The phason energy gap has been observed to increase on going from the incommensurate to higher-order commensurate phases in the "devil's staircase" compound ${\left[\mathrm{N}{\left(\mathrm{C}{\mathrm{H}}_3\right)}_4\right]}_2$Zn${\mathrm{Cl}}_4$. The gap was determined via the phason-induced ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ spin-lattice relaxation contribution, which was obtained from the variation of the effective spin-lattice relaxation rate over the inhomogeneous incommensurate frequency distribution. Here, as well as in ${\mathrm{Rb}}_2$Zn${\mathrm{Cl}}_4$ and ${\mathrm{Rb}}_2$Zn${\mathrm{Br}}_4$, the phason gap in the incommensurate phase is defect induced.

We introduce an analog, in classical phase space, of the fundamental building block of time dependence in quantum mechanics, i.e., the superposition state. Applications to two-level systems and quantum beats illustrate this conceptually pleasing phase-space picture of the phase interference between energy eigenstates which comprises pure-state quantum dynamics.

The nucleation and growth stage of oxide formation on Rh field emitters has been investigated by imaging atom-probe mass spectroscopy. Quantitative atom-probe analysis of samples heated in 1-Torr ${\mathrm{O}}_2$ indicated that stoichiometric ${\mathrm{Rh}}_2$${\mathrm{O}}_3$ is formed at temperatures as low as 500 K and suboxides are formed at lower temperatures. The activation energy for the process was determined from the temperature dependence of the surface oxygen accumulation rate to be 4-5 kcal/mole.

This work is concerned with the Rayleigh-Schrödinger perturbation expansions associated with Hamiltonians of the form $\hat{H}\left(\lambda \right)=\frac{1}{2}{\hat{p}}^2-\frac{Z}{r}+\lambda r^m$, $m=1,2,3,\dots$. These Hamiltonians are important in atomic and molecular physics as well as in confinement studies of quantum chromodynamics. A method of calculating the Rayleigh-Schrödinger coefficients $E^{\mathrm{}\left(n\right),m}$, based on an so(4,2) Lie algebraic reformulation of the Schrödinger equation, is outlined. Nonrigorous Bender-Wu WKB methods are employed to determine the high-order behavior of the $E^{\mathrm{}\left(n\right),m}$.

We numerically study the excitation mechanism of the hydrogen atom in a microwave field and show that quantum mechanics imposes limitations to the classical chaotic diffusion. In particular a multiphoton resonance pattern has been found. We suggest that a direct laboratory experimental verification of these phenomena should be possible.

Large-$N$ three-dimensional ${\left({\overrightarrow{\phi }}^2\right)}^3$ field theory is studied by means of a composite-field effective potential $V_{\mathrm{eff}}(\overrightarrow{\phi },\chi )$. $V_{\mathrm{eff}}(\overrightarrow{\phi },\chi )$ is shown to be renormalizable in each order of the $\frac{1}{N}$ expansion. When we take the first two orders into account it is found that $V_{\mathrm{eff}}(\overrightarrow{\phi },\chi )$ is unbounded below. As a result the theory lacks a strict continuum limit for nonzero ${\phi }^6$ coupling and large but finite $N$. Our results may be of relevance to tricritical phenomena.

Deep-inelastic lepton-nucleus scattering has shown that the momentum distribution of quarks in a nucleon is modified by the nuclear environment. Several different explanations have been proposed. These can be distinguished by proton-nucleus scattering experiments which measure the production of lepton pairs in certain kinematic regions.

Momentum spectra of protons from the ($\gamma$, $p$) reaction have been measured at ${\theta }_p=30\mathrm{}°$ for ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ with a tagged photon beam with energies from 187 to 427 MeV. By analysis of these inclusive proton spectra, cross sections for the pion photoproduction from the quasifree nucleon in the target nucleus and those for the photodisintegration of the quasifree two-nucleon system have been obtained as a function of the photon energy and of the target mass number.

We have measured the energy and angle of coincident $\mathrm{np}$ pairs from the reaction ${}_{}{}^6{}_{}{}^{}\mathrm{Li}(\gamma ,np)\alpha$ with 67-MeV bremsstrahlung. Quasideuteron correlations were observed as a function of photon energy. The measured quasideuteron transverse momentum distribution was fitted with a $2S$ harmonic-oscillator momentum wave function, yielding a full width at half maximum of 89.4 ± 5.6 MeV/c. Coincident $\mathrm{np}$ events from the reaction ${}_{}{}^6{}_{}{}^{}\mathrm{Li}(\gamma ,p){}_{}{}^5{}_{}{}^{}\mathrm{He}$ were also observed.

The particle stability of ${}_{}{}^{19}{}_{}{}^{}\mathrm{B}$ and the particle instability of ${}_{}{}^{18}{}_{}{}^{}\mathrm{B}$ are demonstrated. Light neutron-rich nuclei were searched for among the projectile fragments resulting from interactions of a $670A$- MeV ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$ beam in a 7.9-g/${\mathrm{cm}}^2$ Be target. A detector consisting of a magnetic spectrometer, threshold Cherenkov paddles, and scintillator paddles was used to determine the charge and mass of the projectile fragments.

Self-pulsing instability is reported in an experiment in which two-level atoms couple to a single mode of a traveling-wave resonator to produce optical bistability. The threshold for the onset of oscillation together with the characteristics of the dynamical state are described. The self-pulsing instability observed is the analog in an absorbing system of the single-mode laser instability.

Significant new evidence is reported for resonant transfer and excitation in ion-atom collisions. This process, which is analogous to dielectronic recombination, occurs when a target electron is captured simultaneously with the excitation of the projectile followed by photon emission. Strong resonant behavior with structure, in agreement with theoretical calculations, is observed in the cross section for projectile $K$ x rays coincident with single electron capture for 100-360-MeV ${}_{20}{}^{}{}_{}{}^{}\mathrm{Ca}_{}^{16+,17+,18+}{}_{}{}^{}$ and 180-460-MeV ${}_{23}{}^{}{}_{}{}^{}\mathrm{V}_{}^{19+,20+,21+}{}_{}{}^{}$ ions colliding with helium.

A technique has been developed to observe collisions in a regime where the rotational energy-level separation is greater than the thermal energy. Pressure-broadening parameters of trace quantities of CO, thermalized by collisions with helium at 4 K, were measured and found to be 10.1 and 11.2 MHz/Torr (half width at half maximum) for the $J=0-1\mathrm{}$ and $J=1-1\mathrm{}$ transitions, respectively. At 4 K only a small number of collision channels are energetically available and comparisons between experiment and theory are facilitated.

A zero-frequency magnetostatic mode is excited externally in a fully ionized, magnetized plasma. Space-time-resolved measurements of magnetic fluctuations indicate that the magnetostatic mode decays with a typical time scale characterizing electron-ion collisions in agreement with theoretical predictions.

Solid spherical layered targets have been uniformly illuminated at irradiances of ${10}^{14}$ to ${10}^{15}$ W/${\mathrm{cm}}^2$. Extensive diagnostics including time-resolved x-ray emission and optical probing were used to determine the plasma ablation rate and the plasma blowoff conditions. Comparisons with hydrodynamic simulations show that the thermal conduction is well characterized by a flux limit $f_e=0.08\mathrm{}\pm 0.02$, with a steep temperature gradient. The steepness of the heat front was confirmed with time-resolved spectroscopy.

We have investigated the stability of electrons above saturated films of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on both insulating and conducting substrates. Electron densities of up to ${10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$ have been observed to be stable, nearly 2 orders of magnitude more than on bulk ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Charging the films to such high densities leads to a drastic reduction in film thickness.

The minimal base of muffin-tin orbitals is transformed exactly into a tight-binding base. The linear transformations, the orbitals, and the Hamiltonian, overlap, and Green's function matrices are expressed in terms of one matrix, the canonical structure matrix $S_{\mathrm{ij}}$. It vanishes beyond second-nearest neighbors and is tabulated. Tight-binding two-center forms with transfer integrals proportional to $S_{\mathrm{ij}}$ are derived.

The vibrational-rotational relaxation of molecular hydrogen in the solid phase is measured by the transient optothermal lensing effect. We have obtained the relaxation rates for the first, second, and third vibrational levels leading to $k_v\approx v\times 1.89\times {10}^5$ ${\sec }^{-1\mathrm{}}$, where $v$ is the vibrational quantum number. This relationship is consistent with the energy-gap law and is a manifestation of the multiphonon relaxation mechanism.

Two classes of artificial semiconductor heterostructures, differing only in the inversion symmetry of their internal quantum wells, are studied via magnetotransport. The samples consist of GaAs/(AlGa) As layered structures containing two-dimensional hole systems. The results reveal a lifting of the spin degeneracy of the lowest hole subband in the samples with inversion asymmetric quantum wells. In those structures with symmetric wells the subband remains doubly degenerate.

We show that the surface-enhanced Raman scattering of pyridine on quenched or island Ag films is strongly reduced by Pd submonolayer coverages. A partial tenfold reenhancement is obtained by subsequent deposition of less than a silver monolayer.