Radiative decay rates connecting the lowest four energy levels of ${}_{}{}^{198}{}_{}{}^{}\mathrm{Hg}_{}^{+}{}_{}{}^{}$ have been derived solely from an analysis of the fluctuations (quantum jumps) of the laser-induced fluorescence of the 194-nm first resonance transition of a single ion confined in a Paul trap. The natural linewidth of the 194-nm first resonance transition was also measured. The measured decay rates and branching ratios are in satisfactory agreement with theory.

Photoionization in atomic beryllium has been studied in detail near the $1s$ threshold. Multiple excitation to the $1s\left(2s2p{}_{}{}^{1,3}{}_{}{}^{}P\right)$ states, i.e., conjugate shakeup, is found to be unusually strong, 40% of single excitation. The ${}_{}{}^3{}_{}{}^{}P$ component rapidly loses strength with increasing photon energy, while the ${}_{}{}^1{}_{}{}^{}P$ component persists. The $1s2s2p$ states are highly aligned, as shown by the Auger-electron angular distribution. The $A_2$ value of -0.65 implies a $d$-wave contribution of 0.39. Our results indicate that resonances reported for the solid-state photoabsorption are of atomic origin.

Periodic spatial forcing can strongly influence selection of stationary patterns near a symmetry-breaking bifurcation point. The following effects are demonstrated by use of simple model equations of long-scale thermal convection: (a) transition between alternative patterns; (b) emergence of spatially quasiperiodic patterns; and (c) evolution of patterns due to rotating phases.

We have observed highly nonlinear localized wave packets propagating as flexural modes of thin shells. They maintain their structure in a collision and have envelopes in quantitative agreement with the soliton solutions to the nonlinear Schrödinger equation.

A silicon surface-barrier detector was used to measure energy spectra of hot electrons coming out of an end cell of the GAMMA 10 tandem mirror. A hump was observed in an energy spectrum and it was attributed to enhanced velocity diffusion along heating characteristics of the third-harmonic resonance of the incident waves. Hot electrons were produced by second-harmonic electron-cyclotron-resonance heating and scattered into the loss cone. Good agreement was obtained among the observation, Fokker-Planck simulation, and the prediction from the relativistic resonance condition.

We have experimentally observed the growth and saturation of a single wave on a warm-beam, slow-wave-structure system. When the wave growth is sufficiently weak, the time-averaged beam distribution function develops a localized plateau. When the growth is stronger, the entire distribution function can become distorted. The electric field at saturation scales as the square of the linear growth rate of the wave.

The embedded-atom method has been combined with Monte Carlo simulations to study the structure and order-disorder transformation of Au and Pt (110) surfaces. We find that the missing-row structure of Au disorders at 570 K, in close agreement with the experimental value of 650 K. For Pt, we predict a transformation at 750 K. Relaxations and vibrations are found to be essential for the accurate calculation of the critical temperature.

Molecular-dynamics simulations of insertion of self-interstitials into a single fcc crystal reveal a crystal-to-amorphous transition which is dependent upon both insertion rate and defect-concentration accumulation. The resulting amorphous structure is found to be similar to that of a rapidly quenched liquid. In terms of elastic properties, amorphization brings about a further decrease in $C_{11}$ and $C_{44}$ relative to a defective crystal.

The effective-medium theory is used to study the chemisorption and incorporation of hydrogen on the Cu(110) surface. The hydrogen-induced relaxations of the Cu lattice and their role in stabilizing hydrogen just under the surface are studied in particular. We find that hydrogen induces local lattice distortions even when chemisorbed. The relaxations are largest when hydrogen is just under the surface, but not large enough to make this site more stable than the chemisorption site.

The initial dissociative chemisorption probability of ${\mathrm{N}}_2$ on Fe(111) is found to increase by a factor of ≃${10}^5$ with increasing kinetic energy, rising from ≃1×${10}^{-6\mathrm{}}$ at ≃0.09 eV to over 1×${10}^{-1\mathrm{}}$ at 4.3 eV. This probability also increases with decreasing surface temperature, suggesting that reagent energy provides access to an intermediate state, such as $\alpha$-${\mathrm{N}}_2$, whose population also increases with kinetic energy, consistent with the accepted mechanism at low energies.

We report the results of a high-resolution x-ray scattering study of the structure of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ as a function of temperature. We find that there is an anomaly in the orthorhombic splitting, $b-a$, near the superconducting transition, whereas there is little or no anomaly in the unit-cell volume. Our results suggest that the superconducting wave function has a uniaxial anisotropy within the $a-b$ plane.

The method of concentration waves is applied to structural transformations in superconducting Y-Ba-Cu oxide. The method predicts an ordering reaction that results in a tetragonal-to-orthorhombic phase transformation, and also yields the correct structure of the Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ orthorhombic ordered phase. The results suggest that at low temperature the off-stoichiometric orthorhombic phase is thermodynamically unstable with respect to secondary decomposition into a mixture of ordered orthorhombic and disordered tetragonal phases, whose microstructure can be anticipated.

We propose a simple mechanism, anchored in weak-coupling BCS theory, which ties together the following facts: high $T_c$; quasi two dimensionality; orthorhombic distortion and/or disordered lines of oxygen; proximity to a metal-insulator transition; and anomalously small isotope effects.

We have studied the excitation spectrum for a one-half filled Landau level in a two-layer system of interacting electrons, by finite-size calculations in a periodic rectangular geometry. The interlayer interaction is found to be important for obtaining a unique ground state and the required gap structure in the spectrum. On the basis of these observations, we predict that the fractional quantum Hall effect might be found at this filling in such a layered system.

Spin-resolved photoemission from polycrystalline Cr${\mathrm{O}}_2$ films shows a spin polarization ($P$) of nearly +100% for binding energies near 2 eV below the Fermi level ($E_{\mathrm{F}}$). Despite the metallic resistivity behavior, extremely low intensity is observed in the photoemission spectra near $E_{\mathrm{F}}$. Our findings are in contrast to recent band-structure calculations, predicting Cr${\mathrm{O}}_2$ to be a half-metallic ferromagnet with $P=+100\%$ at $E_{\mathrm{F}}$.

Vibrationally excited N${\mathrm{H}}_3$ molecules are observed in the scattering of N${\mathrm{H}}_3$ ($v=0\mathrm{}$) from Au(111). We observe up to three quanta of vibrational excitation in the ${\nu }_2$ "umbrella" vibrational mode of N${\mathrm{H}}_3$. The vibrational excitation probability scales linearly with the kinetic energy of the incident beam above a threshold energy corresponding to the quanta excited, and is independent of the surface temperature, $T_s$, over the range $300 \mathrm{K}\le T_s\le 800 \mathrm{K}$. These results are discussed in terms of an electronically adiabatic collisional mechanism for vibrational excitation.

Measurements of the time dependence and magnitude of fracture-induced changes in electrical conductivity in single-crystal Si are presented. A transient increase in conduction is observed during crack growth due to the production of free charge carriers. The presence and characteristics of this transient depend on the nature of the fracture.

It is shown that transition at the Curie point and uniaxial crystalline anisotropy exhibit catastrophes which have the same morphology as well-known elementary catastrophes like the cusp and the swallowtail. It is possible to understand micromagnetics as conflict between attractors of dynamical systems. Catastrophe theory allows great geometrical insight into the mathematical background of micromagnetics. It appears as a geometrical study of metastability that can have general significance.

We have made the first capacitance measurements of the density of states of quasi one-dimensional electronic systems. We observe oscillations reflecting the discrete energy levels of doubly confined electrons squeezed beneath submicron lines (0.2, 0.3, and 0.4 μm wide) patterned on the surface of $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterojunction capacitors. The spacings of the oscillations and their dependence on potential well width can be explained theoretically by quantum size effects and the quasi one-dimensional nature of our samples.

A new type of tunneling phenomenon in forward-biased single-barrier $n^{+}\left(\mathrm{InGa}\right)\mathrm{As}-n^{-}\left(\mathrm{InGa}\right)\mathrm{As}-\mathrm{InP}-n^{+}\left(\mathrm{InGa}\right)\mathrm{As}$ heterostructures has been observed when a quantizing magnetic field is applied in the plane of the InP barrier. The occurrence of two distinct series of resonances in the voltage or field dependence of the current is interpreted in terms of electron tunneling from a 2D electron gas in the $n^{-}$ layer into interfacial Landau states in the $n^{+}\left(\mathrm{InGa}\right)\mathrm{As}$. These states correspond to classical skipping orbits of an electron along the tunnel-barrier interface.