A fluid flows parallel to the strata of an $n$-layer stratified system. Particles suspended in the fluid are carried along with the flow, and additionally make random jumps into neighboring strata, where the flow velocity is different. The spatial dispersion of the particles in the flow direction, for long times, is calculated exactly for arbitrary $n$, jump rates, and velocity field. The result is applied to a variety of phenomena, e.g., chromatography, Taylor diffusion, and random walks.

An exact solution of the Einstein equations in vacuum representing two pairs of gravitational solitons propagating on an expanding universe is given and studied. It is suggested that the solitons evolve from quasiparticles to pure gravitational waves. Two of the four solitons collide and the focusing produced on null rays is studied. Although the spacetime following the collision is highly distorted, null rays do not focus to a singularity.

The average lifetime of bottom hadrons was measured with the Mark II vertex detector at the storage ring PEP. The lifetime was determined by measuring the impact parameters of leptons produced in bottom decays. ${\tau }_b=({12.0}_{-3.6\mathrm{}}^{+4.5\mathrm{}}\pm 3.0)\times {10}^{-13\mathrm{}}$ sec was found.

The ($n$, $p$) reaction in ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ at 60 MeV reveals large structures at high excitations which are the analogs of states in ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ at 29 and 31 MeV, respectively. These new structures, which exhibit giant dipole strength, are not seen in photoneutron data. They do not appear to contain appreciable dipole spin-mode strength. The exhaust large sumrule fractions, and largely answer the longstanding question of missing giant dipole resonance strength in these nuclei.

The 0° differential cross sections of (${\pi }^{+}$, ${\pi }^0$) reactions on a series of nuclei to isobaric analog states were determined at 165, 230, and 295 MeV. For each energy, the $A$ dependence is well described by a power law of the form $\sigma =g\left(E\right)\mathrm{}(N-Z)\mathrm{}{A}^{-\alpha \mathrm{}\left(E\right)\mathrm{}}$, where $\alpha \mathrm{}\left(E\right)\mathrm{}$ is a decreasing function of energy. The 0° excitation functions peak around the (3,3) resonance energy for all nuclei except ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$. A possible explanation in terms of neutron and proton densities is discussed.

The reaction ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}\mathrm{}(p,n)\mathrm{}{}_{}{}^{71}{}_{}{}^{}\mathrm{Ge}$ has been studied at $E_p=35\mathrm{}$ MeV with a high-resolution time-of-flight system. Individual cross sections to the states at 0.0, 0.175, and 0.500 MeV, which are directly related to solar-neutrino absorption in a proposed ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$ detector, have been determined. The cross sections for the 0.0- and 0.175-MeV states are nearly equal, contrary to systematics in this mass region. As a result the absorption of solar neutrinos is estimated to be ≳40% greater than previously calculated.

Atomic stopping cross sections for low-velocity light ions have been evaluated for the atomic numbers $1<~Z_2<~36$ on the basis of the kinetic theory of electronic stopping. Valence electrons make the main contribution. A pronounced $Z_2$ structure in stopping cross sections is found, with minima for noble gases where our estimates agree well with experimental results and with the Lindhard-Scharff formula. Pronounced maxima are predicted for alkalis, where large gas-solid differences are to be expected.

A configuration of magnetic fields is exhibited which can harmonically trap paramagnetic particles in a shallow field minimum, superposed on a nearly uniform field which simplifies spectroscopic interactions with the particles. The potential energy of a trapped neutral particle depends on the particle's internal quantum number, as well as its position in the trap, permitting a combined rf-laser optical pumping cycle which can cool particles by a factor of 2 per cycle. Application of these ideas to the trapping of Na atoms is discussed.

High-resolution three-photon laser spectroscopy has been performed for the first time to gain new spectroscopic information. Starting from the $6s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ ground state of Yb, $6s15p{}_{}{}^{1,3}{}_{}{}^{}P_1^{}{}_{}{}^{}$ Rydberg states were populated by two-photon-resonant, three-photon absorption via the $6s7s{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ intermediate level. As a result of two-photon line narrowing Doppler-free spectra, showing well resolved isotope shifts and hyperfine structures, were obtained. From the hyperfine coupling constants ${}_{}{}^{171,173}{}_{}{}^{}A\left({}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}\right)$ the singlet-triplet mixing was derived.

When a homonuclear diatomic molecule is photodissociated to give one ground state and one electronically excited atom, the wave function of the two-atom systems must reflect an uncertainty as to which atom is excited. Consequently, the wave function of the photon for a photodetector viewing the resulting fluorescence must be written as a superposition of states. Detection of such photons is shown to give rise to an interference signal in the detector voltage at a frequency depending on the Doppler shift from the atomic recoil. Photodissociation of a random distribution of molecules gives rise to a detector voltage whose power spectral density is determined by the angular distribution of photofragments.

Based upon the analysis of electron correlations in hyperspherical coordinates, a classification scheme for all doubly excited states of two-electron atoms is presented. A new set of correlation quantum numbers, $K$, $T$, and $A$, is introduced, where ($K$, $T$) describes angular correlations and $A$ describes radial correlations. It is shown that states with different $L$, $S$, and $\pi$ but identical ${\mathrm{}(K,T)\mathrm{}}^A$ have isomorphic correlations. Such isomorphism is shown to result in the general supermultiplet structure of doubly excited states.

The wavy vortex state of Taylor flow has been examined with use of a novel linear optical scanning apparatus which allows the wavy vortex flow to be studied in considerable detail: Measurements of the wavy mode amplitude and pair-size distribution show that characteristic lengths scale with the size of the apparatus.

Measurements of the rate at which 1.05-μm light ablates material from spherical targets are reported for absorbed intensities between 4×${10}^{13}$ and 4×${10}^{14}$ W/${\mathrm{cm}}^2$. At 4×${10}^{14}$, the ablation rate is twice as high as has been reported for planar targets. A steady-state model which includes inhibited thermal transport matches the data well.

Oscillatory multilayer relaxation of both interlayer spacing and registry at certain high-index metal surfaces is predicted via minimization of a simple model for the total energy of a semi-infinite crystal. Results for the (210) and (211) surfaces of bcc and fcc simple metals indicate that the relaxation parallel to the surface plane moves the surface layers toward more symmetrical configurations with respect to adjacent layers.

Rotating superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-$B$ is found to possess a new contribution to the NMR frequency shift, which changes sign on reversal of either the angular velocity of rotation or the magnetic field. For $p=29.3\mathrm{}$ bars this gyromagnetic effect shows a discontinuity in magnitude at the first-order phase-transition temperature $\frac{T}{T_c}=0.6\mathrm{}$, at which a change in the vortex-core structure takes place. These observations support the conclusion that the vortex core possesses a spontaneous intrinsic magnetization.

A new roughening transition has been observed on hcp ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ crystals at $T=0.365\mathrm{}\pm 0.03$ K, where a third kind of facet appears at 60±3° from the basal planes. The authors relate this experimental observation to the arguments presented recently against the existence of quantum roughening. Some preliminary results concerning the growth of facets are also briefly mentioned.

Equations of motion for excitons in molecular aggregates obtained from stochastic Hamiltonian models are known to be applicable only at infinite temperatures. It is argued that the problem lies in the omission of dissipative contributions that must be present if the excitons are to achieve thermal equilibrium. Stochastic equations of motion are constructed that are applicable at finite temperatures. As an application of the model optical line shapes are considered.

Dynamic nonlinear behavior is reported at high currents in the quantum Hall regime of GaAs heterostructures, resulting from breakdown of the dissipationless current flow. It is demonstrated that this breakdown is spatially localized and transient switching is observed on microsecond time scales among a set of distinct dissipative states. A simple macroscopic picture is proposed to account for these novel phenomena.

Photoelectron core-level spectra of condensed nitric oxide exhibit intense new lines not present in the gas phase. These satellites result from intermolecular interactions arising from the formation of weakly bound ON-NO dimers in the solid. Generalized valence-bond calculations demonstrate that core-level ionization produces radically different ion states for the dimer and monomer. A simple physical picture shows the importance of intermolecular charge-transfer screening in the lowest-energy $\mathrm{N}\mathrm{}\left(1s\right)\mathrm{}$ ion final state.

For the first time the dc magnetization of a metallic nuclear magnet, Pr${\mathrm{Ni}}_5$, has been directly measured down to well below the nuclear magnetic ordering temperature in magnetic fields up to 37.6 mT. The ratio of the Curie constant to the saturation magnetization is found to be (32±9)% larger than the same ratio expected for the localized hyperfine enhances moments with the known enhancement factor $1+K=12.2\pm 0.5$. Contributions of moment fluctuations are discussed.