Expressions for the zeroth, first, and second translational spectral moments are reported for the rotovibrational collision-induced absorption (RVCIA) spectra of atom-diatom pairs, which take into account the dependence of the intermolecular interaction potential on the vibrational excitation of the molecule involved. From these, numerical results are obtained for the fundamental CIA band of ${\mathrm{H}}_2$He. Consistency with a previous ab initio line-shape computation is observed which has also accounted for this dependence. However, especially at the higher temperatures considered (150–300 K), the first and second moments differ significantly, by up to 30%, from values calculated from existing, commonly used sum formulas which neglect the vibrational-state dependence of the interaction potential. The work thus corrects an existing inexactitude of the moment expressions in general use. When these expressions are combined with suitable model profile functions, a simple and yet very accurate modeling of RVCIA spectra is possible, with rms deviations from exact quantum profiles of no more than a few percent.

The effect of a sudden rotation of electric field on dielectronic recombination cross section has been investigated. Field-induced shifts of the high-Rydberg-state quantum numbers were found to partially explain the discrepancy between theory and the experiment of Müller et al. [Phys. Rev. Lett. 56, 127 (1986)] on Mg ii in the high-n end of the spectrum. The effect of n mixing was estimated for moderate and high values of n, and found to be negligible.

Observed cusp spectra for electron transfer to the continuum (ETC) of 0.6-MeV/u ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{+}$, and ${\mathrm{He}}^{2+}$ ions from hydrocarbon gaseous targets (${\mathrm{CH}}_4$, ${\mathrm{C}}_3$${\mathrm{H}}_6$, and ${\mathrm{C}}_7$${\mathrm{H}}_{16}$) are fitted to a generic expression of Meckbach, Nemirovsky, and Garibotti [Phys. Rev. A 24, 1793 (1981)]. Six leading terms of this formula are given here analytically. They were derived after the convolution of the assumed expression for the ETC cross section with an analytical function that accounts for the electron transmission through the electron spectrometer of specified longitudinal and transverse velocity resolution. With given angles that determine the angular acceptance as instrumental parameters, formulas for these terms are expressed as universal functions of a single variable which is the ratio of electron speed v to ion speed $v_i$. The least-squares fit of these functions to experimental cusps allows for reconstruction of the generic ETC cross section, which is not ‘‘polluted’’ by finite resolution of the electron spectrometer.

Key characteristics, the asymmetry and relative width of deconvoluted cross sections, are discussed. The skewness in observed cusp spectra is traced to the scattering angles that, relative to the spectrometer’s half-angle of acceptance ${\theta }_0$, are small; this is particularly seen in electron capture to continuum (ECC) data, while the spectra dominated by electron loss to continuum (ELC) are more symmetric and less dependent in their asymmetry on the scattering angle θ. The relative full widths at half maximum, (Δv/v${)}_{\mathrm{FWHM}}$, for the deconvoluted ECC cross sections rise significantly faster with the increasing θ than for the ELC cross sections; this difference is obscured in observed cusp spectra by the finite velocity resolution of the spectrometer. Relatively narrow resolution results in the preferential detection of the highly forward scattered electrons whose velocity distribution conforms with the leading term, 1/‖${\mathrm{v}\mathrm{-}\mathrm{v}}_i$‖, for ETC cross sections.

Hence, for all projectile-target combinations, the observed spectra have the relative widths which are of comparable magnitude and in very good agreement with Dettmann’s prediction, i.e., 1.5${\theta }_0$. A comment on wakes as possible contributors to cusps observed in ETC from large hydrocarbon molecules is made.

The polarizability of the ${\mathrm{CO}}_2$ molecule in excited vibrational states has been studied using molecular-beam laser Stark spectroscopy techniques. The analysis yields the polarizability anisotropies Δα=2.215(7) A${̊}^3$ for the 00°1 state, 2.244(7) A${̊}^3$ for the 02°0 state, and the difference of the isotropic polarizabilities (${\alpha }_{00°1}$-${\alpha }_{02°0}$)=-0.0124(8) A${\mathrm{̊}}^3$. .AE

The response to external electric fields of the ${}_{}{}^1{}_{}{}^{}\mathrm{P}_{}^{\mathrm{o}}{}_{}{}^{}$ resonance ‘‘dip’’ in the ${\mathrm{H}}^{\mathrm{-}}$ photodetachment continuum cross section below the n=3 hydrogenic excitation threshold is investigated. Using the relativistic (β=0.806) 650-MeV ${\mathrm{H}}^{\mathrm{-}}$ beam at the Clinton P. Anderson Meson Physics Facility (LAMPF) in Los Alamos, the fourth harmonic (266 nm) of a Nd:YAG laser (where YAG denotes yttrium aluminum garnet) is Doppler shifted to provide a continuously tunable photon beam in the rest frame of the ions. The magnetic field from pulsed Helmholtz coils, surrounding the photon-${\mathrm{H}}^{\mathrm{-}}$ interaction point provides a Lorentz-transformed barycentric electric field. As the magnitude of the external field is increased, there is no observed shift in the photon energy to excite the resonance $E_o$, 12.650±0.001 eV, until quenching occurs at a field strength of 2.36 MV/cm. The effective strength of the resonant state is consistent with a linear decrease with increasing field. The line shape index q is constant within statistical uncertainty, until the resonance disappears. There is evidence that the width of the resonance remains constant at fields below 1.25 MV/cm. It then broadens at higher fields before the resonance quenches.

We present a derivation of closed-form algebraic expressions for two-body operators in the context of the special-unitary-group approach to complex spectroscopy. The method involves the use of a graphical representation of the permutational symmetry-adapted irreducible-representation basis which facilitates the decomposition of the matrix elements into simple factors. The results afford significant reductions of computational complexity in cases pertaining to higher-than-spin-1/2 N-particle systems.

We apply the random-phase approximation to two-photon ionization. We develop a formalism which, by means of screened electron-photon interactions, includes the nonlinear response and, in particular, double-excitation processes. The theory is illustrated by numerical calculations of the two-photon ionization cross section of the external (5p and 5s) shell of xenon, over a large energy range: from the two-photon ionization threshold, in the discrete resonance region, and above the one-photon ionization threshold. We show the influence of different many-electron effects, for example, ground-state and final-state correlation, and linear and nonlinear screening. Compared with an independent-electron picture, screening effects lead to a large reduction of the ionization cross section at low photon energy and in contrast, to an important enhancement at higher photon energy, due to the influence of double-excitation and -ionization processes.

The nonrelativistic formulation of Cherepkov [J. Phys. B 14, 2165 (1981)] of the spin-resolved photoionization cross sections is extended from the Hund’s coupling case (a) to case (c) for the description of the ion molecular state. The spin-polarization parameters are analyzed in terms of a generalization of the partitioning scheme developed by Thiel [Chem. Phys. 77, 103 (1983)] for the study of the β asymmetry parameter. This scheme allows a detailed analysis of different l contributions to spin-polarization parameters and, in the case of a single predominant term, a straightforward evaluation of these parameters. In the framework of the frozen-core static-exchange approximation, this formulation is used for the calculation and the interpretation of the spin-polarization parameters in the case of the πnp outer-shell and d inner-shell photoionizations of the HBr and HI molecules in the energy region of Cooper minima and shape resonances.

A laser with parameters modulated at a frequency f may respond not only at that frequency and its harmonics nf, but also at that of its subharmonics f/n. When the order n of its subharmonics increases indefinitely, the response of the laser becomes irregular, though the system remains deterministic. Many approaches have been used to characterize these different behaviors and the associated attractors, in a ${\mathrm{CO}}_2$ laser containing an elasto-optic modulator. A quite important similitude has been remarked between the bifurcation diagram of the laser and that of the logistic map $x_{n+1\mathrm{}}$=1-μ$x_n^2$. More complex chaotic features have also been observed, e.g., generalized bistability between different attractors, and crisis when a chaotic attractor collides with an unstable periodic cycle. The influence of the rate of change of the driving parameters has also been studied. The experimental results show excellent agreement with those provided by the two-level model of the laser.

The semiclassical quantum-mechanical treatment of Raman scattering with an infrared-active mode is given and solitary wave solutions are derived. Optical attenuation and vibrational damping are taken into account. The soliton properties of the waves are touched upon.

A semiclassical description for dye lasers is proposed, where the energy-level diagram of the dye molecule is assumed to consist of a continuous bandlike ground state and an excited singlet state. Unidirectional single-frequency (s.f.) operation is discussed. The linear-stability analysis for this operation reveals a very low threshold instability, which may appear generally in practical lasers. The ratio of the instability threshold to the lasing threshold may be of any value greater than 1, depending mainly on the bandwidth and the distribution of the dipole moments on the band, but it is independent of the cavity loss. This instability may account for that observed in recent experiments by Hillman, Krasinki, Boyd, and Stroud [Phys. Rev. Lett. 52, 1605 (1984)]. A general approach to analyzing the linear stability of the s.f. operation of the Maxwell-Bloch equations is also proposed, which states that only the eigenvalues of a 2×2 matrix are relevant: one concerns the stability of the s.f. operation near the lasing threshold, the other determines the instability threshold of this operation.

The steady-state spatial polarization distribution of counterpropagating plane waves in a ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ medium is numerically investigated using a Stokes vector formalism. We consider all those material symmetry classes whose equations of motion are thought to be nonintegrable. Poincaré plots indicate that all these cases exhibit chaotic behavior. We also identify new analytic solutions for certain initial conditions. This completes the classification of the polarization dynamics of counterpropagating beams for all rotation symmetries about the axis of propagation, in parity- and non-parity-invariant media.

Interaction of two coherent fields of arbitrary intensity with a general homogeneously broadened three-level molecular system for both an amplifier as well as a laser oscillator under general conditions of pumping is discussed. The amplifier solution is obtained through the time-averaged density-matrix formalism with two coherent fields treated classically. A detailed consideration of both real and imaginary parts of the electric susceptibility at pump and emission frequencies, corresponding to gain and dispersion in the active medium has been taken into account. The results obtained are used to study a number of situations for weak and strong pumping in the laser amplifier. Subsequently these are applied to a mode analysis of a unidirectional single-mode laser oscillator to determine the output characteristics of the laser and to establish conditions under which emission becomes dynamically unstable resulting in sustained oscillatory emission. The results obtained are found to be in good agreement with those of a full numerical analysis of the atomic and field coupled equations describing the system.

An experimental and theoretical study of the temporal correlations between the intensities of a broadband pump laser and Stokes light in stimulated Raman generation has been carried out. Direct measurements have been made of the pump laser and Stokes light intensity autocorrelation functions and of the intensity cross-correlation function between the two. Strong cross correlations are found. Nevertheless, the correlations are not perfect; that is, the Stokes intensity does not follow exactly the intensity variations of the laser. Transient effects and quantum fluctuations are shown, by theoretical modeling, to be responsible for the absence of perfect correlations.

A thermodynamic formalism is developed for a liquid microcluster which is so small that the homogeneous properties of the bulk liquid are not attained even at the center. The system is carefully defined by a mathematical boundary, and then the thermodynamic variables are introduced in such a manner that the interaction of molecules inside and outside the boundary is taken into consideration. It is shown that the variables may be classified into the extensive and the intensive ones and the Gibbs-Duhem relation may also be derived. It is clarified that both the surface tension in Nishioka’s formalism with the equimolecular dividing surface and that in Gibbs’s formalism with the surface of tension represent the reversible works of a very similar nature. Finally, complication due to translation and rotation of a microcluster in vapor is discussed, and the use of the internal free energy of a microcluster to get the cluster-size dependence of the surface tension in Nishioka’s formalism is shown to be reasonable.

Monte Carlo simulation is used to study the elastic properties of the classical hard-sphere solid. The elastic constants $C_{11}$, $C_{12}$, and $C_{44}$ are computed for the fcc lattice structure at the melting density. Poisson’s ratio is computed and is shown to be positive. These ‘‘exact’’ results are compared with those of simple models of the hard-sphere solid.

Theoretical profiles of (3/2)${\omega }_0$ emissions from laser-produced carbon plasmas, at plasma temperatures varying from 600 to 250 eV, corresponding to laser intensities varying from 1.5×${10}^{14}$ to 1.7×${10}^{13}$ W/${\mathrm{cm}}^2$ have been calculated. The peak shifts of these emissions have been compared with the experimental results obtained using a 1.0641-μm Nd:glass laser and they are found to be in good agreement. Theoretical estimates of wavelength shifts taking into account the variation of plasmon wave number have also been compared with the experimental values. Profiles of (3/2)${\omega }_0$ emissions from planar slab targets of carbon, aluminum, and copper have been reliably measured and are compared with the theoretical estimates. Experimental results strongly suggest the saturation of two-plasmon–decay convective instability.

Modulated Rayleigh-Bénard convection is analyzed for high frequencies and large modulation amplitudes. The linear theory of Gershuni and Zhukhovitskii is generalized to the nonlinear domain, and a subcritical bifurcation to convection is found in agreement with the experiments of Niemela and Donnelly. The crossover between the high-frequency (‘‘Stokes layer’’) regime and the low-frequency regime studied previously is analyzed.

In order to study the phase-separation process near the critical point of fluid mixtures (spinodal decomposition) without the influence of the Earth’s gravity, we have used a carefully density-matched system of deuterated cyclohexane, cyclohexane, and methanol. Such a system is known to behave as a real binary fluid [C. Houessou, P. Guenoun, R. Gastaud, F. Perrot, and D. Beysens, Phys. Rev. A 32, 1818 (1985)], and our own previous microgravity experiments have demonstrated that the gravity influence was negligible during the whole separation. A light-scattering analysis has been performed, which provided the three-dimensional structure factor S mainly in the first stages of the separation. For the late stages a necessary alternative is the study of the images of the separating fluid. The interest of such a direct observation lies in the possibility of extracting not only statistical properties analogous to those obtained with the light-scattering technique, but also of analyzing the morphology of the phase-separation process and the motion of interfaces. The origin of such images is not straightforward. It is shown that they reproduce the section of the pattern of interfaces between phases, in a plane located close to the exit window. Because the domains are interconnected, and because only the interfaces nearly parallel to the light direction are detected, this pattern exhibits the same periodicity as that of the domains. From these images, a numerical treatment using video techniques and computer analysis allows a two-dimensional structure factor Ŝ to be obtained. The similarities and differences with the corresponding three-dimensional factor S are discussed. The scaling properties of the phase separation—the time invariance of the reduced structure factors F and F^, of the reduced second moments r and r^, the universal behavior of the typical wave vector $K_m^{\mathrm{*}}$—can be verified in the whole available range, up to the final equilibrium state. Here wetting forces and the residual gravity effects compete to give to the system its equilibrium morphology.

Molecular-dynamics simulations have been carried out on a ‘‘soft-sphere’’ model for binary alloys quenched into supercooled and amorphous states. The main emphasis of the work is on the static and dynamic characterization of the glass transition. A comparison between molecular-dynamics data and the results of a self-consistent integral equation shows that the equation of state bifurcates into ‘‘glass’’ and ‘‘fluid’’ branches below a glass transition temperature $T_g$. The static pair structures differ significantly along the two branches. The structurally relaxed ‘‘fluid’’ branch leads to a phase separation at very low temperatures. Close to the glass transition, the atomic mean-square displacements of the two species go over more and more slowly to the asymptotic diffusive regime, due to the emergence of an intermediate time scale linked to the slowing down of structural relaxation. The diffusion constants of the two species follow closely a scaling law, as predicted by mode-coupling theory, except in the immediate vicinity of the glass transition where activated processes lead to residual diffusion.