We present a microscopic simulation of convoy-electron production and transport through solids. Using a classical transport theory, we calculate the forward-ejected electron spectrum for 3-MeV/u ${\mathrm{H}}^0$ projectiles traversing carbon foils of different thicknesses. We find excellent agreement with experiment in both shape and absolute magnitude of the convoy-electron spectrum over a wide range of electron energies and foil thicknesses. Our study provides detailed information on the relative importance of competing pathways contributing to convoy-electron emission.

Ballistic collisions is a term used to describe atom-diatom collisions during which a substantial fraction of the initial relative translational energy is converted into the internal energy of the diatom. An exact formulation of the impulse approach to atom-diatom collisions is shown to be in excellent agreement with the experimental results for the CsF-Ar system at 1.1 eV relative translational energy for laboratory scattering angles of 30° and 60°. The differential cross section for scattering of CsF peaks at two distinct recoil velocities. The peak centered at the recoil velocity corresponding to elastic scattering has been called the elastic peak. This peak is shown to consist of several hundred inelastic transitions, most involving a small change in internal energy. The peak near the center-of-mass (c.m.) velocity is called the ballistic peak and is shown to consist of highly inelastic (ballistic) transitions. It is shown that transitions comprising the ballistic (elastic) peak occur when an Ar atom strikes the F (Cs) end of CsF. When one is looking along the direction of the c.m. velocity, the signal from a single transition, which converts about 99.99% of the relative translational energy into internal energy, may be larger than the signal from any other ballistic transition by as much as an order of magnitude. This property may be used to prepare state-selected and velocity-selected beams for further studies. It is also pointed out that the ballistic peak may be observed for any atom-molecule system under appropriate circumstances.

Large-scale classical-trajectory Monte Carlo calculations are executed in three-dimensional space for proton-hydrogen collisions at 2.8 and 5 MeV. The Thomas peak that has been confirmed in the experimental data and in the quantal calculations is invisible in the classical calculations. This unexpected result is due to the peculiar character of classical bound states that have no minimum binding energy. The Oppenheimer-Brinkmann-Kramers-type capture by means of the velocity-matching mechanism dominates even in the MeV region owing to this classical peculiarity, and the Thomas double-scattering contribution is embedded in this background.

We report on a precision measurement of the 2 ${}_{}{}^3{}_{}{}^{}$S–3 ${}_{}{}^3{}_{}{}^{}$P transition in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Doppler-free spectra of the transition were observed by exciting metastable helium atoms, produced by a chopped rf discharge, using 389-nm radiation generated by a frequency-doubled Ti-doped sapphire laser. A frequency measurement of the Doppler-free lines was made by interferrometric comparison of the Ti-doped sapphire frequency with that of an iodine-stabilized He-Ne laser using a 1-m evacuated Fabry-Pérot etalon. The measurement uncertainty was 2 parts in ${10}^9$ (3 standard deviations), which gives an improvement of a factor of 60 in the term value of the 3 ${}_{}{}^3{}_{}{}^{}$P level with respect to earlier measurements. This result makes it possible to determine the one electron Lamb-shift correction to the 3 ${}_{}{}^3{}_{}{}^{}$P state with an accuracy of 0.8%.

In connection with experiments aiming at the improvement of the cesium atomic beam clock by means of a fountain of laser-cooled cesium atoms, we present expressions for the line shift and line broadening due to collisions between cesium atoms. The coherent collision cross sections occurring in these expressions are calculated by means of the coupled-channels method. The magnitude of the calculated shift implies that spin-exchange collisions are an important factor in limiting the anticipated accuracy of a future cesium fountain clock and of other intended fountain experiments.

A model for nondegenerate cascade laser instabilities is given. The linear stability analysis of the resonant stationary solution reveals the existence of two independent Hopf bifurcations and a codimension-2 point. The system exhibits a rich dynamic behavior with transitions to chaos through the Ruelle-Takens-Newhouse scenario.

A classical study of the dynamics of atomic Rydberg states interacting with both elliptically and circularly polarized microwave fields is presented. In recent experiments, enhanced ionization rates were observed in elliptically as opposed to circularly polarized fields. A classical phase-space simulation provides good qualitative agreement with experimental findings. These results are explained in terms of the breakdown of almost conserved quantities as the polarization of the field is changed from circular to elliptical.

Coherent π-pulse propagation with pulse breakup has been observed in an erbium-doped fiber waveguide amplifier. The waveguide was pumped by a 1.48-μm ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{-}\mathit{y}}$ laser diode. The waveguide had an erbium ion concentration of 8900 ppm and was cooled to 4.2 K. The pulse source was a 1.53-μm mode-locked Er-doped glass laser with a pulse width of 400 ps. A pulse advance of approximately 130 ps was observed between a low-intensity transmitted pulse with a low pump power and an amplified π pulse in a 1.5-m-long fiber.

Numerical simulation data for the rate of thermally activated escape from a metastable well are presented for a range of parameters extending from low-damping to strong-damping Kramers limits. To this purpose the Langevin equation is transformed into a set of difference equations optimized for numerical calculations. With this algorithm the range of barrier heights explorable is considerably larger than in previous work. For sufficiently high barriers the data are found to be in good agreement with a recent analytical solution [H. Grabert, Phys. Rev. Lett. 61, 1683 (1988)] of the Kramers turnover problem.

A scaling analysis of the Kardar-Parisi-Zhang equation for driven interface growth, as a function of the hydrodynamical parameters λ, D, and ν, is presented in d=2, which predicts the existence of universal amplitude ratios as well as a universal scaling function for the surface width w(L,t) on length scales L at time t. These predictions are confirmed by simulations of three different models as well as a mode-coupling calculation with which good agreement is found. This scaling analysis is expected to be useful in establishing a more detailed connection between continuum equations, microscopic models, and experiments.

Light scattering was used to study the time evolution of phase separation of a critical isobutyric acid and water mixture in the spinodal region. A shoulder is observed in the larger-scattering-angle region of the main dominant peak in the structure factor, and the time dependence of the position of shoulder is well scaled by the same characteristic wave number and interdiffusion coefficient as those for the main peak. The tail in the large-scattering-angle region of the structure factor is well expressed by the Porod law in the intermediate and late stages of spinodal decomposition.

We show, using constant-pressure molecular-dynamics calculations, that the deviation from Arrhenius behavior in transport properties in the supercooled states of Lennard-Jones (LJ) fluids and soft-sphere (SS) systems starts to occur when the time scale for obtaining ergodicity starts to increase dramatically. The temperature dependence of the ergodicity diffusion parameter for both the SS mixtures and the two-component LJ system follows the Vogel-Fulcher (VF) equation. The self-diffusion constants for the SS mixtures follows the Arrhenius law, whereas for the LJ system a VF behavior is found. Our results also demonstrate that relaxation processes in supercooled liquids are dominated by fluctuations in domains of finite length.

As a counterpart of the Porod law, the scattering intensity from bilayers of amphiphilic systems behaves as ${\mathit{q}}^{\mathrm{-}2}$ at large wave numbers q. We derive this result and its curvature correction and then find a version of Tomita’s sum rule. We also examine the large-q behavior of the scattering intensity in general anisotropic cases in both bilayer systems and usual two-phase systems.

Measurements of reflectivity versus ac electric field E were performed in the nematic phase of octylcyanobiphenyl homeotropically aligned between two indium tin oxide–coated glass slides. The change in reflectivity δR was found to be linear in E, with d(δR)/dE increasing monotonically toward the nematic-isotropic transition temperature. In addition, the phase of δR at the front and back surfaces differed by π. Such an effect will arise from t/Iorder electricity, whereby a term proportional to the product of the electric field and the gradient of the nematic order parameter appears in the bulk free energy. This term integrates to yield a pair of surface terms such that there is a y/Ilinear change in the order parameter close to the interface. Similar terms will also arise from ferroelectric interactions at the surface.

We present an experimental analysis of the variance of photon-correlation and photon-structure functions obtained from dynamic light-scattering measurements. The results are compared with theoretical predictions. The noise evaluation of the statistical estimators has been obtained by using a fluctuation analyzer, specifically built for this purpose, which uses linearly spaced sampling time. The noise dependences on the experimental bounds (mean count rate and sampling time) are investigated. The predicted behavior of the estimator variances has been confirmed. However, the measured accuracies of correlation and structure functions show a discrepancy between their analytical expressions and the experimental results in the high-intensity limit of the scattered light. The results confirm that the structure function has better noise performance for relatively high mean count rates. This strongly suggests a wider use of this statistical estimator in measurements requiring a good accuracy, such as in the analysis of polydispersed microparticle size distribution, in the case of uniform sampling.

We present an exact solution for the percolation probability of directed compact percolation near a nonconducting wall. The model is solved for clusters grown from a seed of arbitrary width on an isotropic square lattice. In the case where clusters grow from a seed containing a single site, the exact result confirms a recent conjecture on the percolation probability by Bidaux and Privman (unpublished).

The potential gain of luminosity by both focusing and colliding ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ beams in an underdense plasma is investigated. We first suggest the possibility of creating the plasms by the beam self-induced ‘‘tunneling ionization’’ of a gas. We then study the beam optics in the continuous focusing environment provided by the plasma, and the subsequent ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ beam-beam interaction in the same plasma. When applied to a range of beam parameters based upon the present Stanford Linear Collider conditions, we find that a gain by more than a factor of 10 in luminosity is possible. The sensitivity of various nonideal situations is discussed.

A semiclassical formula that gives scattering cross sections down to an angular resolution of order ${\mathrm{ħ}}^{4/5}$ is derived. The formula describes rainbow peaks, which have widths of order ${\mathrm{ħ}}^{2/3}$, but not other interference effects, which give structure on a scale of order ħ. For cross sections with many rainbow peaks, it can be easier to evaluate than conventional uniform approximations. As an example, the formula is applied to a simple two-dimensional model.