First measurements are reported of the near-threshold behavior of the vacuum-ultraviolet emission cross section for Ar + Ar atomic collisions. The experimentally observed threshold is 18±2 eV. The absolute emission cross section is presented for center-of-mass energies from threshold to 150 eV. The monitored radiation results from the decay of the lowest excited resonance state of the Ar atom and individual substate contributions are determined over most of the energy range investigated.

We present results of an experimental survey of the $n=15\mathrm{}$ state of Na in electric fields from 0 to 4 kV/cm. Both linear and higher-order Stark effects play important roles. States with low $\mathrm{}\left|m\right|\mathrm{}$ show marked differences from hydrogenic behavior, particularly in the vicinity of level crossings. Computed values for the energy levels are in good agreement with the data. The experimental method provides a simple technique for populating selected Stark or angular momentum levels of a Rydberg state.

A new class of density fluctuations has been observed in the adiabatic toroidal compressor (ATC) tokamak by spectral analysis of scattered microwaves. The observed frequency spectrum is consistent with that of drift waves with amplitudes that are maximum in the wavelength range 0.5-1.0 cm where the transverse inertia of ions is important for plasma stability. The total density fluctuation is ${\tilde{n}}_e\gtrsim 5\times {10}^{-3\mathrm{}}{\overline{n}}_e$. We estimate that these fluctuations could account for a large fraction of the electron energy losses of the ATC discharge.

A strong radio-frequency field applied to an expanding plasma is found to produce a shock at the surface where the local plasma frequency matches the applied frequency. The shock propagates towards the upstream overdense region with the ion-acoustic speed relative to the plasma flow. The density jump at the shock front is found to be proportional to the field amplitude.

The importance of recognizing a 1 × 1 structure preceding a superstructure in a chemisorption experiment is demonstrated by the system N-Ti(0001). Low-energy-electron-diffraction analysis of the 1 × 1 structure reveals that the N atoms are located in the octahedral holes 1.22±0.05 Å below the first layer of Ti atoms. Interatomic distances are such as to justify the view of the surface structure as a single double-layer of TiN.

Neutron inelastic scattering techniques have been used to measure the phonon dispersion curves along the chain direction in TTF-TCNQ. Most of the branches are rather flat and probably stem from excitations within the TTF or TCNQ molecules. Longitudinal and transverse acoustic modes were also measured and a large Kohn anomaly that seems to stem from the longitudinal branch was observed at 0.295 ($\frac{2\pi }{b}$).

We announce a new method for proving the existence of phase transitions and of spontaneous symmetry breaking. This method is then applied to various models, including a multicomponent ${\left[{\left(\overrightarrow{\varphi }·\overrightarrow{\varphi }\right)}^2\right]}_3$ quantum field model and classical, isotropic Heisenberg model.

Neutron scattering from the uniaxial ferroelectric ${\mathrm{Pb}}_5$${\mathrm{Ge}}_3$${\mathrm{O}}_{11}$ shows a quasielastic component in addition to the soft ferroelectric mode. At low temperatures, this scattering increases but can be suppressed by an applied electric field and probably arises from scattering by static domain walls. This scattering also has a maximum near $T_c$ which is largely field independent and which we suggest may arise from scattering by the walls of moving domains or clusters.

We present the results of high-precision electrical resistivity measurements in the vicinity of the metal-nonmetal transition in TTF-TCNQ. We find a sharp negative divergence in the derivative of the resistivity, $\frac{R^{-1\mathrm{}}\mathrm{dR}}{\mathrm{dT}}$, which suggests the existence of a second-order phase transition at about 53 K. A critical-exponent analysis of this divergence implies that the onset of long-range order occurs with a classical exponent (i.e., $\beta =\frac{1}{2}$) within a three-dimensional region which extends to approximately $\frac{|T-T_c|}{T_c}=t={10}^{-1\mathrm{}}$.

We compare optical-homogeneous-linewidth measurements for the resonant ${}_{}{}^5{}_{}{}^{}D_0^{}{}_{}{}^{}-{}_{}{}^7{}_{}{}^{}F_0^{}{}_{}{}^{}$ transition of ${\mathrm{Eu}}^{3+}$ -doped silicate glass and YAl${\mathrm{O}}_3$, using fluorescence line narrowing. The linewidth in the amorphous host is two orders of magnitude larger than in the crystal and exhibits an unusual temperature dependence. This behavior indicates an additional ion-lattice linewidth mechanism probably arising from an interaction between the Eu ions and "disorder modes" in the glass. Possible models are discussed.

We show that lattice-dimensionality crossover phenomena may be usefully analyzed by expansions in $R=\frac{J^{\prime }}{J}$, where $J$ and $J^{\prime }$ are the interaction constants in $d$ and $3-d$ lattice directions ($d=1\mathrm{} or 2$). The nature of the crossover phenomena is predicted to depend on the thermodynamic function considered, on $\mathrm{sgn}{J}^{\prime }$, and on $\mathrm{sgn}J$. Comparison with experiment confirms our predictions, and we present the first unambiguous experimental demonstration of crossover in the ferromagnetic susceptibility.

The nonlinear $\sigma$ model is renormalizable and asymptotically free in two dimensions. We show here how to construct this model in $2+\epsilon$ dimensions. Renormalization-group equations follow and exhibit a nontrivial uv stable fixed point, which corresponds in the language of Heisenberg ferromagnets to a critical point. The existence of systematic expansions in powers of $d-2\mathrm{}$ follows from this analysis. In the presence of a source information about long-distance behavior above the critical coupling constant is obtainable.

Recent experiments at SPEAR indicate an unexpectedly large number of $1^{--}$ states in the energy range 3.9-4.4 GeV. We show how the existence of exotic $c\overline{q}\overline{c}q$ mesons can account for these states as well as the rise in $R$ and the missing $\psi \mathrm{}\left(3.7\right)\mathrm{}$ decays. The width of these states does not require that they lie above the, as yet unobserved, $D\overline{D}$ threshold. Predictions of the model are readily testable.

I suggest that the structure observed between $\sqrt{s}=3.9\mathrm{} \mathrm{and} 4.5$ GeV at SPEAR is due to the presence of exotic mesons composed of a charmed diquark and antidiquark. Arguments for the existence of such mesons are reviewed. The characteristic features, i.e., masses, narrow widths, and abundance, which enable them to explain the data are presented. Further properties and yet another narrow meson with $m\approx 6.7$ GeV/${\mathit{c}}^2$ is predicted.

We observe a resonancelike structure in the total cross section for hadron production by $e^{+}{e}^{-}$ colliding beams at a mass of 4414 ± 7 MeV having a total width $\Gamma =33\mathrm{}\pm 10$ MeV. From the area under this resonance, we deduce the partial width to electron pairs to be ${\Gamma }_{\mathrm{ee}}=440\mathrm{}\pm 140$ eV. Further structure of comparable width is present near 4.1 GeV.

We present a partial-wave analysis of the $K^{\pm }{\pi }^{+}{\pi }^{-}$ system in $K^{\pm }p\to K^{\pm }{\pi }^{+}{\pi }^{-}p$ at 13 GeV. Evidence is given for the existence of two $J^P=1^{+}$ mesons: one at ∼ 1300 MeV ($\Gamma \sim 200$ MeV) coupling principally to $\rho K$ and the other at ∼ 1400 MeV ($\Gamma \sim 160$ MeV) coupling principally to $K^{*}\pi$.

The momentum transfer ($t^{\prime }$) dependence of the $J^P=1^{+}{K}^{*}\pi$ and $\rho K$ partial waves in the $K^{\pm }{\pi }^{+}{\pi }^{-}$ system is presented. The production of the $Q_1$ meson ($m\sim 1300$ MeV), which has a large $\rho K$ decay mode, obeys approximate $s$-channel helicity conservation. In contrast the production of the $Q_2$ meson ($m\sim 1400$ MeV), which decays predominantly to $K^{*}\pi$, satisfies approximate $t$-channel helicity conservation. Furthermore the $Q_1$ production distributions are virtually identical, whereas the ${Q_2}^{\pm }$ distributions exhibit a distinct cross-over for $\left|t^{\prime }\right|\sim 0.18$ Ge${\mathrm{V}}^2$.

Four events have been observed with an $e^{+}$ and a ${K_s}^0$ decay in the final state induced by a neutrino beam incident on the Fermilab 15-ft neon bubble chamber. Conventional backgrounds are unable to account for these events. Two of the events have in addition a ${\mu }^{-}$ in the final state clearly identified by the external muon identifier. The other two have ${\mu }^{-}$ candidates.

A general SU(3) analysis of vector meson and pseudoscalar meson radiative decays is carried out. Two solutions are obtained corresponding either to an inverse-square mass formula or to a linear mass formula for vector mesons that lead to reasonable agreement with most of the radiative decays.