Relativistic coupling terms at crossings between hydrogen Stark levels are computed with numerical Stark wave functions. Over a small interval of the electric field at crossings between a slowly and a rapidly ionizing state, the ionization rate of the former is increased. The effect is calculated to be observable with available techniques at more than thirty crossings between two $m_L=0\mathrm{}$ levels in the range 7 to 140 kV/cm, $n=7\mathrm{} \mathrm{to} 16$.

We have measured the $1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ interval in positronium (Ps) to be 1 233 607 185 ± 15 MHz, in agreement to within 1% of the ${\alpha }^3{R}_{\infty }$ QED prediction. The quoted 12-ppb uncertainty has equal contributions from the measurement of the Ps resonance relative to a ${\mathrm{Te}}_2$ absorption line and the calibration of the ${\mathrm{Te}}_2$ line relative to the deuterium $2S_{\frac{1}{2}}-4\mathrm{}{P}_{\frac{3}{2}}$ Balmer line.

A vacancy-buckling model is proposed for the (2×2) structure of the GaAs(111) surface. The model is confirmed by analysis of low-energy electron diffraction spectra. A reconstruction mechanism, basic to III-V compound surfaces, is proposed which favors equal numbers of dangling bonds on the nearest Ga and As neighbors. This model explains the (2×2) and (1×1) periodicities observed on (111) and (110) surfaces, respectively.

The coadsorption of low coverages of CO and potassium (${\theta }_{\mathrm{K}}<0.10\mathrm{}$) on Ru(001) is characterized by a single adsorption state with a C-O stretching frequency of ∼ 1460 ${\mathrm{cm}}^{-1\mathrm{}}$. Vibrational overtone spectroscopy and isotopic scrambling in thermal desorption reveal anomalously weak C-O bonds. Comparisons with metal carbonyls, molecularly adsorbed acetone, and oxygen on transition metals suggest a side-on-bonded molecule with its molecular axis parallel to or inclined towards the metal surface.

Additional dissipation of oscillatory superflow has been observed in uniformly rotating ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$. The damping increases linearly with angular velocity and is interpreted in terms of vortex-induced mutual friction similar to that observed in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. In the nonrotating state the damping is amplitude dependent, and may contain a contribution from remanent vorticity.

We demonstrate that the flow of excitations in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in restricted geometries is strongly influenced by Andreev scattering at very low temperatures. The effective viscosity is calculated in a simple model and shown to agree with the one observed in a torsional-oscillator experiment.

We have used picosecond white-light spectroscopy to study evolution of absorption in NaCl after pulsed band-gap excitation. Spectra at 80 and 300 K, initially very similar, subsequently develop features of well separated $F-H$ pairs (300 K) or of self-trapped excitons (80 K). We interpret the spectra in terms of diffusion of the $H$ center in the neighborhood of the complementary $F$ center, corresponding to reversible interconversion on the lowest potential surface connecting the exciton and the $F-H$ pair.

We present results of Monte Carlo simulations of odd-site polyacetylene chains, described by the Su-Schrieffer-Heeger Hamiltonian with on-site and nearest-neighbor Coulomb interaction terms. Starting with initial uniform or dimerized configurations, a soliton always forms in the system, even in the presence of Coulomb interactions. We study in particular the spin polarization, and by comparison with ENDOR and other experiments we discuss possible microscopic parameters characterizing polyacetylene.

We study the effect of electron-electron correlations on the energetics of solitons in electron-phonon models of quasi one-dimensional materials. In these combined Peierls-Hubbard models, by use of quantum Monte Carlo techniques, we (1) establish that the ground state of an odd chain, singly charged or neutral, is a soliton; (2) calculate neutral-soliton creation energies; and (3) prove that "soliton doping" persists in the presence of correlations. We discuss the relevance of our results for trans-polyacetylene.

The transverse magnetoresistance of Nb${\mathrm{Se}}_3$ and ${\mathrm{Fe}}_x\mathrm{Nb}{\mathrm{Se}}_3$ ($x<~0.01$) at magnetic fields in the range 100-220 kG shows a strong dependence on applied electric field in the temperature range 20 to 60 K. At 20 K and 206 kG, $\frac{\left[\rho \mathrm{}\right(H)-\rho \mathrm{}(0\left)\right]}{\rho \mathrm{}\left(0\right)\mathrm{}}=4.5\mathrm{}$ at 10 mV/cm and is reduced to 0.9 at 100 mV/cm for pure Nb${\mathrm{Se}}_3$ while the threshold for charge-density-wave sliding motion is ≥ 100 mV/cm. Pinning and unpinning of the charge-density wave with changes of temperature or Fe impurity does not affect the magnetoresistance reduction induced by the electric field.

An inflationary model which satisfies constraints from particle physics and cosmology is presented. The key feature is an SU(5)-singlet field which drives inflation, leads to proper density fluctuations, and solves the strong $\mathrm{CP}$ problem by the invisible axion mechanism. Thus, for the first time, all reservations about the inflationary universe are removed.

The lengths of molecular trajectories in an argonlike liquid simulated by molecular dynamics have been found to be fractal over a substantial range of scales with a Richardson coefficient of $\alpha =0.65\mathrm{}$ (Mandelbrot dimensionality of $d=1.65\mathrm{}$). The results obey a simple empirical relation with three parameters. We believe $\alpha$ to be a useful new parameter to describe the form of trajectories of molecules in fluids where none has previously been available, and that it is a state function.

Conformal invariance and unitarity severely limit the possible values of critical exponents in two-dimensional systems.

The spin van der Waals model may be regarded as an infinite-lattice-dimensional limit of the spin-½ anisotropic Heisenberg model. By solution of the generalized Langevin equation, time-dependent behavior is obtained. The geometry of realized Hilbert spaces lends a simple interpretation for critical dynamics.

Quasiparticles at the fractional quantized Hall states obey quantization rules appropriate to particles of fractional statistics. Stable states at various rational filling factors may be constructed iteratively by adding quasiparticles or holes to lower-order states, and the corresponding energies have been estimated.

The quantum Hall effect is shown be equivalent to a chiral anomaly in quantum electrodynamics. The integers $n_1$ and $n_2$ in the rationally quantized Hall conductance $g=\left(\frac{e^2}{2\pi \hslash }\right)\left(\frac{n_1}{n_2}\right)$ arise from vortex excitations carrying $n_1$ electrons and $n_2$ flux quanta. It is found that $n_2=2j$, where $j=l+\frac{1}{2}$ is the angular momentum per electron in the vortex. This explains the odd-denominator rule $n_2=2l+1\mathrm{}$.

The reactions Ca + Ca and Nb + Nb at 400 MeV/nucleon have been studied at the Bevalac using the "Plastic Ball" spectrometer. A global analysis of the events shows two nontrivial collective flow effects: the bounceoff of the projectile fragments, and the side-splash of the intermediate-rapidity fragments for the higher-multiplicity Nb + Nb events. Neither effect is seen in a knockon cascade calculation. A simulation with an event-generating statistical model has been done in order to extract the magnitudes of the effects.

A kinetic-energy—flow analysis of multiplicity-selected collisions of ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(E_{\mathrm{lab}}=400A \mathrm{MeV})+{}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ is performed on the basis of the nuclear fluid dynamical model. The effects of finite particle numbers on the flow tensor are explicitly taken into account. Strong sidewards peaks are predicted in $d\frac{N}{d}cos{\theta }_F$, the distribution of event by event flow angles. This is in qualitative agreement with recent data from the "Plastic Ball" electronic detection system. Cascade simulations fail to reproduce the data.

Using methods developed in field theory and statistical mechanics, especially in the context of the Anderson model, we formulate a novel and unified approach to both the statistical theory of nuclear spectra and the statistical theory of nuclear reactions. This approach leads to a separation between global (or mean) properties of nuclear spectra, and local properties (fluctuations). It yields novel results for the Gaussian orthogonal ensemble, for the embedded Gaussian orthogonal ensemble, and for the statistical theory of nuclear reactions.

This paper reports on betatron acceleration experiments using the space-charge-neutralized electron rings in the RECE-Christa device. Magnetic-probe and x-ray-absorption measurements indicate that electron ring currents of up to 2 kA were accelerated to 3.3 ± 0.3 MeV without indication of instabilities. A similar neutralization and acceleration method also appears applicable to electron rings generated in $B_{\theta }$-free configurations.