The time-resolved cooperative emission from a system of correlated neutral dissociation fragments, or molecular collision products in beams, is investigated. The investigation is focused on emission at large fragment separations (between 1 nm and a few emission wavelengths), exceeding the domain of short-range interactions within the reactive or collisional molecular complex. A master-equation approach is used to obtain a general expression for the cooperative emission rate, which consists of nonexponential decay factors multiplied by temporal ringing patterns. These features result from the time-dependent radiative coupling between the receding fragments; they depend in an essential manner on the initial electronic state of the parent molecular complex and its symmetry which determine the correlations between the fragments. In the model system of a pair of identical two-level fragments two cases are considered separately. (a) A single photon shared by the fragments, where the emission is initially superradiant or subradiant (radiation trapping), depending on the spin and inversion symmetry of the parent molecular system and of the nascent fragments. The ringing pattern depends on the electronic angular momentum state of the parent molecule and on the polarization of the emitted light. (Such a ringing has been observed recently by Grangier, Aspect, and Vigué [Phys. Rev. Lett. 54, 418 (1985)] in the emission of photodissociated ${\mathrm{Ca}}_2$.) (b) Two initially excited fragments, where the ringing pattern is of smaller amplitude, and is weakly dependent on the electronic angular momentum of the parent molecule. All the aforementioned cooperative features generally last until the fragments recede several radiation wavelengths away from each other. The application of this time-resolved analysis to various diagnostic problems is discussed, especially with regard to the identification of excited electronic states of the parent molecular complex, and the stereospecificity of the fragmental orbitals.

We investigate the dynamics of two-component solitary waves in the Antonchenko-Davydov-Zolotariuk model for hydrogen-bonded chains. Two-component solitonlike solutions are found numerically in the whole velocity range between 0 and the characteristic velocity $v_0$, while for v>$v_0$ energy flows continuously from the proton sublattice to the heavy ions so that free solitary waves do not exist. The results are in good agreement with a theoretical analysis that treats the heavy-ion motion as forced by a kink in the proton sublattice. We investigate numerically the dynamics of the ionic defects in the presence of an external force and damping. Abrupt discontinuities and hysteresis phenomena are observed. They are quantitatively explained by our theoretical analysis.

It is shown in the present paper that if operators of a boson Green’s function involve arbitrary c numbers, then the corresponding correlation function obtained from the spectral theorem is independent of the c numbers, which is in contradiction with the definition of the said correlation function. In order to solve this problem we have introduced an undetermined-constant method and applied it to ferromagnetic and ferroelectric systems, respectively. It is pointed out that the introduction of the undetermined constant is an intrinsic characteristic of the boson Green’s-function method and that contradictory results can often be obtained if it is not introduced.

The Mellin transform of the correlation integral is introduced and proved to be equal to the energy integral whose divergence abscissa is a lower bound to the Hausdorff dimension. For some Julia sets exact results are obtained. For the linear Cantor sets on the real axis it is shown that the energy integral is meromorphic, and the real pole, determining the divergence abscissa, has a sequence of satellite poles equally spaced on a line parallel to the imaginary axis, which explain the oscillations observed in numerical calculations of the correlation integral. The order-d generalized energy integrals are introduced as Mellin transforms of the order-d correlation integrals and for the Cantor sets they are proved to have the same singularities as the ordinary energy integrals. Letting $r_d$ be the residue of the real pole corresponding to the divergence abscissa it is proved that ${\lim }_{\mathrm{d}\to \mathrm{\infty }}$(-${\mathrm{d}}^{\mathrm{-}1}$ln$r_d$) is the second Renyi entropy. Some numerical results obtained for the energy integrals are discussed.

The observation of no photons emitted by a fluorescing multilevel atom dramatically affects its future evolution. This collapse of the quantum state due to measurements with a null result is the cause of intermittent atomic fluorescence even when the exciting field is arbitrarily coherent.

The classification of physical systems according to their level density is studied. Here a large class of (bounded and unbounded) Lanczos-type Hamiltonians which have a very general level-density function are identified. This function is given in integral form and only depends on a few parameters which govern the asymptotic behavior of the matrix elements. These may behave differently according to whether the index is even or odd. Several previous findings of various authors are found as particular cases. Enlarged subclasses of Hamiltonians having for their level density functions of regular, uniform, Wigner, Weyl, Karamata, and hypergeometric types are explicitly given. There exist a rich variety of physical systems characterized by these kinds of Hamiltonians.

The elastic cross section for the scattering of hydrogen atoms by electron impact has been calculated using a perturbative approach. Two eigenstates (1s,2s) and two pseudostates (2p¯,3s¯) have been employed to calculate the second-order term. Higher-order terms up to five are evaluated using eikonal approximations. The effect of exchange has been included using the Born-Oppenheimer and Born-exchange approximations. The scattering parameters in the energy range 30–680 eV are reported. Results are found to be in excellent agreement with the measured values for incident energies E≥100 eV. Results have also been compared with recent theoretical predictions.

Extensive study is made to obtain the total γ-ray cross sections with an accuracy close to 1% in Pb at 276.4, 295.96, 302.86, 316.508, 356.04, 383.86, 468.07, 484.58, 588.584, 604.414, and 612.465 keV. The basic idea behind these measurements is that the data available on total γ-ray cross sections are rather meager, especially at higher energies with γ-ray sources showing highly complex energy distribution. For measuring the cross sections, a transmission experiment is conducted using a good geometry setup and a high-resolution hyperpure germanium detector. In a transmission experiment, there is a chance of underestimation of these cross sections due to small-angle scattering. Photopeaks evaluated must be free from scattering contributions as well as background. Measures are taken to account correctly for the included small-angle scattering. The measured values are compared with the available theoretical, semiexperimental and other experimental data and are in good agreement. The effect of collimation and good geometrical arrangements on the cross section is discussed.

It is shown that for circularly polarized incident light, the 2ω scattered radiation is circularly polarized for small scattering angles, θ; for θ=π/2, the scattered radiation is plane polarized. In the general case, the scattered 2ω light vector traces out an ellipse with eccentricity sinθ and is elliptically polarized.

We recently presented nonequilibrium molecular dynamics simulations of coupled heat and matter transport in a binary liquid mixture. While these simulations were carried out exactly as indicated, the relations given between the microscopic and macroscopic formalism were strictly wrong. Here we correct these errors and indicate how they limit the possibilities for comparison of our simulation results with experiment.

The effective ionization and recombination rate coefficients Ar xii–xvi were obtained by studying the modulation of line emission by sawtooth oscillations in the Texas Experimental tokamak (TEXT). Experimental data are matched with the simulation of argon ion emission by a one-dimensional code. The rate coefficients were determined for $T_e$∼400 eV and $N_e$∼4×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The uncertainty of ionization rates is estimated to be less than 30%, while the accuracy of the recombination rates is better than a factor of 2.

It is shown that the magnetic-electron-drift vortex wave can parametrically decay into a daughter wave and an ion-acoustic wave in a nonuniform unmagnetized plasma. From the fluid and Maxwell equations, a pair of coupled nonlinear equations is derived. A normal-mode analysis leads to the nonlinear dispersion relation which governs the decay and modulational instabilities. The growth rate of the decay interaction is presented.

The statistical uncertainty in the calculation of velocity autocorrelation functions and self-diffusion coefficients from molecular dynamics is empirically determined from the spread of the molecular-dynamics results for an ensemble of macroscopically identical systems. The ‘‘experimental’’ uncertainties of the velocity-autocorrelation-function values at equilibrium and in the presence of flow agree well with theoretical predictions. The uncertainty of the self-diffusion coefficient is found to decrease as the inverse square root of the averaging time.

We present a molecular-dynamics simulation of a two-dimensional system of coupled rotators [O(2) planar Heisenberg model]. We observe that if the maximal characteristic Lyapunov exponent for the system is plotted versus the temperature, a clear-cut ‘‘knee’’ appears precisely in correspondence with the Kosterlitz-Thouless transition temperature. A new order parameter is therefore available which reflects the marked change in the stochasticity properties of the motion of the system as the temperature is varied in the vicinity of the critical temperature.

Explicit solutions are obtained for evolution equations for explosively unstable situations. These solutions include the effects of diffusion with linear or quadratic density dependence of the diffusion coefficient. As a result of balance between the diffusion and nonlinear terms, explosive growth in time can occur with a preservation in shape of certain spatial distributions. The solutions are generalized to cases of two interacting populations.

The electron affinity of (${\mathrm{H}}_2$O${)}_2$${\mathrm{}}^{\mathrm{-}}$ is estimated to be 17 meV, based on electric-field-detachment measurements. The shift of the electric field necessary for electron detachment from (${\mathrm{H}}_2$O${)}_2$${\mathrm{}}^{\mathrm{-}}$ clustered with Ar, Kr, and Xe is interpreted in terms of Fermi’s theory of the pressure shift of Rydberg spectra, extended to a dipole-bound electron. The implications of these results for electron localization in gaseous and bulk water are briefly discussed.

A simple combination of quantum defect theory with a nonperturbative Fermi-type analysis is shown to reliably determine a class of molecular Born-Oppenheimer potential curves. Applications to alkali-metal-atom–rare-gas-atom dimers verify that their potential curves are determined to semiquantitative accuracy using two pieces of information: the alkali-atom quantum defects and the electron–rare-gas scattering phase shifts.

We have compared the impact of loss, pump, and frequency modulations on a single-mode homogeneously broadened laser under varying detunings and modulation frequencies. In the case of loss and gain modulations the impact on intensity shows a minimal behavior at the line center for gain values more than twice the lasing threshold. The system relaxation time with respect to any transient perturbation depends on the magnitude of detuning and is minimal at the line center. In the case of frequency modulation, the impact on intensity vanishes at the line center irrespective of the gain values. These results are based on linear analysis.

Recent calculations for the elastic properties of the hard-sphere crystal gave quite peculiar results, including negative values for the Poisson’s ratio. Here we claim that these results were due to the density-functional expansion used in these calculations. We report the elastic constants given in a different functional approach, which are in full disagreement with the previous calculations.

The covariant coupled equations for plasma dynamics and the Maxwell field are expressed as a phase-space–Lagrangian action principle. The linear interaction is transformed to the bilinear beat Hamiltonian by a gauge-invariant Lagrangian Lie transform. The result yields the generalized linear susceptibility directly.