover from a $\ln V$ to a $V^{\frac{1}{2}}$ correction to the density of states at a thickness-dependent energy. In the $V^{\frac{1}{2}}$ range, where the sample exhibits a three-dimensional density-of-states anomaly, its transport properties may still be two dimensional. These expectations are experimentally confirmed.

The observation of strong electric-dipole spin resonance of donor electrons in ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$ in far-infrared magnetotransmission experiments is reported. The resonance is shown to be allowed through corrections to the effective-mass Hamiltonian due to the uniaxial and acentric crystal structure, and constitutes a powerful new tool for the study of diluted magnetic semiconductors with uniaxial symmetry.

A one-dimensional quantum mechanical model defined by the "quadratic mapping Hamiltonian" which is almost periodic is presented. The model admits a Cantor spectrum of Lebesgue measure zero with a singular continuous measure and produces extended states displaying an unexpected chaotic behavior at large distances.

The quenched Eguchi-Kawai model is studied by using the Monte Carlo technique. The data indicate that in the large-$N$ limit the quenched Eguchi-Kawai model is identical to the Wilson theory throughout the whole range of coupling constants. The first-order phase transition is observed at $\frac{\beta }{N}=0.29\mathrm{}\pm 0.02$ for the gauge group U(20).

A new instrument capable of precisely comparing the polarization of positrons from the decay of different nuclei is described. The decays of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{68}{}_{}{}^{}\mathrm{Ga}$ have been compared to 1% accuracy with a first-generation instrument. Systematic studies indicate that an eventual accuracy of 4×${10}^{-4\mathrm{}}$ can be reached, allowing improved limits to be set on parity-symmetric formulations of the weak interactions.

High-transverse-energy events produced in $\mathrm{pp}$ collisions at 400 GeV/c were studied using the large-acceptance Fermilab multiparticle spectrometer. The cross sections for such interactions exhibit an exponential dependence on transverse energy with values of the slope increasing with the decreasing azimuthal acceptance of the trigger. The majority of events selected with full azimuthal acceptance are nonplanar; hence they do not show jet-like event structure. The fraction of planar events is small and remains constant as a function of transverse energy.

The population hole burned within the inhomogeneously broadened ir fundamental absorption band of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ in solid ${\mathrm{N}}_2$ by a narrow-band pulsed laser is detected as an antihole in the transient absorption of a tunable cw probe beam whose frequency is swept through the $v=1\mathrm{}\to v=2\mathrm{}$ resonance. The time development of the transient profile demonstrates that spectral diffusion is governed by an energy-dependent process, contrary to what has been observed in many disordered systems.

Heterodyne detection of backward four-wave mixing in resonant gas media allows one to explore both real and imaginary parts of the reemitted electromagnetic fields. It provides the first experimental proof of the dispersive character of nonlinear susceptibilities in the saturated regime. It also allows one to demonstrate the strong directional anisotropy of the saturation induced by either forward or backward pump wave.