The rotational transitions R(1), R(2), and R(3) of HD have been studied in absorption with 0.06-${\mathrm{cm}}^{\mathrm{-}1}$ resolution in mixtures of HD with He, Ne, Ar, Kr, Xe, ${\mathrm{H}}_2$, and ${\mathrm{N}}_2$ at 295 K and at densities between 3 and 62 amagat. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced moment to the allowed moment, showed a dependence on system size and ranged from +10×${10}^{\mathrm{-}3}$ ${\mathrm{amagat}}^{\mathrm{-}1}$ for light perturbers to -24×${10}^{\mathrm{-}3}$ ${\mathrm{amagat}}^{\mathrm{-}1}$ for the heavier ones. These interference effects in the pure rotational band are an order of magnitude smaller than those of the fundamental vibrational band and show a strong J dependence not predicted within current theory. The sign of the isotropic overlap component of the induced moment was determined experimentally in an unambiguous way and its trend was found to be within theoretical expectation; specifically, the sign changes as the mass increases. The line shapes were mostly symmetrical and Lorentzian; asymmetry was observed mainly for the R(1) transition with the low-frequency side of the lines super-Lorentzian and the high-frequency side sub-Lorentzian. Investigation of the effect of collision duration on the autocorrelation function of the allowed moment showed it was not important to the integrated allowed absorption.

Energy-gain spectra for single-electron capture by ${\mathrm{O}}^{2+}$ ions colliding with He have been measured at laboratory energies between 60 and 200 eV, and scattering angles between 0° and 4°. The spectra indicate that the predominant exit channel is the (2$p^3$ ${\mathrm{}}^2$P) excited state of ${\mathrm{O}}^{+}$. Differential cross sections for capture into the ${(}^2$P) state were also measured. The data show an oscillatory structure. The measurements are in qualitative agreement with recent theoretical calculations based on a full quantum-mechanical description.

The coherence and alignment in electron excitation of hydrogen (1s-2p) is studied for electron energies of 35 and 54.4 eV using a two-potential approach. We also study the spin asymmetry in the scattering of spin-polarized electrons with spin-polarized hydrogen atoms and compare our results with other available calculations.

Based on a synthetic scattering function describing the interaction of thermal neutrons with molecular gases, we developed analytical expressions for cross sections of interest in reactor-physics calculations; these are the zero- and first-order angular moments of the double-differential cross section, ${\sigma }_0$($E_0$,E) and ${\sigma }_1$($E_0$,E), respectively, and the total scattering cross section σ($E_0$). The formulas presented here allow then a very fast and reliable evaluation of thermalization parameters and transport coefficients at any desired temperature of the moderating media. One of the main applications of this study is the generation of (thermal) nuclear data libraries for reactor codes, using a minimum set of input information. In the following paper, calculations based on our formulas are compared with experimental data and other theories for some commonly used moderators under different physical conditions.

The formulas derived from the synthetic model which were presented in the preceding paper are here applied to the evaluation of partial and total cross sections at thermal neutron energies for the commonly used moderators ${\mathrm{H}}_2$O, ${\mathrm{D}}_2$O, and ${\mathrm{C}}_6$${\mathrm{H}}_6$. The results of our analytical expressions for the scattering kernels ${\sigma }_0$($E_0$,E) and ${\sigma }_1$($E_0$,E) are compared with standard code calculations for well-separated incident neutron energies $E_0$. The related integral quantities, total cross section σ($E_0$), and average cosine of the scattering angle 〈cosθ($E_0$)〉, respectively, are also compared with experimental data and results from other theories. Within the framework of neutron diffusion theory, transport coefficients are readily evaluated using the model expressions and they are compared with available information over a wide range of temperatures.

In inner-shell photoionization followed by Auger decay, the Auger electron initially screens the ionic Coulomb field ‘‘seen’’ by the photoelectron. This screening phenomenon affects the post-collision interaction that shifts the Auger energy and distorts the line shape. The analytical Auger electron line-shape formula which we derived previously on the basis of asymptotic Coulomb wave functions is now modified to account for the screening effect. The ionic charge seen by the photoelectron is reinterpreted on the basis of asymptotic properties of the continuum wave function pertaining to two outgoing electrons. It is shown that this procedure is consistent with semiclassical models which account for the time required for the Auger electron to overtake the photoelectron. Our modified Coulomb line shapes are found to agree perfectly with the semiclassical line shapes for photon excess energies $E_{\mathrm{exc}\mathrm{\gtrsim }({\Gamma }_i{\varepsilon }_A{)}^{1/2}}$, where ${\Gamma }_i$ is the width of the initial hole state and ${\varepsilon }_A$, the Auger electron energy. Agreement between the present theory and recent synchrotron-radiation measurements consequently becomes excellent in this range of photon energies as well.

We consider an atomic beam at normal incidence with a standing wave of light with phase fluctuations and we evaluate the momentum distribution of the diffracted atoms after passing through the light beam. We consider the independent-increment model or the random-jump model for the phase of the light, and the random-jump model for the light frequency. In each of these three cases we distinguish the situation of independent or identical phases of the two counter-running light waves that compose the standing wave. For identical phases, the momentum distribution seems to be sensitive mainly to the spectrum of the light, whereas for independent phases higher correlation functions are also important.

Cross sections for the elastic scattering of 88.03-keV γ rays through 125° by aluminum, gold, lead, and bismuth have been measured with a semiconductor detector. Simultaneously, information has been obtained regarding resonance Raman scattering and Compton scattering in the case of bismuth, and K-shell photoeffect cross sections in the case of gold and lead. Values accurate to about 8% were determined with the help of a normalization technique relying on a comparison with the Compton scattering counts from the low-atomic-number aluminum target. The elastic scattering cross section of aluminum at the chosen photon energy, which is far above the aluminum K-shell threshold, is in reasonable accord with form-factor and S-matrix calculations based on the independent-particle approximation. For gold, 7.3 keV above its threshold, the experiment agrees with the S-matrix prediction. However, for lead some 25 eV above threshold, the experimental value is about 40% larger than predicted, while for bismuth (2.5 keV below threshold), experiment lies 70% above theory. The measured K-shell photoeffect cross section of lead is within 10% of the prediction based on the independent-particle approximation.

We investigate different processes which may be responsible for ejection of two electrons from the external shell of xenon through the absorption of two and three photons, namely two-photon direct double ionization and three-photon sequential double ionization via the ion in an excited state or in the ground state. We calculate the cross sections of the ionization processes involved, using many-body perturbation theory. These cross sections are then used within a rate equation model for comparing the relative efficiencies of the different double-ionization mechanisms.

We report a quantitative investigation of competition between dissociation and ionization in four-photon excited ${\mathrm{H}}_2$. The C ${}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ v=0–4, N=1–3 states are prepared by three-photon excitation from the ground state. Absorption of a fourth photon from the same laser pulse leads either to ${\mathrm{H}}_2$${\mathrm{}}^{+}$+e or H+H(n=3,4). This branching is monitored in photoelectron spectra, taken with a novel magnetic bottle electron spectrometer based on a permanent magnet design. The competition between dissociation and ionization shows a pronounced dependence on the symmetry, vibrational, and rotational quantum numbers of the C state. We attempt to explain this dependence in terms of excitation of dissociative ${\mathrm{H}}_2$ states in the ${\mathrm{H}}_2$${\mathrm{}}^{+}$ ionization continuum from which both autoionization and dissociation may occur.

We demonstrate the existence of Hanle resonances in four-wave mixing with pump and probe fields derived from the same fluctuating source. A theory of these fluctuation-induced Hanle resonances is developed following the work of Agarwal and Kunasz [Phys. Rev. A 27, 996 (1983)] on four-wave mixing in stochastic fields. Such Hanle resonances are compared with those induced by dephasing collisions.

The time-space behavior of the charges produced in typical laser multiphoton ionization experiments is analyzed from the point of view of the fluid approximation. On the basis of analytic methods, first the equation system describing the one-dimensional motion of a single-charged fluid in an external uniform and constant electric field is solved. It is shown that the solution can be extended to the three-dimensional case, also including momentum-exchange collisions and time-varying electric field and temperature. Moreover, the method is extended to analyze the motion of both ions and electrons, coupled by the self-generated electric field. Finally, the consequences on the time-space behavior of the collected current, in typical multiphoton ionization experiments, are pointed out and the range of validity also discussed.

A phenomenological (classical) and quantum treatment of the laser light-intensity-induced Faraday effect in isotropic media and crystals is given. The effect, being universal (occurring in all matter), is shown to act as a stimulator controlling the self-squeezing of light: depending on the helicity of circular polarization of the light wave and on the sense of the dc magnetic field, the sign of the pseudoscalar parameter of nonlinear coupling changes leading to an enhancement or weakening of self-squeezing. Moreover, by varying the state of polarization of the light wave or the magnetic field strength, control of the efficiency of light-squeezing generation can be achieved.

We have measured the coexistence curve of ethylene, ${\mathrm{C}}_2$${\mathrm{H}}_4$, near the liquid-vapor critical point using two optical techniques. The data cover a reduced temperature range 1.5×${10}^{\mathrm{-}6}$${10}^{\mathrm{-}2}$, where t=($T_c$-T)/$T_c$ and $T_c$ is the critical temperature. The data are analyzed using a power series incorporating corrections-to-scaling terms to give values for the critical exponent β and the corrections-to-scaling exponent Δ; we find β=0.327(2) and Δ=0.46(4). Some limitations to the usefulness of this analysis procedure are discussed.

We have measured profiles of optically thin Balmer α and γ lines of He ii at 164 and 108.5 nm in a homogeneous hydrogen plasma using a microchannel-plate detector. The electron density of the plasma is between 5×${10}^{23}$ and 8×${10}^{23}$ ${\mathrm{m}}^{\mathrm{-}3}$ at temperatures of about 10 eV. The measured line profiles and full widths at half maximum are compared with various calculations and measurements. We find agreement for the Balmer α line within the experimental errors of approximately 25%. The Balmer γ line is less broadened than expected from the calculation of Kepple, as found in two earlier experiments.

The disorder-order transition observed in aqueous suspension of charged macroions is investigated with use of both simulations and a simple theoretical model. Molecular-dynamics (MD) simulations were performed by assuming that the interaction between the charge stabilized particles interact via a modified Debye-Hückel potential. The parameters of the potential were chosen to mimic the experimental study of Lindsay and Chaikin. Simulation results predict that the liquid freezes into a bcc phase in accord with the experimental findings. For comparison, the phase diagram with use of the self-consistent phonon theory (SCP) is presented. It is shown that the predictions of the SCP theory in the weak screening limit are in disagreement with both the MD results and the experiments. Possible reasons for this failure are pointed out. Finally, similar calculations have been carried out for the Yukawa potential. It is found that for the parameters considered here, this is an unphysical model for the suspensions of polystyrene spheres. No evidence for reentrant transitions is found in either of these models. This is in agreement with the predictions of the self-consistent phonon theory.

The ‘‘Coulomb logarithm’’ for inverse-bremsstrahlung laser absorption is examined for plasmas of different ionic charge, spanning the classical and quantum-mechanical limits. Previously, this term has not been calculated exactly for the conditions of interest in laser fusion experiments; it has only been estimated from physical considerations. For short-wavelength irradiation (e.g., 0.35 μm), uncertainties in the ‘‘logarithmic’’ factor can produce variations of 20–50 % in the laser absorption coefficient. A more exact treatment of this term is presented here. For low-Z plasmas, a modified Born approximation is used that reproduces previous results for long-range interactions that cannot be described by a single electron-ion collision, and it simultaneously treats the short-range electron-ion encounters. For high-Z plasmas, the Coulomb logarithm is calculated in terms of the classical, nonlinear electron trajectory in a self-consistent electrostatic potential; strong ion-ion correlations are treated by the nonlinear Debye-Hückel model. There are no indeterminate quantities in the calculations.

The optogalvanic effect induced in a Hg-Ar discharge by light at 546.1 nm is studied experimentally and theoretically. The plasma and its response to a disturbance in the atomic-level populations are described entirely in terms of electron collision cross sections and other fundamental parameters. In a detailed description of the laser interaction with the atomic system, it was necessary to take into account velocity-changing collisions (VCC). These collisions (VCC) strongly modify the interaction between the Doppler-broadened system and the optical field. In some Doppler-free optogalvanic spectra the dramatic influence of VCC on the velocity distribution of atomic-level population is evident. Measurements of the magnitude of the optogalvanic signal confirm most of the theoretical results. Some anomalous features of the impedance change due to a weak beam were observed when a strong counterpropagating beam was introduced.

We have calculated a weak localization contribution to the diffusion constant of electrons in a gas. The contribution arises from the quantum interference between pairs of electron trajectories which are in the form of a closed loop traversed in opposite time directions. The contribution is determined by the mean free path of the electrons due to their collisions with the atoms as well as by the mean variation of electron energy in the course of such collisions. Both quantities, as well as the total weak localization contribution, depend on the density and on the temperature of the gas. The quantum contribution can be separated from the leading classical term by application of a relatively weak magnetic field. We have obtained formulas describing its variation as a function of the magnetic field. Our estimates show that at room temperature and for a density corresponding to a pressure of some 10 atm the relative magnitude of the quantum contribution can be of the order of several per thousand.

A family of extrapolants somewhat more general than Padé approximants is presented and applied to the low-density ground-state energy expansions which systematic quantum-field-theoretic infinite-partial-summation techniques have produced for many-boson and many-fermion systems of hard spheres. The existence of a quantum close-packing ultimate system density, as well as monotonic-increasing energy as function of density, is assumed in narrowing down the possible extrapolants appropriate for moderate to high densities. Of those remaining, excellent agreement is found at physical densities, and almost unique selections at higher densities, which are consistent both with available Green-function Monte Carlo data as well as with a feasible overall picture of ultimate-density pole placements.