Spontaneous radiative recombination between protons and electrons to form hydrogen atoms with 8≤n≤19 has been measured. ${\mathrm{CO}}_2$ laser light has been shown to induce stimulated radiative recombination to the n=11 and 12 levels with an inferred gain in the cross section of 1720±860 and 4790±2830 for a laser power of 12.6 and 15.3 W, respectively. This is in line with that predicted theoretically.

Absolute, cascade-free excitation cross sections in an ion have been measured for the resonance ${}_{}{}^2{}_{}{}^{}$S${\to }^2$P transition in ${\mathrm{Zn}}^{+}$ using electron-energy-loss and merged electron-ion beams methods. Measurements were carried out at electron energies of below threshold to 6 times threshold. Comparisons are made with 2-, 5-, and 15-state close-coupling and distorted-wave theories. There is good agreement between experiment and the 15-state close-coupling cross sections over the energy range of the calculations.

We report on optically induced spin-echo phenomena associated with Zeeman coherences in a sublevel manifold. The experiments on the ground state of sodium show multiple echoes occurring periodically after a double-pulse excitation sequence in an inhomogeneous magnetic field. The echo formation mechanism is attributed to a light-shift-induced transfer of sublevel coherence among the nondegenerate substates. This mechanism is verified experimentally, using a modulated excitation scheme with subsequent phase-sensitive detection.

How large must the input signal to an amplifier be to dominate the quantum noise introduced during amplification? We attempt to answer this question for a Raman amplifier with an experiment that utilizes a modified Mach-Zender interferometer with an amplifier placed in each leg. The quantum noise added by the amplifiers manifests itself by degrading the visibility of the output fringes from the interferometer. Only an average of a few photons per mode were necessary to achieve good visibility.

High-resolution measurements for the dissociative recombination of ${\mathrm{H}}_2^{+}$(v=0) are presented and compared with theoretical calculations. The observed structure is explained with a model that invokes the coupling to intermediate electronic states belonging to two separate Rydberg series.

We report application of Hoffnagle’s broadband cooling concept to cool Na atoms in an atomic beam. The rms velocity spread of the cooled atoms is 0.75±0.05 m/s (1.5±0.2 mK) with a flux of (9.5±1.0)×${10}^8$ atoms/s. 1D broadband optical molasses is also demonstrated. Potential applications of the cooling scheme are discussed.

Astrophysical and cosmological arguments constrain the properties of a 17-keV Dirac neutrino. It must have interactions beyond those of the standard electroweak theory to reduce its cosmic abundance by a factor of 200. A predicament arises because the additional helicity states of the neutrino necessary to construct a Dirac mass must have interactions strong enough to evade the astrophysical bound from SN 1987A, but weak enough to avoid violating the bound from primordial nucleosynthesis.

The peak pattern of a magnetic fluid, in both a vertical dc and an additional vertical ac magnetic field, behaves as an oscillator. Using the ac-dc field ratio as a control parameter, we study the dynamics of this oscillator. For low values, the peaks oscillate at the ac field frequency, whereas for a large enough value the oscillation is subharmonic (period doubling). The phase diagram is interpreted in terms of a bounded parametric resonance driven by the nonlinearity of the oscillator. In 2D this transition occurs with symmetry breaking from a triangular to a square pattern.

Deformation and breakup of immiscible fluids in deterministic chaotic flows is governed by self-similar distributions of stretching histories and stretching rates and produces populations of droplets of widely distributed sizes. Scaling reveals that distributions of drop sizes collapse into two self-similar families; each family exhibits a different shape, presumably due to changes in the breakup mechanism.

Damping of the plasma toroidal velocity owing to parallel viscosity is observed in the Compact Helical System-torsatron device with tangential neutral-beam injection. The toroidal velocity profile is dominated by perpendicular viscosity when the magnetic-field modulation is weak near the axis. However, parallel viscosity is found to be dominant when the modulation is strong enough, and to increase in proportion to the square of the modulation of the magnetic field. The absolute values of the viscosity agree with the neoclassical prediction within a factor of 3.

Fluctuations are experimentally studied using Langmuir probes and magnetic probes in the a/2≲r≤a region of the REPUTE-1 reversed-field-pinch plasma. An electron-heat flux ${\mathit{q}}_{\mathit{E}}^{\mathit{e}}$ due to electrostatic fluctuations is too small to account for the total electron-heat flux ${\mathit{q}}_{\mathrm{total}}^{\mathit{e}}$. It is implied that magnetic fluctuations mainly determine the electron energy confinement in the interior region.

Plasma-size scans have been performed in the Tokamak Fusion Test Reactor to investigate the effect of major and minor radius upon energy confinement in high-recycling plasmas heated by neutral-beam injection. The major radius, minor radius, and aspect ratio were varied over a large range (a=0.4–0.9 m, R=2.08–3.2 m, R/a=2.8–8.0). The energy confinement measured with diamagnetic and kinetic diagnostics scales strongly with major radius, and weakly or negatively with minor radius.

The acceleration of beam ions due to their interaction with lower hybrid (LH) waves is investigated. The critical coupling density, above which the LH power is absorbed mostly by ions rather than electrons, is found to depend strongly on the LH frequency but not on the refractive index parallel to the toroidal magnetic field. These dependences are in good agreement with the predictions of a simple, clear theory based on a dispersion relation in the electrostatic cold-plasma approximation.

An exactly solvable model with an infinite number of modulated phases is proposed. Its relation to the anistropic next-nearest-neighbor Ising model is established and possible applications are discussed.

We perform computer simulations of Eden-like growth models in two dimensions with random reactions that either allow or inhibit further growth. By varying the probabilities of such reactions, the cluster develops a variety of fractal interfaces related to the hull and the external perimeter of the usual percolation cluster. However, a blocking mechanism leads to different fractal dimensions. Reaction probabilities away from the critival value were also found to give different effective dimensions due to slow crossovers. These results may explain the nonuniversal surface roughness observed in heterogeneous materials.

We present experimental proof that in rotating ${}_{}{}^3{}_{}{}^{}\mathit{B}$ the vortex-core transition temperature ${\mathit{T}}_{\mathit{V}}$ separates axisymmetric vortices above ${\mathit{T}}_{\mathit{V}}$ from vortices with spontaneously broken axial symmetry below ${\mathit{T}}_{\mathit{V}}$. The nonaxisymmetry is observed in the presence of coherent spin precession as a new soft Goldstone mode, manifested as oscillations and spiral twisting of the core anisotropy axis. These are driven by the precessing spin via spin-orbit coupling and lead to magnetic relaxation from viscous losses, which depend on vortex pinning.

We study transitions to periodic structures in diblock copolymer melts including self-consistently the concentration fluctuations, and expanding the density in multiple harmonics. The fluctuations induce a shift in the maximum scattering wave vector in the isotropic state which suggests that the chains stretch near the transition. Transitions to 3D hexagonal lattices, 2D hexagonally packed cylinders, and bcc structures are predicted. We find the periodicity near the transition: The chains contract in the bcc, while they stretch in the other structures.

We present a simplified polymer model for twist-boundary phases in liquid crystals. The thermodynamic phases of this model are characterized by an angle 2πα̃. There are incommensurate phases with α̃ an irrational number and commensurate phases with α̃=P/Q, with P and Q relatively prime integers. The latter have quasicrystalline symmetry for Q=5 or Q>6. Equilibrium phases with all values of α̃ can be produced by varying a control parameter α. The curve α̃(α) is an incomplete devil’s staircase with finite locking intervals about every rational α̃.

Using the preliminary results for pp¯→WγX from the Collider Detector at Fermilab, we obtain information on the magnetic moment of the W boson. At 90% C.L. we find the bound -9.9≤κ≤12.3, which is consistent with the standard model value κ=1. We also consider the radiative decay W→eνγ.

Low-temperature thermal conductivity and time-dependent specific heat have been measured on mixed crystals of ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$${)}_{\mathit{z}}$${\mathrm{H}}_2$${\mathrm{PO}}_4$ (x=0.35,0.72), in the composition range in which an electric dipole glass state forms below ∼100 K. Glasslike excitations have been observed. In the fully deuterated isomorph with x=0.62, however, the thermal conductivity is that of a crystal, and glasslike excitations are only seen in the specific heat at very long times. This striking isotope effect shows that the low-energy excitations in these crystals relate to tunneling of protons or deuterons, respectively.