We report the observation of long-lived (τ>${10}^{\mathrm{-}3}$ sec) doubly charged negative ions of ${\mathrm{C}}_{60}$ in the gas phase. ${\mathrm{C}}_{60}^{\mathrm{-}}$ and ${\mathrm{C}}_{60}^{2\mathrm{-}}$ ions are produced by laser desorption from a surface covered with ${\mathrm{C}}_{60}$ molecules and are believed to be produced by negative surface ionization from the laser-heated surface. Pseudopotential calculations indicate that ${\mathrm{C}}_{60}^{2\mathrm{-}}$ is slightly bound (electron affinity of ${\mathrm{C}}_{60}^{\mathrm{-}}$≃0.1–0.4 eV) relative to ${\mathrm{C}}_{60}^{\mathrm{-}}$.

Delayed annihilation of antiprotons stopped in liquid helium has been observed, revealing that about 3.6% of stopped antiprotons are trapped in long-lived metastable states. No delayed component was found either in liquid nitrogen or in liquid argon. The observed time distribution of delayed annihilation shows fast-decaying components followed by a major part with a decay time constant of 3 μsec.

The stability of ${\mathrm{Pb}}_{\mathit{n}}^{2+}$ clusters is studied by solving exactly for the valence 6p electrons a realistic many-body Hamiltonian with intratomic and interatomic Coulomb interactions. We obtain that ${\mathrm{Pb}}_3^{2+}$ has a metastable ground state and isosceles shape (bond angle θ≃${124}^9$). The energy barrier against dissociation ${\mathit{E}}_{\mathit{B}}$=0.13 eV yields a very long lifetime. This explains the recent experimental observation of ${\mathrm{Pb}}_3^{2+}$. Furthermore, comparison with self-consistent Hartree-Fock calculations shows that the metastability of ${\mathrm{Pb}}_3^{2+}$ results from electron-correlation effects.

Integral cross sections for ${\mathrm{ArH}}^{+}$ production in the ${\mathrm{Ar}}^{+}$+${\mathrm{H}}_2$ reaction were measured by a novel high-resolution apparatus, at low collision energies (0.025–1 eV). The observed features are interpreted as due to successive openings of vibronic levels of the intermediate Ar-${\mathrm{H}}_2^{+}$ charge-transfer complex. The proposed potential-energy surfaces involve an accurate description of the entrance channels at long range including fine-structure and Σ-Π splitting effects.

The phase dynamics of wavy vortex flow is studied by applying a forced modulation to the upper boundary of a large-aspect-ratio concentric cylinders system. The experimental results are consistent with a model based on coupled phase equations. We find that the perturbations propagate as traveling waves parallel with the axis of the system when there is a strong coupling between the axial and azimuthal phase variables, and diffusively for weak coupling. The propagating wave speed is measured for a range of Reynolds numbers.

The linear term in the density expansion of the dynamic structure factor has been determined experimentally for the first time. We used neutron-scattering data at very low momentum transfer (0.3<k<1.2 ${\mathrm{nm}}^{\mathrm{-}1}$) on room-temperature ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ at low density (0.06–0.25 times the critical). Only for k<0.95 ${\mathrm{nm}}^{\mathrm{-}1}$, the experimental data agree qualitatively with a corresponding hard-spheres system. We conclude that the linear term is a very sensitive probe for studying the two-particle interaction potential in dilute gases.

Rapid temperature quenches have long been known to produce metastable thermodynamic phases. We study Landau-Ginzburg models of phase transitions for free energies having three local minima and show that metastable phases can be formed via a dynamic instability that splits the front separating the stable high- and low-temperature phases. Even though quenching may nucleate the stable phase, the splitting instability we discuss will favor the formation of the metastable phase.

We have observed the distortion of the gap and associated excitation spectrum in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ with increasing magnetic field up to the first-order transition to the A phase at a temperature of ${\mathit{T}}_{\mathit{c}}$/T of about 7 (≤140 μK at P=0). The pair-breaking critical velocity of a vibrating-wire resonator is monitored as a function of field. By plotting the critical velocity we can follow changes in the B-phase excitation spectrum with increasing magnetic field, which are in excellent agreement with the calculations of Ashida and Nagai. The sudden transition to the A phase at the critical field is clearly seen.

The zero-temperature lower critical field ${\mathit{B}}_{\mathit{c}1}$(0) between the two nuclear antiferromagntic phases of bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has been determined as a function of molar volume from pressure measurements down to 120 μK. In a volume range between 22.45 and 24.06 ${\mathrm{cm}}^3$/mol, the Grüneisen parameter for ${\mathit{B}}_{\mathit{c}1}$(0) is anomalously small compared to typical values for other physical quantities associated with the exchange interactions. The result suggests that the two-spin exchange frequency decreases more rapidly with decreasing volume than three- and four-spin exchanges in the terms of the multiple-spin exchange model.

We demonstrate that the early stage of spinodal decomposition and continuous ordering is supersymmetric in systems with long-range interactions. Indeed, the Cahn-Hilliard-Cook linear theory can be described equivalently as an evolution of the order parameter (bosons) or objects that obey an exclusion principle (fermions). Coupled with a cluster representation of the Cahn-Hilliard-Cook theory, supersymmetry suggests a new physical picture of the early stage of spinodal decomposition and continuous ordering that provides a greater understanding of early-stage morphology, and could provide a basis for an improved description of the nonlinear regime.

The diffusion length of positrons in Ga and Bi was measured using a positron beam. For Ga, there is a drop from 1200(100) to 60(5) Å upon melting; this is interpreted in terms of creation of centers that trap the positron. As the temperature of the liquid is increased, the diffusion length increases, suggesting that the size of the traps changes. For Bi, the diffusion length decreases from 500(30) to 190(10) Å upon melting, and increases slightly as the temperature is increased. This drop is interpreted as increased positron-ion scattering, and not positron trapping as in Ga.

The structure of a-C prepared by plasma-arc deposition has been determined by neutron scattering and analyzed to estimate the fraction of tetrahedrally and trigonally bonded carbons. The structure factor and reduced radial distribution functions are similar to those for a-Si and a-Ge, indicating a high proportion of tetrahedral bonding. Also, the mean value of the nearest-neighbor C-C distance is close to that of crystalline diamond and a fit to the data gives about 86% tetrahedral bonding. This form of diamondlike carbon therefore appears to be the structural equivalent of a-Si and a-Ge.

Vicinal Si(001) surfaces are believed to undergo a phase transition between single and double atomic height steps as either temperature or angle of miscut is varied. Here we calculate the full temperature-angle phase diagram, which is found to be quite different than previously believed. In particular, there is a critical point above which there is no phase transition at all. The results appear to explain the rather continuous behavior seen in a variety of experiments.

The dynamical behavior of the reconstructed Au(111) surface is studied by a molecular-dynamics simulation based on a well-tested many-body force model for gold. By calculating the surface spectral densities through time-dependent correlations of molecular-dynamics trajectories, we have constructed the surface-phonon dispersion curve of the reconstructed Au(111) surface. The results are in excellent agreement with the measured surface vibrational spectra, and yield important information on the effect of surface reconstruction on the observed anomalous surface phonons.

The multilayer relaxation of the Pt(210) stepped and kinked surface is analyzed by low-energy-electron diffraction. This is the first application of the new real-space multiple-scattering theory of LEED, designed specifically for such open surfaces where conventional theories fail. Combined with an automated tensor LEED method, it efficiently detects nonalternating atomic relaxations which are oriented primarily perpendicular to the surface. These relaxations are in qualitative agreement with new embedded-atom-method results.

Domains and discommensurations have been directly observed by transmission electron microscopy in some dark-field images using certain reflections in a Mn-Si-Al octagonal quasicrystal. The invisibility and visibility of domains and discommensurations separate the diffraction spots into basic and satellite reflections. The domains and discommensurations observed in satellite reflections are interpreted as due to the ordering of Si and Al atoms in the quasilattice of the quasicrystal. A simple model which gives satisfactory agreement with the experimental results is given.

The energies and wave functions of both the ground and excited states are calculated for a proton in KDP (${\mathrm{KH}}_2$${\mathrm{PO}}_4$) and a deuteron in DKDP (${\mathrm{KD}}_2$${\mathrm{PO}}_4$), respectively. On the basis of these calculations, the isotope effects on the saturated polarization and the transition temperature of the ferroelectric phase are examined. It is shown from these results that remarkable isotope effects are caused by the isotope dependence of changes in the ground-state energy induced by a distortion of ${\mathrm{PO}}_4$ tetrahedrons.

We report on the first experimental observation of far-infrared emission within the reststrahlen band of a semiconductor. We have performed Fourier-transform emission experiments on narrow-band-gap ${\mathrm{Hg}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Te (x≊0.17). Under ${\mathrm{Ar}}^{+}$-laser excitation, we find an emission spectrum at the longitudinal-optical-phonon energies of the material (137.5 and 157.5 ${\mathrm{cm}}^{\mathrm{-}1}$). The appearance of the optical emission signal is caused by interband polaron effects arising from the resonant matching between longitudinal-optical phonons and electron-hole excitations, as suggested in recent theoretical calculations.

Penning deexcitation spectra of alkali atoms adsorbed on metals show an intense peak at the Fermi level for coverages around the work-function minimum. This strong electron emission is assigned to the decay of a core-excited negative ${\mathrm{He}}^{\mathrm{-}}$ ion created by resonance transition of a substrate electron into the affinity level of the metastable rare-gas atom. The formation of this species also easily provides an explanation for the observed ${\mathrm{He}}^{\mathrm{*}}$ singlet-triplet conversion.

Xe absorbed on Pd(111) in the dilute phase, in the (√3 × √3 )R30°, and in the (√7 ×7R19.2° structure has been studied by spin-resolved photoemission with normally incident circularly polarized light and for normal electron emission. Similar to the free atomic case, no splitting was observed for the dilute phase in the peak corresponding to the ${\mathit{p}}_{3/2}$ hole state. Subthreshold electronic resonances were found for the two ordered layers, but could not be detected for the dilute phase. Consequences regarding the splitting mechanism and the excitation and emission processes for the resonances are discussed.