Single magnesium ions at superthermal velocities (200 eV energy) are detected for the first time by the photon-burst method. An optical isotopic resolution of ${>10\mathrm{}}^3$ is demonstrated.

The interionic pair potential derived from the measured structure factor S(Q) of liquid Pb at 613 K using the modified hypernetted-chain approximation was tested in the molecular-dynamics simulation and found to be inadequate. It is shown that the same simple model of the potential with three parameters, having been optimized in an iterative procedure involving the large-scale molecular-dynamics simulation, provides a very good fit to the original structure.

Using Brillouin scattering from an evanescent wave, an interfacial fluid mode has been observed at a solid-liquid interface. Its dispersion relation depends on the component of the wave vector parallel to the interface and is, within the accuracy of the experiment, linear with a speed equal to the bulk sound speed. Also observed is the Rayleigh mode at the solid side of the interface whose width is consistent with mode-coupling theory.

This paper presents a laboratory x-ray laser that lases on several 4d→4p transitions ranging from 54 to 89 Å in nickel-like erbium (Z=68) and that is resonantly photopumped by the 2$p_{1/2}$→1$s_{1/2}$ emission line, Lyman-α, in hydrogenlike aluminum. The Al-Er scheme has a tremendous advantage in that it enhances the gain of laser lines that would be expected to lase from collisional excitations and recombination processes; therefore it should be much easier to observe the photopumping process, as it will increase an already finite exponential gain. The resonant photopumping process is also shown to create gain on one additional laser line that has not previously appeared in the Ni-like spectra. Calculations of the gain of these laser transitions in steady-state equilibrium plasmas are presented.

A search for ferroelectric nematic liquid crystals is carried out on the basis of a simple theoretical model viewing the nonspherical polar molecules as hard ellipsoids of revolution with an embedded permanent dipole oriented along the symmetry axis. The density-dipole moment-aspect ratio phase diagram of the dipolar hard ellipsoids has been determined with the aid of an approximate density-functional theory that was shown recently to lead to good quantitative agreement with the computer simulations of the isotropic-nematic transition of hard ellipsoids. For the dipolar hard ellipsoids, two types of stable ferroelectric nematic phases are found for dipole moments and aspect ratios exceeding some reasonable threshold values.

We consider the escape rate in a bistable potential driven by exponentially correlated noise. Our focus is on the crossover between the small- and large-correlation time behavior. Precise numerical results obtained by using a matrix-continued-fraction technique are compared against recent theoretical predictions.

A base pair in DNA comprises two ringed planar structures joined together by hydrogen bonds. The opposite rotation of these two structures about their lengthwise axis is referred to as propeller twist. We examine the effect of propeller twist on our calculation of the far-infrared absorption in a homopolymer DNA, specifically, B poly(dA)-poly(dT). In our previous calculation [Phys. Rev. A 39, 3173 (1989)], we found agreement with experimental infrared data except for one mode. We now give evidence that this mode may be due to the propeller-twist conformation.

It is shown that the fermion quasiparticle in many nonrelativistic quantum field theories has a maximum velocity.

The dependence of the energies of high-n Rydberg states of ${\mathrm{CH}}_3$I on the molar composition of helium-argon mixtures (in the number density range 1.3×${10}^{20–}$5.6×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$) is reported. The energy shifts, when normalized to a given density value, are found to vary linearly with the mole fraction of either component of the binary, rare-gas mixture. The observed change in sign of the energy shift is attributable to the different signs of the electron scattering lengths for the two rare-gas components. As a result, there exists a mixture composition, at a mole ratio [He]/[Ar]=2.0, at which the shift is null. The experimental results for the gas mixture agree with the Fermi formula, as modified to include the Alekseev-Sobel’man polarization term. Effective electron scattering lengths and cross sections, polarizabilities, and thermal velocities are used to characterize the effects of the binary gas perturber system.

We are reporting electron spectra resulting from the inelastic scattering of 45-eV electrons from helium atoms through a scattering angle of 12° in the presence of an intense carbon dioxide laser. The spectra presented are in the region of the 2 ${}_{}{}^1{}_{}{}^{}S$ and 2 ${}_{}{}^1{}_{}{}^{}P$ states of helium. These data show additional structure when the laser is present at scattered electron energies which correspond to an increase or decrease in electron kinetic energy of the equivalent of one laser quantum. These measurements demonstrate the simultaneous electron-photon excitation of an atom at high incident-electron energies.

The role of phase dynamics in laser systems is investigated. A detailed analysis of a bidirectional ring laser shows that, besides a trivial shift of the average frequency, a diffusion originated by the chaotic motion can be detected. In this case, even if the evolution of the average phase is removed by a suitable change of reference, the fractal dimension of the attractor remains unchanged, at variance with the ordered motion, where the same choice would reduce the dimension by one.

High-resolution heat-capacity measurements have been carried out on several pure compounds and binary mixtures in the homologous series of n-alkyl-4’-n-decyloxybiphenyl-4-carboxylate. The measured smectic-A–hexatic-B heat-capacity anomalies can be fitted to a power law over two decades in the reduced temperature with a large value for the critical exponent (α=0.60). The value of α remains fairly constant when the hexatic-B phase temperature range varies from 0.8 to 22 K.

Initial-value problems are investigated numerically for a two-dimensional (2D) nonlinear long-wavelength equation including effects of instability, dissipation, and dispersion. It is found that one-dimensional structures are always unstable and localized 2D structures are generated. For relatively weak dispersion, the localized structures are not distinct and the overall evolutions are irregular. For strongly dispersive cases, a number of bell-shaped pulses of equal amplitude exist stably in the time asymptotic state and they form a quasistationary lattice for certain initial conditions.

We measured the temporal behavior of the electroclinic effect in a chiral smectic-A liquid crystal by monitoring the optical response to a voltage pulse. A simple formula was shown to describe the time behavior of the induced tilt angle Θ. The electroclinic response time τ increased for decreasing values of the applied electric field E and the cell gap d. The static value of Θ did not depend on d and remained linear in E. We conclude that bulk interactions determine the equilibrium value of Θ. The increase of τ is attributed to a hindrance by the boundaries of the switching along the entire thickness of the cell.

The vacuum decay rate ${\lambda }_T$ of orthopositronium ($1^3$$S_1$) formed in a gas has been measured to be ${\lambda }_{\mathit{T}}$=7.0514±0.0014μ${\mathrm{s}}^{\mathrm{-}1}$. Measurements of ${\lambda }_T$ in four different gases, all in agreement, are averaged to obtain this result. As systematic tests, two entirely separate digital timing systems are simultaneously used throughout the experiment; the cross section for collisional quenching of the long-lived 2 ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ excited state is determined; ${\lambda }_T$ is redetermined in two of the gases (${\mathrm{N}}_2$ and Ne) using only high-gas-density measurements; and the collisional quenching rate of water vapor, the major residual gas contaminant, is directly measured. The final value of ${\lambda }_T$ from this gas experiment, which represents a factor of 4 improvement in accuracy over previous measurements, is 9.4 experimental standard deviations above the theoretical value.

We present a method for efficiently calculating the one-particle Green’s function (OPGF) for the interaction of a particle with a static potential using an iterative procedure that avoids the necessity for inversion of large matrices. The OPGF for a hierarchy of successively larger systems converging to the full system under study are thereby also obtained. We demonstrate the usefulness of the method, applying it to determine the bound-state spectrum and density of states of one particle interacting with mesoscopic size and extended two-dimensional systems. Applications to a variety of physical systems are briefly discussed.

A study on the energy-level distribution of the Pariser-Parr-Pople Hamiltonian has been performed. In particular, relations between the values of the electron interaction parameters, the symmetry of the core potential, and the form of the secular distribution of the energy-level density have been investigated. The density is shown to be Gaussian in the physically significant range of the parameter values. However, very strong deviations from the Gaussian behavior have been observed at the limit of the very weak electron-core interaction. The secular density distribution functions have been used to predict the Hamiltonian spectra in very good agreement with the results of the matrix diagonalization. Spacings of the nearest-neighbor levels behave mainly in a Poisson-like form, showing the existence of approximate hidden symmetries even in the case of highly nonsymmetric core potentials.

We calculate the spontaneous-emission spectrum and the spectrum of weakly driven fluorescence for a two-level atom coupled to a resonant-cavity mode. For strong atom-cavity coupling the spectra split into two peaks that can have subnatural linewidths. If the cavity linewidth is negligible, the spontaneous-emission spectrum has half the radiative linewidth of the atom; the spectrum of weakly driven fluorescence shows an additional 36% squeezing-induced narrowing. These effects can be observed using coupled-field and collective-polarization oscillators excited in a cavity containing N two-level atoms.

We calculate the hyperfine structure of the ground state of the muonic ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atom by the variational approach, employing an improved set of the Hylleraas-type basis functions to deal with the nonrelativistic three-body wave functions of the muon, the electron, and the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ nucleus. The convergence of the series is very fast, thus eliminating the problem with the propagation of the truncation errors of the computer machine. Using the recoil correction, ${\delta }_{\mathrm{rec}}$ =Δ${\nu }_F$(3α/π)($m_e$/$m_{\mu }$) ln($m_{\mu }$/$m_e$), we arrived at the value Δν=4464.87±0.05 MHz for the total hyperfine-structure splitting of the ground state of the muonic ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atom including relativistic and other radiative corrections. But if we follow the treatment of Borie [Z. Phys. A 291, 107 (1979)] for the recoil corrections by treating the (μ${\mathrm{-}}^4$He${)}^{+}$ system as a pseudonucleus, we will arrive instead at the value Δν=4464.00±0.05 MHz. The former seems to be in better agreement with the experimental result of 4465.004±0.029 MHz.

Perturbative corrections to the wave functions of the 16 n=2 positronium states are used to derive the n=1⇆n=2 radiative transition rates of a positronium atom moving in arbitrarily oriented electric and magnetic fields. Excess annihilation radiation resulting from laser excitation and magnetic mixing in n=2 positronium is calculated and found to be in accord with a recent experiment.