An observation of the simultaneous emission of a characteristic K x ray and a continuum photon has been made for 70-keV electrons bombarding target atoms of Fe, Cu, Y, and Ag. The absolute cross section for Y and Ag agree well with a model that considers a contribution from both double-bremsstrahlung and electron-electron bremsstrahlung processes. The data for Fe and Cu, on the other hand, are about an order of magnitude higher than the model, suggesting the possibility of a resonant polarization bremsstrahlung contribution to the double-bremsstrahlung process.

It is shown that the appearance of the plateau on the atomic electron density n(r) gives rise to the dip in the photoabsorption cross section of a many-electron atom. The dip at frequency ω is formed if (a) there is a point of plasma resonance ${\mathit{r}}_0$ where the local plasma frequency inside the atom is equal to ω or (b) the derivative of electron density at this point equals zero: n’(${\mathit{r}}_0$)=0. Another result of the appearance of the plateau on electron-density distribution is the effect of complete screening of the external electric field inside the atom behind the resonance point ${\mathit{r}}_0$.

On the basis of a classical treatment, we present analytical and numerical evidence that Rydberg atoms in a strong laser field are stable provided the orbital momentum is sufficiently large. We surmise that this effect persists in the quantum case also and we suggest its observation in laboratory experiments. The possibility that such atoms may radiate high-frequency photons is discussed.

We put forward a simple, feasible scheme for the preparation and subsequent detection of macroscopic quantum superposition (MQS) states. It is based on the two-photon model which obtains when a cascade of two atomic transitions is resonant with twice the field frequency. The initial conditions amount to a field in a mixed state characteristic of lasers or masers and an excited atom. The MQS is generated by a conditional measurement of the atomic excitation after an interaction time that determines the relative phase of the MQS components. Remarkably, the MQS is subsequently detected and its phase is inferred by measuring the excitation probability of a second, ‘‘probe,’’ atom, as a function of its interaction time. The realization of the scheme in the optical domain, using dielectric microspheres, is discussed.

Fluctuation phenomena, to be anticipated in all systems for which the internal fluctuating variable is a multivalued function of the observable quantity, are reported. Analog electronic experiments on a model optically bistable Fabry-Pérot cavity have shown that the observable may be described in terms of a Boltzmann distribution with a multibranch effective potential. The distribution was observed to possess up to four peaks (two twin peaks) within the range of bistability, and to become singular at outer boundaries corresponding to the branching points of the potential. The experimental results are compared with theory.

We propose directed percolation as a generic mechanism for interface pinning in a two-dimensional medium with quenched point disorder. The roughness of the interface just below a critical depinning transition is found to obey a power law with an exponent ${\mathrm{\zeta }}_{\mathit{c}}$=${\nu }_{\mathrm{\perp }}$/${\nu }_{\mathrm{\parallel }}$≃0.63. A moving interface near the transition is not self-affine, but may nevertheless be assigned an effective roughness exponent comparable to the values determined in fluid displacement experiments in porous media.

We report measurements of the interface formed when a wet front propagates in paper by imbibition and we find anomalous roughening with exponent α=0.63±0.04. We also formulate an imbibition model that agrees with the experimental morphology. The main ingredient of the model is the propagation and pinning of a self-affine interface in the presence of quenched disorder, with erosion of overhangs. By relating our model to directed percolation, we find α≃0.63.

The reflection coefficient of linear traveling waves in binary-fluid convection is calculated for the experimental situation of rigid, impermeable boundaries. Results for a range of parameters of interest to experiments in ethanol-water mixtures are displayed and compared with experiment.

Interaction of solitons belonging to different modes is analyzed in the framework of a system of nonlinear Schrödinger equations with incoherent and coherent nonlinear couplings with different group velocities. It is demonstrated that the two solitons can form a strongly bound state with coinciding centers and several weakly bound states with far-separated centers. The bound states of the latter type can be produced only by the coherent nonlinear coupling, provided it is stronger than the incoherent one. The results obtained are employed to explain qualitatively recent experiments with interactions of solitary pulses in the subcritical traveling-wave convection.

Time-resolved light scattering was used to study the time evolution of phase separation in a critical mixture of 2,5-hexanediol and benzene in the spinodal region. The early-stage behavior of spinodal decomposition is observed. The existence of the time region where the linearized theory of spinodal decomposition is valid is ascertained for the simple binary liquid mixture.

Water injected into aqueous solutions of polyethylene oxide in a Hele-Shaw cell shows a wide variety of patterns as four control parameters are independently varied. Despite the strong variation of patterns, both the initial unstable length scale and the mean and standard deviation of Fourier modes of averaged late-time patterns show a smooth variation with a dimensionless number constructed from the ratio of the imposed shear time scale to the polymer-relaxation time.

We have experimentally observed a dense-branching morphology (DBM) in an ${\mathrm{NH}}_4$Cl dendritic crystal growth. The DBM forms a randomly and densely tip-split structure, and the envelope of interface is stable. The measured interfacial velocity of our DBM decreases with a supersaturation. This behavior stabilizes the interface of DBM. The averaged length between tips corresponds to the diffusion length.

Trivelpiece-Gould modes (TG modes) have been observed with frequencies up to the background plasma frequency in a plasma-filled X-band backward wave oscillator. The TG mode emission was broadband with observed frequencies ranging from 2.6 GHz, the lower limit of our frequency diagnostic, to ${\mathit{f}}_{\mathit{p}}$, the plasma frequency. Mode competition between the TG modes and the X-band ${\mathrm{TM}}_{01}$ mode prevailed when the background plasma density was below 6×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$ (${\mathit{f}}_{\mathit{p}}$∼7 GHz). At a critical background plasma density of ${\mathit{n}}_{\mathrm{cr}}$∼8×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$ (${\mathit{f}}_{\mathit{p}}$∼8 GHz) a sudden power enhancement appeared simultaneously in both X band and the TG modes. Power enhancements in S and J bands were up to 8 and 3 dB, respectively, with the X band enhanced up to a factor of 6.

Coherent transition radiation has been observed in the wavelength range from 0.5 to 5.5 mm emitted from bunched electrons of 42 MeV passing through an Al foil in vacuum. The radiation intensity at λ=4 mm is enhanced by a factor of 3×${10}^6$ in comparison with incoherent transition radiation. The intensity is proportional to the square of the beam current, and it increases with the emission length, or the length of the trajectory of the electrons between the foil and a mirror to observe the radiation. The divergence of radiation increases with the wavelength and it is qualitatively in agreement with theory. A sheet of Eccosorb is also used as a radiator, and a similar spectrum to that from the Al foil has been observed. The bunch shape is discussed on the basis of a bunch form factor derived from the spectrum.

We have measured the dielectric properties of concentrated solutions and gels (≤30 mg/ml) of random-sequenced DNA from E. coli in the 400-MHz–26-GHz range. Two Debye-type relaxations are evident, one with a relaxation time near 9 ps and attributable to the classical Debye relaxation of water. More noteworthy is a second relaxation process with a characteristic time in the 20–200-ps range, i.e., a relaxation frequency in the 0.8–8-GHz range, depending upon the species of the counterions and the temperature. The slower relaxation process has an enthalpy of 3.3 kcal/mol and is accounted for by a counterion-based relaxation process. These experimental results are considered in terms of two models from polyelectrolyte theory, one by Oosawa [Biopolymers 9, 677 (1970)] and Wyllie [in Dielectric and Related Molecular Processes, edited by M. Davies (American Chemical Society, Washington, D.C., 1972), Vol. 1], and the other by Manning [Q. Rev. Biophys. 11, 179 (1978); Acc. Chem. Res. 12, 443 (1979)], and we propose that different ion-based relaxation mechanisms dominate in different hydration regimes.

We simulate the expolration in conformation space of an RNA molecule with random uncorrelated sequence and subject to intrachain Watson-Crick-type interactions. The folding pathway that leads to the equilibrium conformation follows consecutively two relaxation regimes, both rooted in a random-energy model. The first regime is kinetically controlled and, thereafter, detailed balance prevails.

We prove here that the eigenvalues, eigenfunctions, and time evolutions for a broad class of spatially bounded, finite particle number, undriven, quantum systems can be computed by algorithms containing logarithmically less information than the quantities themselves. Algorithmic complexity theory asserts that such quantal systems are nonchaotic. These results are shown to be valid independent of the size of system parameters such as mass or Planck’s constant, provided they are not set equal to zero or infinity. However, rather than confronting quantum mechanics with classical mechanics via the correspondence principle, we suggest a direct comparison of quantum mechanics with macroscopic laboratory reality. Specifically, we here propose the double pendulum—a simple, two-degree-of-freedom, macroscopic system exhibiting a transition to chaos as its amplitude increases—as a model suitable for testing whether a nonchaotic quantum mechanics can accurately predict laboratory observation. Neither theory nor experiment appears to offer insurmountable difficulties to the performance of this comparison.

There is a long-range parity-nonconserving (PNC) interaction between a charged particle and a composite system (atom, nucleus) induced by the usual contact weak interaction inside this system: U=(H×E)⋅J${\mathrm{\beta }}_{\mathit{v}}$∼1/${\mathit{R}}^4$ or 1/${\mathit{R}}^5$, where R is distance, E and H are electric and magnetic fields of the particle, and J and ${\mathrm{\beta }}_{\mathit{v}}$ are the angular momentum and PNC vector polarizability of the system. The mechanisms of enhancement of PNC vector polarizability are considered. PNC effects in atoms, molecules, and solids are discussed (P-odd and T-even electric dipole moment, P-odd electric field, etc.).

We analyze the quantum version of the Landau-Lifshitz equation for damped spin motion using a properly tailored Gisin approach to the dissipative quantum mechanics. Coherent- and squeezed-spin-state evolution for a single spin S and a ferromagnetically coupled spin chain are discussed in detail. The classical limit of the quantum dissipative dynamics is shown to be equivalent to the conventional description, which uses only the diagonal matrix elements for spin operators. The homogeneous and wave-vector-dependent decay of a squeezing parameter in the neighborhood of the ground state is discussed and shown to exhibit long-wavelength instability.

The two-center shell-model potential admits a two-component eigenfunction due to the presence of the spin-orbit coupling. We carry out a detailed numerical study of its variation with the shape of the potential and also investigate the applicability of the criteria commonly used to distinguish regular and irregular states in systems that possess a clear classical analog. Our results corroborate similar studies on spectral fluctuations. The occurrence of scarred states and a Gaussian amplitude distribution strongly indicate the presence of an underlying classical dynamics. Moreover, since the spin-up and spin-down spatial components of each eigenfunction are significantly dissimilar, there seem to be two sets of closed orbits that give rise to the same spectrum but influence the wave functions in different ways.