The photon spectrum from ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$(${\pi }^{-}$,$\gamma$)${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$ has been measured. The branching ratio to ${}_{}{}^{20}{}_{}{}^{}\mathrm{F}$($1^{+}$, 1.06 MeV) is compared to the analogous $M1\mathrm{}$ decay width ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$($1^{+}$, 11.24 MeV) →${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$(g.s.). This determines the relative importance of spin and orbital magnetization in the transition amplitude. The $M1\mathrm{}$ amplitude is found to contain only (47 ± 16)% spin flip, in agreement with shell-model calculations. Excitation of $2^{-}$ states by ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$(${\pi }^{-}$,$\gamma$) is also observed.

A new method is proposed for studying the semiclassical limit for model Hamiltonians which are polynomials in the generators of the Lie algebra $\mathrm{SU}\mathrm{}\left(n\right)\mathrm{}$. The symmetric representation, characterized by a single independent Casimir invariant is mapped onto a subspace of a set of canonical coordinates, and a systematic expansion in the reciprocal of the Casimir invariant is carried out. WKB calculations for a specific example yield improved accuracy compared to previous work.

Central nuclear collisions are studied with a 1.8-GeV/N ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ beam incident on a Pb target. Momentum and angular distributions of negative pions infer a pion temperature of 86.6±3.9 MeV, and a pion-nucleon ratio of 0.21-0.24. The pion multiplicity suggests a suppression of high-multiplicity events and a non-Poisson distribution. Pion interferometry analyses are performed; the relationship between pion multiplicity and the collision impact parameter is examined.

In coincidence with tritons from the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ (${}_{}{}^3{}_{}{}^{}\mathrm{He}$, $t$)${}_{}{}^{208}{}_{}{}^{}\mathrm{Bi}$ reaction proton spectra corresponding to decay into definite final states in ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ have been measured. The coincidence yield is described as exclusively coming from the decay of two resonances, the isobaric analog state (IAS) and the Gamow-Teller (GT) resonance. An analysis in terms of a formation cross section times a branching ratio gives a good description for the IAS and for the shape of the GT resonance. The yield from the GT resonance is, however, an order of magnitude larger than calculated.

With use of the $B^2{{\Sigma }_{\mathrm{u}}}^{+}$ state of ${\mathrm{N}}_2^{+}$ as an example, this Letter presents the first measurements demonstrating that fluorescence from molecular ions formed by photoionization is partially polarized. Furthermore, it is shown that the degree of polarization reflects the alignment of the molecular ion and the ratio of the dipole strengths for the degenerate channels producing the ionic state. The wavelength-dependent measurements are compared with both quantum mechanical and classical calculations. Temperature-dependent effects are predicted.

The carbon Auger line shapes obtained from three diverse gas-phase transition-metal carbonyls are analyzed. Some features in the line shape directly reflect the amount of $\pi$ backbonding. The analysis indicates that the dominant initial state is the adiabatically relaxed ground state. The final-state holes are either localized on the same ligand or described by a state consisting of one hole on a ligand and the other on the metal atom. The latter state may be viewed as a $\pi$-backbonding—assisted interatomic decay.

The resolution of the supernumerary rotational rainbow structure in state-to-state differential cross sections for ${\mathrm{Na}}_2$-Ne collisions which had been predicted by infinite-order sudden-approximation theory is reported. These data, by proving the existence of such a structure in quantitative agreement with the infinite-order sudden-approximation theory, answer one of the major remaining questions in the physics of rotationally inelastic scattering.

Adiabatic-nuclei resonance theory has been applied to the study of dissociative attachement of low-energy electrons to ${\mathrm{F}}_2$. Stieltjes moment theory was used to derive fixed-nuclei electronic resonance parameters from large-scale configuration-interaction calculations on ${\mathrm{F}}_2$ and ${\mathrm{F}}_2^{-}$. Dissociative attachment cross sections for the four lowest vibrational levels of ${\mathrm{F}}_2$ are reported and compared with available experimental data.

A new approach to the problem of steady-state plane-wave propagation in a Raman-active medium has lead to general analytical solutions. Any arbitrary number of fields can interact via both Raman and four-wave processes, but all fields must satisfy the phase-matching condition. The solutions will be illustrated with a problem of current interest; multiple first Stokes generation from a multiple-mode pump field.

It is predicted with use of a simplified model that a Fabry-Perot cavity filled with atoms with Zeeman sublevels in the ground state should exhibit optical tristability via optical pumping. For linearly polarized incident light, three stable states appear in the polarization of the transmitted light; ${\sigma }_{+}$ dominant, ${\sigma }_{-}$ dominant, and linear polarizations. This optical tristability is discussed in the context of a butterfly catastrophe.

Presented herein is a fully electromagnetic and relativistic stability analysis of the Brillouin-flow equilibrium for magnetic insulation in planar geometry. Instability of TM waves propagating in the direction of the sheared electron flow is found. This instability occurs at short wavelengths at frequencies above the cyclotron and plasma frequencies relevant to the system. It is found that relativistic effects can make the maximum instability growth rate normalized to the cyclotron frequency substantially lower than the nonrelativistic value (0.06).

The effect of external noise on the transition to chaos for maps of the interval which exhibit period-doubling bifurcations are considered. It is shown that the Liapunov characteristic exponent satisfies scaling in the vicinity of the transition. The critical exponent for noise is calculated with the use of Feigenbaum's renormalization group approach, and the scaling function for the Liapunov characteristic exponent is obtained numerically by iterating a map with additive noise.

The effect of noise on systems which undergo period-doubling transitions to chaos is studied. With the aid of nonequilibrium field-theoretic techniques, a correlation-function expression for the Lyapunov parameter (which describes the sensitivity of the system to initial conditions) is derived and shown to satisfy a scaling theory. Since these transitions have previously been shown to exhibit universal behavior, this theory predicts universal effects for the noise. These predictions are in good agreement with numerical experiments.

The geometric structures of the clean 2×1 reconstructed and the hydrogen-stabilized (1×1)2H Si(100) surface have been studied by means of ion channeling and blocking. The latter surface appears to be contracted by 0.08±0.03 Å and has a surface Debye temperature of ∼230 K. For the 2×1 surface it is shown that only the surface dimer models by Appelbaum and Hamann and by Chadi agree well with the backscattering data. The silicon atoms in the very surface are displaced more than 0.45 Å in the surface dimer direction, but those in deeper layers are displaced less than ∼0.2 Å.

The first observation of the predicted anomaly in the static dielectric constant of a pure fluid near its critical point is reported. This anomaly, not observed to date in nonpolar fluids, is now seen in CO, a polar fluid. The anomalous increase in the dielectric constant, $\frac{\delta \epsilon }{\epsilon }$, reaches 1×${10}^{-3\mathrm{}}$ for fluid with density very close to the critical value. The functional form of the anomaly is consistent with current theoretical predictions.

Inelastic neutron scattering measurements demonstrating H tunneling states for O-H pairs in Nb${\mathrm{O}}_{0.013}$${\mathrm{H}}_{0.016}$ are presented. The tunneling matrix element found is 0.19±0.04 meV.

Molecular-dynamics computations for point dipolar particles of the time-dependent polarization fluctuations allow evaluation of the dielectric properties. From these the collective effects in the longitudinal component (dipolarons) analogous to the plasma oscillations (plasmons) in Coulomb systems can be readily identified. The transverse mode, by contrast, is strongly damped.

Measurements of $L_3\mathrm{VV}$ Auger-electron line shapes were made for Ni and Cu at incident electron energies near threshold for $L_3$ subshell ionization to reduce perturbing satellite effects. Large shifts were observed in the position of $L_3\mathrm{VV}$ features as the incident energy was varied. These shifts are comparable to those found in free atoms due to post-collisional interaction. Apparent inconsistencies in the calculated electronic properties of nickel are removed by a new value of $U_{\mathrm{eff}}=2.5\mathrm{}$ eV, the effective energy of two $d$ electrons on the same atom.

The mean-field equations of the Ising model with competing interactions are studied by means of an iteration procedure. It is found that the phase diagram consists of regimes with stochastic behavior, indicating a complete "devil's staircase" with pinned configurations only, and analytic regimes where commensurate pinned phases are separated by incommensurate phases with Goldstone modes.

The critical behavior of the Yvon-Born-Green integral equation for fluids is analyzed by a moment expansion which yields a nonlinear differential equation accurately describing the long-range correlations. Phase plane analyses show that for dimensions $d<~4$ a critical point is characterized by $\eta =4-d$ with $g\left(\mathcal{r}\right)-1\mathrm{}$ negative for large distances, $\mathcal{r}$, in contrast to normal expectations. For $d>\mathrm{}4\mathrm{}$ the differential equation allows $\left[g\right(\mathcal{r})-1]>\mathrm{}0\mathrm{}$ and $\eta =0\mathrm{} or 4-d$. The compressibility never diverges if $d=1\mathrm{}$.