The ground-state inversion splitting of the ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ molecular ion has been determined to be 55.3462±0.0055 ${\mathrm{cm}}^{-1\mathrm{}}$ from the high-resolution laser infrared spectroscopy of the ${\nu }_2$ band. This has enabled us to predict accurately the ground-state inversion spectrum in the far-infrared region and the inversion-rotation spectrum in the millimeter-wave region which may be used to detect this fundamental molecular ion in interstellar space.

The polarization correlation of the two photons emitted simultaneously by metastable atomic deuterium in a true second-order decay process has been measured for the first time. The results are in agreement with quantum mechanics and violate Bell's inequality by nearly 2 standard deviations in an experiment where the process of absorption and reemission is not important.

We report studies of ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$ ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$-orientation velocity-changing kernels with resolution sufficient to observe both classical and wave mechanical features. The measurements employ photon-echo techniques in which the echo intensity depends on the square of an isolated $J=1\mathrm{}$ multipole moment for which the velocity-changing kernel is diagonal.

Experimental results are presented regarding the generation and bistable behavior of a conjugated signal obtained through intracavity nearly degenerate four-wave mixing in a semiconductor laser amplifier injected with both pump ($\omega$) and probe ($\omega -\delta \omega$) frequencies. Conjugated reflectivities at $\omega +\delta \omega$ larger than 500% with 35% efficiency together with bistable behavior in the three signals (pump, probe, and conjugated) are reported.

It is shown that a squeezing pattern is produced in resonance fluorescence from $N$ atoms distributed at regular positions. The squeezing pattern has twice the periodicity of the ordinary diffraction pattern. Directions in which squeezed coherent states are observed alternate with directions in which squeezed vacuum states occur. The squeezing effect is $N$ times stronger than in resonance fluorescence from a single atom.

The universal nature of self-induced polarization rotation in nonlinear optical media having a third-order nonlinearity is depicted in terms of phase-space trajectories characterized by separatrix orbits. The existence of nonlinear eigenpolarization, and of periodic synchronized spatial as well as chaotic spatial distributions of the polarization of light, are found in the mutual interaction of counterpropagating laser beams (collinear degenerate four-wave mixing geometry). Such spatial polarization turbulences cause a frustrated optical instability in input- versus output-intensity characteristics, when a fixed output polarization is selected.

The spontaneous decay of coherent monochromatic sound is dramatically self-enhanced by parametric amplification. We observed this effect for the first time with 3.25-GHz sound in superfluid helium at 85 mK. Starting with a typical intensity of 10 W/${\mathrm{m}}^2$ the sound beam is depleted by more than 99.9% over just 1 mm of propagation distance. In addition we observed, in contrast to Landau-Rumer processes, an exponential decay coefficient proportional to the square root of the initial intensity.

On the basis of a van der Waals theory of wetting, we propose an asymptotic method by which the surface free energy can be studied analytically as a function of large coverage. In this Letter zeroth-order results for the exponential model are given, but higher-order terms will not alter the qualitative conclusions. We establish new results on the order of the wetting transition, and find a nonuniversal critical exponent, $\nu$, for the parallel correlation length.

We discuss the defects and the macroscopic properties which can result when a biaxial nematic is chiralized. A "biaxial" cholesteric phase is possible, where two axes of the triad of unit vectors spiral around the third one. In contrast to "ordinary" cholesteric liquid crystals, $\lambda$ defects have a singular core. Probably this type of cholesteric liquid crystal has been observed very recently.

We report the first observation of the magnetic-field-dependent electronic specific heat in GaAs-GaAs multilayers. With a heat-pulse technique oscillations of the sample temperature on the order of millikelvins were observed. Both intra- and inter-Landau-level contributions could be distinguished. Theoretical fits to the data reveal a density of states consisting of Gaussian peaks on a flat background.

We study wetting phenomena in which the wetting layer is (nearly) critical and intrudes between two noncritical phases. Finite-size scaling theory predicts an interaction, identical in range to that due to the van der Waals forces, between the interfaces bounding the wetting layer. This finite-size interaction leads to new wetting phenomena near critical end points, e.g., in ternary mixtures. The interaction amplitude and its possible universality can be observed directly in experiment.

We give a mean-field, continuum treatment of ballistic aggregation on a seed and a line. The treatment is deterministic, except for one statistical assumption, the so-called tangent rule which determines the mean direction of growth. Our treatment represents progress toward the explanation of the columnar microstructure.

Within the framework of the $\epsilon$ expansion, we establish a symmetry criterion allowing us to predict if a phase transition is driven first order by the fluctuations. It is expressed in the form of group-theoretical conditions imposed on the Hamiltonian symmetry. In particular, we specify a sufficient condition for the lack of a stable-fixed-point Hamiltonian. The effectiveness of this condition is illustrated by its application to Hamiltonians based on four-component order parameters.

We report the first direct observation of phosphorus and arsenic donor states in hydrogenated amorphous silicon and germanium by electron-spin resonance. The states are identified via their characteristic hyperfine structure, and are attributed to neutral donors. A comparison with crystalline silicon and germanium shows that the disorder potential of the amorphous phase leads to a stronger localization and a wide distribution of donor states.

Resistivity measurements on epitaxial, single-crystal films of Co${\mathrm{Si}}_2$ grown on Si show little dependence on film thickness down to ∼60 Å. This dimension is much less than the bulk transport scattering length of ∼ 1000 Å as determined from magnetoresistance measurements at liquid-He temperatures. Boundary scattering of the carriers is therefore essentially specular in this system.

The optical modulation technique is used for direct determination of energy levels and the effective correlation energy $U_{\mathrm{eff}}$ of dangling-bond defects. With an accuracy of 0.1 eV we found for the dangling-bond defect in $a$-Si: H, $U_{\mathrm{eff}}=0.5\mathrm{}$ eV; in $a$-${\mathrm{As}}_2$${\mathrm{S}}_3$, $U_{\mathrm{eff}}=-1.0\mathrm{}$ eV; in ${\mathrm{As}}_2$${\mathrm{Se}}_3$, $U_{\mathrm{eff}}=-0.7\mathrm{}$ eV; and in trans-${\left(\mathrm{CH}\right)}_x$, $U_{\mathrm{eff}}=0.95\mathrm{}$ eV.

Metal-oxide-silicon field-effect transistors (MOSFET's) were fabricated with ${\mathrm{Na}}^{+}$ ions in the gate oxide. Large peaks in the tunneling conductivity through the oxide were observed which are interpreted as resonant tunneling of electrons through localized ${\mathrm{Na}}^{+}$ impurities. It was demonstrated that the tunneling current was spatially localized. The temperature dependence of several peaks was measured and found to be consistent with a simple model for resonant tunneling through localized states. The position of the state in the direction of tunneling can be determined from the temperature dependence.

Using the total-energy general-potential linearized-augmented-plane-wave method, we have tested the adequacy of the local-spin-density approximation for describing the magnetic and structural properties of iron. We find that there are fundamental deficiencies in the local-spin-density approximation, both quantitative and qualitative, in that there are substantial errors in the calculated ground-state volumes and magnetic moments, as well as a prediction of the wrong ground-state crystal structure.

Angle-resolved photoelectron spectroscopy has been used to study the changes in the bonding of CO when coadsorbed with alkali-metal atoms. On metal surfaces [Cu(100) and Ru(0001)] various features in the photoemission spectra, including reduced site symmetry and a large shift of the $1\pi$ molecular orbital of CO, indicate a $\pi$-bonded configuration of the CO molecule. $\pi$ bonding involves bonding through both the $1\pi$ and $2{\pi }^{*}$ molecular orbitals of CO. All of the data indicate a short-range interaction between the K and CO.

The quantum path-integral molecular-dynamics method was applied to explore the structure, energetics, and dynamics of an excess electron interacting with an alkali-halide cluster. Four distinct modes of electron localization were established, which depend on the cluster composition, size, and structure; they involve an internal $F$-center defect, an external surface state, dissociative detachment of an alkali atom, and structural isomerization induced by electron attachment.