We report the first direct observation of the effect of Doppleron resonances on the velocity distribution of atoms in an intense optical standing wave. These observations are in reasonable agreement with the simulations based on a continued-fraction solution for the force.

An atomic beam of Li was transversely cooled using an intense standing-wave radiation field. A dramatic change in the transverse velocity distribution was observed. Structure in the resulting velocity distribution was found to be due to velocity-tuned multiphoton ‘‘Doppleron’’ resonances. The force due to seven-photon resonances is clearly resolved in the data. The data are in good agreement with theoretical predictions.

We present a modal analysis of the paraxial wave equation for plasma x-ray lasers. Gain guiding and refractive antiguiding are identified as the essential mechanisms which influence the coherence of the output radiation. Scaling laws for the number of guided modes and the coherence are given, depending on three parameters: the gain-dependent Fresnel number, the strength of refraction relative to gain, and the amplification length. The importance of controlling excess spontaneous emission to obtain coherence is identified. We suggest an experimental effort to verify various guiding regimes and to produce coherent output in the range between 45 and 200 Å.

We examine optical phase conjugation for a shaped-microparticle suspension maintained in a uniform electric field. Theory asserts that the presence of a Stark field decreases the grating response time and enhances the composite’s four-wave-mixing coefficient. These changes arise from and reflect the narrow angular range in which the particles are constrained to point, giving rise to reduced rotational times and the appearance of a new coherent scattering mechanism.

We have observed the 1S-2S transition of atomic hydrogen to a resolution of 5 parts in ${10}^{11}$ by means of Doppler-free two-photon spectroscopy in an atomic beam. The significantly asymmetric line shape is strongly affected by the relativistic Doppler effect. The new high resolution can be used for improved measurements of the Rydberg constant, the 1S Lamb shift, and the isotope shift of hydrogen and deuterium.

A heurisitic model for evolution of the probability distribution (PDF) of transverse velocity gradient s in incompressible Navier-Stokes turbulence is distilled from an analytical closure for Burgers turbulence. At all Reynolds number scrR, the evolved PDF is ∝‖s${\mathrm{\Vert }}^{\mathrm{-}1/2}$ exp(-const×‖s‖/〈${\mathit{s}}^2$${\mathrm{〉}}^{1/2}$) for large ‖s‖. The model suggests that skewness and flatnesses are asymptotically independent of scrR, and that cascade to smaller scales is not a fractal process. For Burgers dynamics, both simulations and the analytical closure give a PDF ∝‖ξ${\mathrm{\Vert }}^{\mathrm{-}1}$ exp(-const×‖ξ‖/〈${\xi }^2$${\mathrm{〉}}^{1/2}$) for large negative velocity gradient ξ.

Microwave phase conjugation was observed in a liquid suspension of elongated microparticles via degenerate four-wave mixing in a collinear waveguide geometry. Longitudinal phase corrections of a double-passed phase shifter were demonstrated by interferometry. Nondegenerate four-wave mixing was demonstrated with a FWHM bandwidth of 70 MHz. The 10% scattering from the induced grating indicates that high conversion efficiency and gain can be achieved with higher pump power.

Using Langmuir probes in the edge plasma of the TEXT tokamak we have identified sawtooth oscillations in density, potential, and temperature. Concurrent with the sawtooth oscillations are increases in the density and potential fluctuation levels which result in significant increases in the fluctuation-driven particle flux and associated heat flux. No changes in the basic turbulence mechanism were observed during the sawtooth passage.

The propagation of a radiation heat wave through a thin foil of solid gold was investigated experimentally. The wave is driven by the intense thermal radiation in 1–3-mm-diam gold cavities heated by an intense laser pulse (duration 0.8–0.9 ns, wavelength 0.35 μm) to temperatures of more than 200 eV. Evidence of the propagating wave was obtained from the delayed onset of intense thermal emission from the outer side of the foil. The results agree with theoretical predictions for a self-similar ablative heat wave and with numerical simulations.

We have determined the effects of imposing dichotomous noise on the magnetic Fréedericksz transition in the nematic liquid crystal MBBA. Various states of the nematic orientation occur, depending on the magnitude of the dc field and the amplitude and correlation time of the noise component. The resultant phase diagrams are in partial agreement with the analytic results of Horsthemke, Doering, Lefever, and Chi for H>${\mathit{H}}_{\mathit{c}}$. A substantial deviation of the experimental result from computer simulations based on their model for low magnetic field (H<${\mathit{H}}_{\mathit{c}}$) is observed.

Transverse spin dynamics have been measured for a very dilute ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixture (${\mathit{x}}_3$=1.82×${10}^{\mathrm{-}3}$) polarized by an 8-T magnetic field. The sample was confined to a small spherical resonator, resulting in well-defined spin-wave modes for temperatures below 50 mK. Results are presented for the spin-rotation parameter ${\mathrm{\Omega }}_{\mathrm{int}}$${\mathrm{\tau }}_{\mathrm{\perp }}$ and spin-wave dispersion ${\mathit{D}}_{\mathrm{\perp }}$/${\mathrm{\Omega }}_{\mathrm{int}}$${\mathrm{\tau }}_{\mathrm{\perp }}$. It is found that ${\mathrm{\Omega }}_{\mathrm{int}}$${\mathrm{\tau }}_{\mathrm{\perp }}$ increases monotonically as the temperature is decreased below the degeneracy temperature ${\mathit{T}}_{\mathit{F}}$, in apparent disagreement with an earlier experiment.

Via ellipsometry, the time-dependent profiles of liquid droplets spreading completely on plane solid substrates are measured in the range 0–200 Å. Two different surfaces are used, a ‘‘high-energy’’ surface (wettable by water) and a ‘‘low-energy’’ surface (Langmuir-Blodgett layer of a double-bond-terminated fatty acid), respectively. We have observed that the shape of spreading droplets of the same fluid (polydimethylsiloxane) are quite different on these two surfaces: On the low-energy surface, the spreading occurs by the macroscopic lateral spreading of essentially one monolayer, away from the central part of the drop. On the contrary, for the high-energy surface, spreading occurs by the macroscopic lateral spreading of several monolayers in a kind of hierarchical process. Nevertheless, in both cases there is a signature of molecular structuring in the fluid flow at the center of the drop.

We calculate the minimum growth probability for diffusion-limited aggregation (DLA) as a function of the cluster mass M, and find a novel singularity of the form -ln${\mathit{p}}_{\min }$(M)∼(lnM${)}^{\mathit{y}}$ with y≊2. We interpret this result in terms of a simple model for DLA structure, which is characterized by a hierarchy of self-similar voids separated by channels whose diameter increases slower than the cluster diameter.

The temporal evolution of nonequilibrium phonon populations in ${\mathrm{LaF}}_3$:${\mathrm{Pr}}^{3+}$ is investigated at low temperatures (1.8 K) utilizing pulsed, tunable, monochromatic generation and time-resolved, tunable, narrow-band detection. High occupation number, narrow-band phonon populations are generated via far-infrared pumping of defect-induced one-phonon absorption. Time-resolved, frequency-selective detection is provided by optical sideband absorption. Nonequilibrium phonon decay times are measured and attributed to anharmonic decay.

A simple method is proposed to describe the thermodynamic properties of disordered grain boundaries in nanocrystalline metals and semiconductors. Based on a free-volume approach at negative pressure of the universal equation of state, the maximum free volume, thermal expansion coefficient, specific-heat capacity, and excess enthalpy, entropy, and free energy of grain boundaries are derived from well-known thermodynamic relationships. The results show good agreement with available experimental data and suggest an intrinsic mechanical instability of a nanograin boundary (Gibbs instability) accompanied by divergences of α and ${\mathit{c}}_{\mathit{p}}$, and the possibility of an entropy stabilization of nanograin boundaries.

${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ nuclear relaxation experiments were performed in different forms of silica (alcogel, aerogel, amorphous, and crystallized) doped with paramagnetic impurities. Magic-angle spinning is used to quench the nuclear-spin diffusion. Under this condition, the saturation recovery of the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ magnetization follows a power law in a very large time range (up to 5 orders of magnitude). This nonexponential relaxation m(t)∼${\mathit{t}}^{\mathrm{\alpha }}$ reflects the mass distribution in the sample: M(r)∼${\mathit{r}}^{\mathit{D}}$ with α=D/6. In aerogels, the measured fractal dimension D≊2.2 is in agreement with that determined by small-angle x-ray scattering. In densified materials, a dimension D≊3 is actually observed.

We propose that the Meyer-Neldel compensation rule can be understood as arising naturally for assisted processes in which the energy for the process is much larger than that of a typical excitation. In this case, the total energy is provided by multiple excitations. The prefactor is then proportional to the number of ways of assembling these excitations. This number increases exponentially with the energy for the process, yielding the Meyer-Neldel rule.

Stimulated by the recent report of Beuhler, Friedlander, and Friedman on the observation of dd fusion under the impact of heavy-water clusters on deuterated solid targets, we undertook a similar study with pure deuterium clusters (${\mathrm{D}}_{200}^{+}$–${\mathrm{D}}_{300}^{+}$) in the same range of incident energy per deuteron (less than 1 keV). We observed no fusion event and our upper limit for the fusion rate is more than 1 order of magnitude below the Brookhaven value. Additional measurements performed with ${\mathit{N}}_{\mathit{n}}^{+}$ projectiles were not conclusive but showed that beam-contamination problems may be very serious.

Carbon cluster ions with a double negative charge, and lifetimes of at least 10 μs, have been produced by sputtering graphite, and observed in a double-focusing mass spectrometer. Ions as small as ${\mathrm{C}}_7^{2\mathrm{-}}$ have been detected. The intensities of the doubly charged clusters (${\mathrm{C}}_{\mathit{n}}^{2\mathrm{-}}$ where n=7–28) follow an alternating pattern, with even-n clusters being more intense than neighboring odd-n clusters. In contrast, even-n singly charged clusters are more intense than odd n for n≤9, and less intense for n≥13, where the clusters are believed to be cyclic.

We measured the energy and density of near-surface defects in hydrogenated amorphous silicon, germanium, and their alloys by total-yield photoemission spectroscopy. The defects lie 0.56 eV above the valence-band edge, independent of alloy composition. The defect density at the clean surface is constant at ∼2×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. These values agree exactly with the equilibrium theory of dangling bonds, so that our experiments furnish the first evidence for thermal equilibration of semiconductor surface defects.