The $\nu =0\mathrm{}\to \nu =5\mathrm{}$ rotational-vibrational spectrum of free HD has been observed by means of photoacoustic spectroscopy. The electric dipole matrix elements are found to depend significantly upon the rotational quantum number. The dependence of the frequency of the $R\left(1\right)\mathrm{}$ line upon density has been determined.

We investigate spatial fluctuations of the cooperative emission of radiation by a two-level-atom system driven by a resonant electric field. For very small absorption length, a directional photon antibunching effect is expected.

It is shown that the introduction of barrier potentials between the plugs and solenoid of a tandem mirror substantially reduce ion energy and density required in end plugs. Several means for creating barriers and some of the important physics issues are discussed.

Relativistic particle simulations of the beam-plasma interaction show that a dc electric field intensifies the interaction and amplifies radiation; radiation in turn clamps the runaway momentum. When the dc field exceeds a critical value, the runaways are detached from collective interactions. The effect of enhanced radiation and clamping may be of significance for devices such as traveling-wave tubes and free-electron lasers and for tokamak experiments.

Necessary and sufficient criteria are obtained for the high-toroidal-mode-number stability of a guiding-center plasma in low-aspect-ratio, D-shaped, beam-induced tensor-pressure tokamaks. The difference between the two criteria is significant for interchange stability, while the difference is small for ballooning stability. The critical $\beta$ value imposed by stability to ballooning modes is higher for perpendicular than for parallel beam injection.

Orientational ordering of nitrogen-molecule overlayers on graphite has been investigated by neutron diffraction. The ordering transition is found to occur at 30 K in both the registered and dense-solid phases despite a 5% difference in the nearest-neighbor distance.

The oscillatory flow of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ through an 18-μm-diam orifice has been studied. A clearly defined critical velocity is seen in both the $A$ and $B$ phases. For velocities greater than the critical value, the superflow involves an excess dissipation. The temperature dependence of the critical velocity is reported for temperatures near the superfluid transition ($\frac{T}{T_c}>0.9\mathrm{}$).

Exact calculations of the angular deceleration of superfluid vortex arrays show momentary speedups in the angular velocity caused by coherent, multiple vortex loss at the boundary. The existence and shape of the speedups depend on the vortex friction, the deceleration rate, and the pattern symmetry. The phenomenon resembles, in several ways, that observed in pulsars.

For "two-dimensional" solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ submonolayer films adsorbed on graphite substrates, clear experimental evidence for the Kosterlitz-Thouless melting mechanism is presented. The regions in the density-temperature plane for the (i) solid, (ii) liquid-crystal, and (iii) fluid phases are determined by nuclear-magnetic-resonance methods. If the melting lines are determined by the dissociation of bound oppositely oriented defect pairs, then these data correlate very well with previous heat capacity measurements.

On the basis of a Korringa-Kohn-Rostoker coherent-potential-approximation calculation of the band structure and the electron-phonon mass enhancement in a number of Pd-rich ${\mathrm{Pd}}_c{\mathrm{Ag}}_{1-c}$ alloys, it is argued that some of these could be superconducting at observable temperatures ($0.01 \mathrm{K}<~T_c<~0.5 \mathrm{K}$). It is suggested that these systems provide a unique possibility for studying the effects of paramagnons on the superconducting state.

The deep impurity character of interstitial positive muons or protons in Si and Ge is shown to result from the valley-orbit interaction of the six conduction-band minima along $\Delta$. This interaction, much stronger for interstitial than for substitutional point charges, leads to a breakdown of the effective-mass approximation and to the formation of a deep state. This is particularly striking in Ge, where the $\Delta$ minima are not the absolute ones.

An interaction mechanism is proposed for the inelastic, grazing-incidence scattering of fast ions by metal surfaces which involves coupling of the ion multipoles with surface plasmons. The monopole-plasmon coupling governs the energy-loss spectrum, whereas the higher-multipole interactions raise the ion into electronic excited states. The proposed mechanism is very efficient in dissociating unstable excited molecular ions and in producing atomic excited states with a high degree of orientation.

This Letter reports self-consistent—field calculations of the electronic structure of the Jahn-Teller distorted vacancy in Si. With use of the tetragonal atomic displacements estimated by Watkins, it is found that the Jahn-Teller splitting of the sixfold-degenerate bound state in the gap is of the order of 0.5 eV. This, together with small breathing-mode displacements, results in a fully occupied doublet in the lower part of the band gap, in agreement with experimental observations.

Pd quench condensed onto a quartz plate in ultrahigh vacuum exhibits a pronounced resistance minimum in an external field. This anomaly has a two-dimensional origin but is not caused by the free surface atoms. A superposition with a small fraction of a Ni or Fe layer causes a similar anomaly as the magnetic field. The two effects are not additive. I suggest that a thin Pd sheet below the surface shows two-dimensional band ferromagnetism.

The Roper resonance photoexcitation amplitude is calculated by use of a quark model with a Lorentz scalar confining potential perturbed by a Coulomb-type vector potential. All the necessary complications due to SU(6) admixture, relativistic effects, and non-single-quark-transition interactions are considered. The result is found to be in good agreement with experiment.

For heavy $q\overline{q}$ bound states in quantum chromodynamics, spin-dependent forces are obtained in a manifestly gauge-invariant formalism. Classical spin-orbit and Thomas precession terms are expressed in general in terms of the static energy, and assuming that the confinement mechanism is electric determines uniquely the spin-dependent forces in terms of the nonrelativistic potential. The Coulomb plus linear potential model gives $P$-state splittings correct to better than 25% and a mass for the ${\eta }_c$ of 3.030±0.020 GeV.

This Letter presents measurements of the phase and magnitude of the $\mathrm{CP}$-nonconservation parameter ${\eta }_{00}$ resulting from a study of the proper-time distribution of $K^0\to {\pi }^0{\pi }^0$ decays in a short-lived neutral beam. We find ${\varphi }_{00}=55.7\mathrm{}°\pm 5.8°$ and $\left|{\eta }_{00}\right|=(2.33\mathrm{}\pm 0.18)\times {10}^{-3\mathrm{}}$. A direct comparison to ${\eta }_{+-}$ measured in the same experiment gives ${\varphi }_{00}-{\varphi }_{+-}=12.6\mathrm{}°\pm 6.2°$ and $\left|\frac{{\eta }_{00}}{{\eta }_{+-}}\right|=1.00\mathrm{}\pm 0.09$.

The phase and magnitude of the $\mathrm{CP}$-nonconservation parameter ${\eta }_{+-}$ have been determined from the proper-time distribution of $K^0\to {\pi }^{+}{\pi }^{-}$ decays in a short-lived neutral beam. We find ${\varphi }_{+-}=41.7\mathrm{}°\pm 3.5°$ and $\left|{\eta }_{+-}\right|=(2.27\mathrm{}\pm 0.12)\times {10}^{-3\mathrm{}}$.

A sensitive test of the charge symmetry of nuclear forces is the angular symmetry about 90° of deuterons produced in the $\mathrm{np}\to d{\pi }^0$ reaction. We predict a front-to-back asymmetry of about 0.8% in the $\Delta \mathrm{}(3,3)\mathrm{}$ resonances region. The asymmetry arises primarily from ${\eta }^0-\eta$ mixing.

Data on $\mu$-pair production by pions are examined as a function of $x$ and $P_T$ for longitudinal photon polarization. Evidence in the form of a ${{sin}^2\theta }^{*}$ term in the helicity angular distribution is observed for $x$ near 1. This is conclusive evidence that production in this region is not predominantly through on-shell quark annihilation. The result is consistent with a calculation based on quantum chromodynamics.