Electron attachment to ${\mathrm{O}}_2$ clusters gives two homologous series, ${{\left({\mathrm{O}}_2\right)}_n}^{-}$ and ${\left({\mathrm{O}}_2\right)}_n{\mathrm{O}}^{-}$. Measured relative attachment-cross-section functions show significant differences, i.e., ${\left({\mathrm{O}}_2\right)}_n{\mathrm{O}}^{-}$ ions show a similar behavior to ${\mathrm{O}}^{-}$ from ${\mathrm{O}}_2$ (a single peak at ∼7 eV), whereas ${{\left({\mathrm{O}}_2\right)}_n}^{-}$ ions show an additional peak at ∼0 eV. The latter result explains the large thermal-electron attachment rates observed in high-pressure and low-temperature swarm experiments.

From generalized-valence-bond calculations on numerous Li-atom clusters (${\mathrm{Li}}_n$ and ${{\mathrm{Li}}_n}^{+}$, $n<~13$), we conclude that the optimum bonding involves singly occupied orbitals localized interstitially (in tetrahedra). Rules based on the calculations are used to predict low-energy isomers (leading for ${\mathrm{Li}}_{13}^{+}$ to low-symmetry structures that are significantly more stable than the icosahedron but retain local fivefold-symmetry axes) and are applied to infinite metallic systems.

We report the observation of anticrossing signals arising from the interaction of 300-K blackbody radiation with Rydberg atoms near avoided crossings of Stark levels. The role of the blackbody radiation is roughly analogous to that of the resonance fluorescence in traditional anticrossing spectroscopy. We present preliminary results for avoided crossings of $\mathrm{ns}$ states with $n-2\mathrm{}$ Stark states in potassium for $n$ in the range 18-21.

We report on instabilities of a single-mode laser operating under homogeneous broadening conditions. Bichromatic emission as recently predicted from the Lorenz model as well as the correspondingly high second thresholds are found.

We present an experimental study of the space and time evolution of the Eckhaus instability, a general mechanism of pattern selection for spatially periodic patterns in nonlinear systems. Using a convecting liquid crystal layer, we observed long-wavelength modulations leading to the nucleation of new roll pairs. The development of this process is studied by time-resolved spatial Fourier analysis and compared with predictions based on an amplitude equation.

Strong spatial asymmetries in the spectrum and magnitude of low-frequency ($\omega \ll {\omega }_{\mathrm{ci}}$) density fluctuations have recently been measured in the Texas experimental tokamak. Large-amplitude, narrow-band (as low as $\frac{\Delta \omega }{\omega }\simeq 0.1$) modes have been observed on the inside of the high-field torus in addition to the typical broad-band ($\frac{\Delta \omega }{\omega }\simeq 1$) microturbulence which is present throughout the plasma cross section. These narrow-band fluctuations are only detected at the plasma interior, being localized towards the midplane, and are not seen at the edge.

The role of runaway electrons in current ramping by lower-hybrid waves is discussed. The back emf induced by lower-hybrid-current ramping produces runaway electrons at a rate ${\dot{n}}_R$ such that the additional inductance (proportional to ${\dot{n}}_R\mathrm{ec}$) can be significant in opposing the current ramping. However, runaway electrons can also destabilize the oblique plasma waves through their anisotropy. The resulting turbulence can greatly reduce the runaway production rate, enabling the current-ramping rate to exceed the above limit.

Tangentially coinjected deuterium beam ions were accelerated from 82 up to 150 keV during a major-radius compression experiment in the tokamak fusion test reactor. The ion energy spectra and the variation in fusion yield were in good agreement with Fokker-Planck code simulations. In addition, the plasma rotation velocity was observed to rise during compression.

Using nozzle beams of $n$-${\mathrm{H}}_2$, $p$-${\mathrm{H}}_2$, $n$-${\mathrm{D}}_2$, and $o$-${\mathrm{D}}_2$ gases, with incident energies and angles in the ranges 20-75 meV and 30°-70°, we have measured the initial sticking probability, $S_0$, on a cold (∼ 15 K) Cu(100) surface. $S_0$ comprises a smooth background and a series of peaks with maxima occurring when the incident energy equals a rotational excitation energy. We argue that background and structure arise primarily from parallel-momentum-nonconserving phonon processes during the initial collision.

High-resolution electron-energy-loss studies of ${\beta }_1$-H and ${\beta }_1$-D on W(100) are reported for experimental conditions under which the first-order diffracted beam emerges from the crystal. A new vibrational mode is observed and assigned to the asymmetric stretch mode of ${\beta }_1$-H. The results yield an independent new determination of the H-W bond distance. The impact-energy dependence of the new asymmetric stretch mode does not agree with theoretical predictions related to cross sections and selection rules that govern them.

Spin-polarized Auger spectroscopy is applied to a study of initial oxidation of Fe(100). The O $\mathrm{KLL}$ lines are found to exhibit positive (i.e., parallel to the Fe magnetization) spin polarization for the singlet and near zero or negative spin polarization for the triplet final states. The finding is relevant for an experimental determination of orbital mixing coefficients. True secondary electrons reveal strongly spin-polarized O-induced structures at 3 and 12 eV above the vacuum level.

Three-photon spectroscopy is introduced as a useful tool for the investigation of electronic transitions in solids. This technique is used to study resonances on the upper polariton branch, and even on the longitudinal exciton. This is the first such application for a system with inversion symmetry. The polariton structure of the lowest exciton in KI is analyzed with use of different wave-vector configurations. The fit of the experimental data yields a longitudinal-transverse splitting of 110 meV. For the more complicated exciton structure of CsI, a correspondingly complicated polariton structure is expected. For CsI, results are presented only for one wave-vector configuration.

By mapping the Edwards-Anderson model onto a nonrandom Ising model we argue that the lower critical dimension $d^{*}$ for the Edwards-Anderson spin-glass is close to 3 and may depend on the probability distribution of the bond strengths. For $d>\mathrm{}{d}^{*}$, the approach to spin-glass order from the paramagnetic phase is characterized by Ising exponents. At $d=d^{*}$, however, the critical exponents defined for $T>\mathrm{}{T}_c$ are twice the corresponding Ising ones. These results are consistent with recent computer simulations.

We report on picosecond luminescence studies of GaAs/AlGaAs quantum wells in the regime of the quantum-confined Stark effect. A drastic increase of the recombination lifetime is accompanied by a Stark shift of the photoluminescence of the lowest free exciton for electric fields perpendicular to the quantum-well layers. A consistent picture of the quantum-confined Stark effect is presented.

A model for the paramagnetic $E_2^{\prime }$ center in alpha quartz is tested by means of semiempirical molecular-orbital calculations. The $E_2^{\prime }$ center is proposed to be a variant of the $E_4^{\prime }$ center (a hydrogen atom substituted for an oxygen atom) with one silicon neighbor relaxed outward, into a "bonded interstitial" position. This silicon is responsible for most of the unpaired spin. The calculations indicate that $E_2^{\prime }$ is slightly more stable than $E_4^{\prime }$, and that there is a large barrier between the two configurations (0.6 eV). Further implications of this model are discussed.

The adsorption of CO on Ni(001) and its coadsorption with K or S as studied with our all-electron full-potential linearized-augmented-plane-wave method show that (i) K modifies the electrostatic potential in the surface layer, produces an increased occupation of the CO antibonding $2{\pi }^{*}$ states, and facilitates the dissociation of CO; (ii) in contrast, S interacts more covalently with the Ni surface and neighboring CO molecules thereby deactivating the surface and inhibiting CO dissociation.

Stable photinos, the photino being the supersymmetry partner of the photon, can explain both the "missing mass" in galactic halos and the cosmic-ray antiproton spectrum up to the highest energies observed so far. This requires a photino mass around 15 GeV; significantly higher masses are cosmologically disfavored. As a consequence, the observed cosmic-ray antiproton-to-proton ratio is predicted to decrease abruptly just above the measured energy range, at $E=m_{\chi }$. If observed, this striking effect would strongly support the hypothesis that photinos make up the missing matter in our galaxy and also lead to a measurement of the photino mass from cosmic-ray data.

A new Monte Carlo method for quantum spins is described. The spin Hamiltonian is mapped onto a model of bosons, which may then be studied by a path-integral formalism. The power of this mapping comes from the fact that, by letting the system evolve through unphysical spin states between time slices, we obtain simple expressions for the needed matrix elements. The method is illustrated by measuring the energy and correlation functions for a spin-½ ring as well as the specific heat for the two-dimensional $\mathrm{XY}$ model.

A general expression for the response function is derived by the method of recurrence relations. Memory effects appear as corrections to the dynamic random-phase-approximation form. The dynamic structure for the three-dimensional electron gas is calculated to third order and compared with Al data at a large wave vector. Also shown is the dynamic local-field term.

The existence of gravitational analog of Dirac's magnetic monopole is speculated on.