This Letter reports the first spectroscopic detection of HeH, the simplest excimer molecule that can exist. He${\mathrm{H}}^{*}$ is produced in electronically excited states in reactions between He and ${\mathrm{H}}_2$ excited selectively by synchrotron radiation into the $C{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}(v^{\prime }>~1)$ or $B{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u}}^{+}{}_{}{}^{}(v^{\prime }>~11)$ state. He${\mathrm{H}}^{*}$ emits a broad characteristic fluorescence continuum between 200 and 400 nm which is attributed to $B{}_{}{}^2{}_{}{}^{}\Pi \to X{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$.

The numerical solution of the Maxwell-Bloch equations describing the propagation of a plane monochromatic wave through a saturable absorber shows the appearance of a new kind of reflected wave generated inside the medium. This new phenomenon is described and its physical origin, related to the saturation-induced refractive index changes, is discussed.

Coherent lattice vibrations in lithium tantalate have been observed directly in the time domain by use of femtosecond optical pulses and the electro-optic effect to coherently excite and detect phonon polaritons. The damping time of the lowest transverse optic phonon has been determined directly from the decay rate of these oscillations. This measurement establishes an intrinsic speed limit for the electro-optic response of this material.

We report new measurements of thermal transport in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films near the superfluid transition. We have done this to check on the universality of this transition as a function of thickness or $T_c$ between 1.37 and 1.88 K. Results from this study show for the first time a thickness or $T_c$ dependence of the two characteristic parameters for the thermal conduction, the ratio of diffusion constant to vortex core radius, $\frac{D}{a^2}$, and the nonuniversal constant $b$. We suggest that the increase of $b$ with film thickness is related to the bulk correlation length.

We report the first reliable determination of the liquid-vapor coexistence curves of the metals cesium and rubidium. Both curves are noticeably different from those of nonmetallic fluids in their extreme vapor-liquid asymmetry which causes the law of rectilinear diameter to break down. The functional form of the diameter is consistent with the suggestion by Goldstein and Ashcroft that the strong state dependence of the effective interparticle forces leads to large amplitudes of the 1 -$\alpha$ anomaly.

It is suggested that the large amplitudes of the singularities in the coexistence-curve diameters of cesium and rubidium, as measured by Jüngst, Knuth, and Hensel arise from the correspondingly strong thermodynamic-state dependence of the screened ion-ion interactions in these systems, especially as the metal-insulator transition is traversed. This state dependence, which is normally negligible for typical insulating fluids, corresponds to the mixing of thermodynamic fields present in certain lattice models and scaling theories.

The reconstruction of the Pt(110) plane from an ordered (1 × 1) surface to an ordered (1 × 2) surface has been directly observed for the first time. The (1 × 1) surface, produced by field evaporation at 78 K, reconstructed at temperatures above 310 K. Field-ion images show clearly that the reconstructed surface consists of alternate missing rows of atoms. The transition was observed for clusters as small as five atoms, implying that the reconstructed surface is stabilized by shortranged, atomic interactions.

Extended x-ray-absorption fine-structure measurements have been made at the Mn $K$ edge of quasicrystalline and crystalline forms of an ${\mathrm{Al}}_6$Mn alloy. Two different quasicrystalline Mn sites are discerned to be populated in the ratio of $\tau$, the golden mean, within experimental error. The more populous site is similar to that in the crystal but with bond-angle distortions and elimination of an unusually short Al-Mn bond, while the other site has additional bond-stretching distortions. The measurements together with density measurements indicate that the volume per Mn site is independent of the type of site.

We study diffusion in the presence of a hierarchical set of barriers. We find a phase transition in the dynamics from ordinary to anomalous diffusion as a parameter controlling the relative barrier heights is varied. Similar behavior is found in suitably defined random systems. Possible connections to glassy dynamics are discussed.

An ion reaction-time amplification scheme is used to study field dissociation of Rh${\mathrm{He}}^{2+}$ formed by low-temperature field evaporation of Rh in He. Field dissociation of Rh${\mathrm{He}}^{2+}$ can occur by atomic tunneling within a few times ${10}^{-13\mathrm{}}$ s in a well-defined spatial zone of width ∼ 150 Å above the emitter surface. The time resolution of this measurement is as good as 20 fs.

Copper-carbon films formed by photodeposition from a vapor-phase copper-organic compound by use of a cw ultraviolet laser beam exhibit periodic ripple structures associated with surface waves. Microscopic analysis of these films shows that composition and order also vary periodically and change abruptly on dimensional scales small compared to optical wavelengths. A simple model including strongly varying fields appropriate to the film corrugation is proposed to explain these unusual features.

The importance of three-dimensional effects in the electronic structure of the quasi two-dimensional layered crystal $1T$-Ti${\mathrm{Se}}_2$ is demonstrated by means of high-resolution angle-resolved photoemission excited by synchrotron radiation. By resolving the spin-orbit splitting of the upper Se-$4p$ valence band in $\Gamma A$ we found strong evidence for hole states at $\Gamma$. The overlap (<120 meV) between these states and the Ti-$3d$ states measured exactly at $\Gamma$ and $L$, respectively, is much smaller than previously suggested and is discussed in relation to the structural phase transition below $T_c\simeq 200$ K.

A self-consistent parquet approximation is shown to give exact results for the Kondo problem in the scaling limit.

A new method has been devised for investigation by transmission electron microscopy of the electrostatic microfield associated with reverse biased $p-n$ junctions. By means of electron holography and optical interferometry it is possible to obtain on the reconstructed images two-dimensional representations of the projected potential distribution inside and outside the specimen.

We present the first direct evidence of hot electrons traversing ballistically a thin GaAs layer. The energy distribution of the hot electrons associated with the momentum in the direction of the current was measured with the use of a tunneling-hot-electron-transfer amplifier as an electron spectrometer. The width of the ballistic peak was found to be about 60 meV for hot electrons with excess energy of some 300 meV above the thermal electrons. Those values are close to the expected initial injection values.

$\mathrm{EL}2\mathrm{}$ is a technologically important deep level in GaAs whose identification has been the subject of intense study. In this paper we present uniaxial stress and magnetic field experiments which establish for the first time that $\mathrm{EL}2\mathrm{}$ has tetrahedral symmetry and is, therefore, an isolated point defect. Combining this result with earlier data, we conclude that $\mathrm{EL}2\mathrm{}$ is an isolated arsenic antisite defect.

For a noninteracting two-dimensional electron gas in a strong perpendicular magnetic field, the excitation energies are multiples of $\hslash {\omega }_c$. These excitation energies are shifted in an interacting system. Because of the singular nature of the noninteracting-system energy spectrum it has previously been possible to evaluate the excitation energies of the interacting system only when all Landau levels are either completely empty or completely full. In this article we suggest a nonperturbative approach which overcomes this difficulty but recovers existing results in the appropriate limits.

The resistivity $\rho$ varies by 7 decades and the intensity of its fluctuations ${\left[S_{\rho }\mathrm{}\right(f\left)\right]}_{1\mathrm{Hz}}$ varies by 17 decades in composite films of Pt and ${\mathrm{Al}}_2$${\mathrm{O}}_3$ for a wide range of metal volume percent $33\%<~p<~100\%$ and temperature $7<T<\mathrm{}300\mathrm{}$ K. The power spectral density $S_{\rho }\mathrm{}\left(f\right)\mathrm{}\propto \frac{1}{f^{\alpha }}\left(\frac{1}{f \mathrm{noise}}\right)$ with $0.8<\mathrm{}\alpha <1.2\mathrm{}$ at all $T$ and $p$. The normalized noise intensity ${\left[\frac{f{S}_{\rho }\mathrm{}\left(f\right)\mathrm{}}{{\rho }^2}\right]}_{1\mathrm{Hz}}$ rises by ${10}^5$ with decreasing $p$ and saturates below a metal-insulator transition, $p<~50\%$. Results implicate a dual conduction mechanism for composites combining noisy tunneling and quiet metallic paths.

We have shown experimentally that in a two-dimensional gas with coexisting electrons and holes the quantum Hall effect is determined by the degree of uncompensation of the system. A classical analysis of low-field magnetotransport measurements in GaSb-InAs-GaSb heterostructures gives individual carrier concentrations and mobilities for both electrons and holes. In the quantum Hall regime, with the Fermi level between electron and hole magnetic levels, the filling factor is the difference between the electron and hole filling factors. Similarly, the carrier density is the difference between the electron and hole densities, in analogy with the infinite-field limit of the classical Hall effect.

We present the results of the first analytical study of finite-size effects in a random-field model. In particular we consider the random-field spherical model in partially infinite geometries (infinite in $d^{\prime }$ dimensions and finite in $d-d^{\prime }$ dimensions) under periodic and antiperiodic boundary conditions in the finite directions. We find a simultaneous dimensional reduction by 2 in $d$ and $d^{\prime }$ for $4<d<\mathrm{}6\mathrm{}$ and $2<~d^{\prime }<~4$. For $0<~d^{\prime }<2\mathrm{}$, we find new power laws describing the approach to bulk in different geometries.