We discuss a model of selective absorption of infrared radiation by an anharmonic molecular vibrational mode. The effects of the rotational sublevels and of the energy dissipation mechanisms are described phenomenologically. We show that in the context of our model the absorption process is described by coupled nonlinear equations for the complex vibrational amplitude and for the average excitation quantum number. Two distinct dynamic regimes (coherent and adiabatic limit) can be identified depending on the relative magnitude of the rotational relaxation time and of the laser pulse duration.

The ${}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ excited states of the cadmium ion are produced through photoionization of cadmium atoms by a beam of Hei 584-Å radiation. The presence of alignment in the ions is detected by measuring the degree of linear polarization $P$ of the resulting emission. It is found that $P({}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{})=-0.052\mathrm{}\pm 0.005$ for the 4416-Å transition, and $P({}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{})=+0.12\mathrm{}\pm 0.04$ for the 3536-Å transition, demonstrating that both ${}_{}{}^2{}_{}{}^{}D$ fine-structure components are aligned. A simple calculation is presented which ignores anisotropic final-state interactions. This calculation correctly predicts the sign of the degree of polarization, but fails to reproduce the experimental results.

We have measured absolute values of the cross sections for ejection of electrons from helium gas by 100- and 200-eV electrons. These cross sections were measured for emission angles from 10° to 150° and for electron energies from 3 eV to a value equal to the primary energy minus the ionization potential. The measurements were made using a static-gas target and an electrostatic analyzer. The resulting cross sections agree reasonably well with those of Opal, Peterson, and Beaty near 90°, but show the need for an adjustment of their angular distributions. Comparison with Born-approximation calculations of Manson et al. at 2 keV indicate that the cross sections of Opal et al. should be divided by $0.53+0.47sin\theta$. A somewhat larger correction is needed to bring their data into agreement with the results of this experiment. The adjustment in the angular distributions also affects the cross sections integrated over all angles.

Excitation energies and line strengths for the low-lying states of ions in the Mg isoelectronic sequence are studied using relativistic multiconfiguration Hartree-Fock techniques. Line strengths for all possible electric dipole transitions between these low-lying states are calculated in both the length gauge and in the velocity gauge along the sequence from Mg to ${\mathrm{U}}^{+80\mathrm{}}$, and the results show good agreement between these two forms of line strength for all cases considered. At low $Z$, the results of the present work are in good agreement with other recent theoretical calculations. The results of this work are expected to give accurate excitation energies and line strengths at high $Z$ as well since relativistic effects are treated nonperturbatively.

The fine-structure intervals of the Na $11f$, $13f$, and $14f$ states have been measured by a microwave-resonance method yielding values of 10.52(21), 6.34(12), and 5.02(8) MHz, respectively. These values are ∼5% lower than the theoretical values for the corresponding hydrogenic states. We have also investigated the use of Ramsey's method of separated oscillatory fields as a means of obtaining narrower resonances.

The Wilcox-Lamb method of deriving rate equations from Bloch equations is outlined using as an example a two-level atomic model. Computer-generated plots directly comparing solutions of these rate equations and of Bloch equations for a three-level atomic model are illustrated. Two cases of interest are discussed: copropagating and counterpropagating laser beams. Some comments concerning collisional accessing and ionizing of atoms in a very broad Doppler profile are made.

A $Z$-expansion procedure is used to obtain eigenvalues and eigenfunctions for the $1s^{2}2p{}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}$, $1s2p^2{}_{}{}^2{}_{}{}^{}P$, and $1s2p^2{}_{}{}^2{}_{}{}^{}D$ states of the lithium sequence. Expansions in inverse powers of the nuclear charge $Z$ are obtained also for the dipole-matrix elements and oscillator strengths of the $1s^{2}2p{}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}-1s2\mathrm{}{p}^2{}_{}{}^2{}_{}{}^{}P$ and $1s^{2}2p{}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}=1s2\mathrm{}{p}^2{}_{}{}^2{}_{}{}^{}D$ transitions, valid for $Z>\mathrm{}3\mathrm{}$. A separate calculation was performed for lithium. The calculated transition probabilities agree with the Hartree-Fock values of Gabriel for large $Z$, but there are considerable discrepancies with the experimental data.

A previously developed diagrammatic method for calculating the nonlinear response of a dilute gas at near resonance with strong classical radiation fields is extended to include velocity-dependent homogeneous and inhomogeneous broadening phenomena. The method is applied to two-level saturated-absorption (Lamb-dip) spectroscopy. Expressions are obtained for the collision-broadened Lamb dip probed by a weak field in the presence of an arbitrarily strong saturating field and are compared to the usual infinite-Doppler-width, weak-saturation expression. We study the effects of velocity and momentum transfer on the line-shape parameters and discuss the effect of phase-changing and strong inelastic collisions on the intensity.

The cooperative spontaneous emission from a system of $N$ identical two-level atoms, excited arbitrarily and interacting with a single mode of radiation, is studied by using the Bonifacio-Schwendimann-Haake approach. The evolution of the probability distribution of the atomic system is considered as a Markov process described by a time-evolution matrix (TEM). The exact analytic expressions for the elements of this TEM are obtained by solving the superradiance master equation. It is proved that the TEM satisfies the normalization condition and the Chapman-Kolmogorov equation as required. The exact general formulas for the expectation values of various physical observables, expressed in terms of an arbitrary initial distribution, are obtained. Finally, these results are applied to a specific example, the superradiant initial state.

The light drag in a rotating fused-silica disk inserted in a He-Ne ring laser was measured at $\lambda =633\mathrm{}$ nm and found to be ${\alpha }_e=0.5424\mathrm{}$ with an rms error ($1\sigma$) of ±2 × ${10}^{-4\mathrm{}}$, including the known error sources. This constitutes a tenfold improvement in accuracy over a previous result. While the given error margin does not include the value given by the Lorentz drag in a linear arrangement, ${\alpha }_{\mathrm{LO}}=0.54182\mathrm{}$, the measured difference of $2\sigma$ is deemed too small to claim a discrepancy.

The scattering of divergent electromagnetic radiation by a dielectric sphere is obtained by assuming that the radiation emanates from an oscillating dipole located an appropriate distance outside of the sphere. The field differs considerably from the case of parallel incident radiation when the dipole is within about ten particle radii, an effect which may be significant for multiple scattering in dense particulate systems.

The first observations of two-photon free-precession and driven transient coherence by Hatanaka et al. are extended by an experimental NMR verification of the Brewer-Hahn exact solutions for two-photon resonance.

Molecular dynamics calculations on a two-dimensional argon liquid at 96°K have been performed taking into account three-body interactions. The three-body forces were assumed to have the Axilrod-Teller form. For the pair interaction the Lennard-Jones potential was chosen. The differences between structural and dynamical correlations which arise from calculations with and without the presence of three-body interactions have been studied. Whereas the differences between distribution functions (pair correlation function and triplet correlation function) due to the three-body forces are not significant, time correlation functions (veloci$\stackrel{\mathrm{´}}{\mathrm{t}}$y autocorrelation function and intermediate scattering function) show systematic deviations. For example, in the case of the velocity autocorrelation function, deviations up to 25% have been observed. Furthermore, the fourth moment of the scattering law is calcualted for wave numbers $0.234\le k\le 2.34$ ${\mathrm{\AA }}^{-1\mathrm{}}$. Here, also, the deviations (up to 12%) from the pair-theory values are systematic. The results indicate that the determination of pair potentials from the fourth moment (a recent method by Rahman) does not give unique pair potentials for argon and argonlike systems since three-body forces are clearly reflected in the fourth moment.

Scattering amplitudes and cross sections for screened Coulomb potentials are examined using the eikonal approximation. We use the method of $h$ transforms to show that if the screening function is sufficiently smooth, then the screened cross section ${\sigma }_{\rho }$ approaches the Coulomb cross section ${\sigma }_C$ as the screening radius $\rho \to \infty$. On the other hand, for a sharply cut-off Coulomb potential ${\sigma }_{\rho }$ does not approach ${\sigma }_C$ as $\rho \to \infty$. These results agree with results obtained earlier using the Born approximation.

This paper studies the problem of a quantum gas of particles whose interparticle potential is attractive enough to support stable bound clusters. The approach to this problem is to employ the time-dependent two-Hilbert-space scattering theory—first developed by Faddeev to treat the three-body problem. From this theory the asymptotic completeness theorem is taken and utilized to give a new definition of the canonical partition function suitable for describing states that are a mixture of free particles and bound clusters. A multispecies cluster expansion is obtained for the grand partition function. The theory of few-particle time delay is employed to determine the values of these cluster integrals. From these solutions are obtained the multidensity virial equation of state, the internal energy, and a density expansion of the chemical mass-action law that gives the effect of interactions between particles and clusters. All of these solutions exhibit the fact that the only aspect of the collision process that has any effect on the macroscopic behavior of the system is the time delay. From another perspective this problem and the solutions obtained represent a specific nontrivial realization of the $S$-matrix description of statistical mechanics put forth by Dashen.

Ab initio calculations of the structure and the thermodynamic properties of simple liquid metals and alloys are presented. The basic ingredients of the theory are as follows: (a) An orthogonalized-plane-wave-based first-principles pseudopotential is used to describe the interatomic forces. The pseudopotential is optimized specifically for binary systems. The $X\alpha$ method is used to construct the electron-ion potential. (b) A system of hard spheres is used as a reference system for describing the liquid structure. Effective hard-sphere diameters are determined by a variational method based on the Gibbs-Bogolyubov inequality. The method of this paper yields encouraging results for the structure factors, the excess volume, and the enthalpy and entropy of formation for alloys with a nearly random distribution of the components. Difficulties arise where nonrandomness has to be expected. The theory provides a basis for a microscopic understanding of the thermodynamics of alloys of simple metals in terms of their electronic structure.

We describe photon autocorrelation studies of both the depolarized and polarized components of light scattered from the critical binary mixture methanol-cyclohexane, carried out to $k\xi \cong 80$, in an effort to differentiate between the Rayleigh linewidth predictions of the mode theories and the dynamic droplet model, and to assess the validity of our treatment of the autocorrelation function of light which undergoes double scattering from a critical fluid. (Here $k$ is the scattering wave number and $\xi$ the Orstein-Zernike correlation length.) We achieved sample temperature control of ±15 μ°C for periods of 24 hours and reduced sample temperature gradietns to ≲ ±40 μ°C/cm. We developed a technique for measuring the effects of laser heating upon the temperature of the fluid within the scattering volume. These latter measurements enabled us to determine the critical temperature of our system to an accuracy of ±60 μ°C. We find agreement between the predictions of our theory of the double-scattered autocorrelation function and the measured depolarized intensity autocorrelation function. The double-scattered contribution becomes significant for the methanol-cyclohexane system at $k\xi \cong 0.6$, and higher than second-order scattering becomes important for $k\xi \gtrsim 1$. Despite the large multiple-scattered component in the light scattered from the fluid in the nonhydrodynamic regime, the Rayleigh linewidth is affected only slightly. We show how to estimate the amount of multiple scattered light from depolarization-ratio measurements, and make qualitative assessments of the modifications of the Rayleigh linewidth due to higher-order scattering. Despite the small effect of multiple scattering upon the Rayleigh linewidth at large values of $k\xi$, the uncertainty introduced is large enough to obscure the differences between the fits of the mode theories and the dynamic droplet model to our experimental results.

The stability of a nonneutral, rapidly rotating electron beam propagating in an axially inhomogeneous magnetic field is considered. It is shown that there exists an instability, driven by an axial shear in the rotation velocity, which does not exist in the uniform limit. Selected numerical results are presented.

The equation of state for a system of molecules with exchange interactions in addition to the spin-independent forces is considered, and explicitly evaluated in one and three dimensions for the case of hardsphere particles with weak long-range forces. Depending upon the ratio $R$ of the integrated strength of the exchange interaction to the spin-independent attractive interaction, five topologically different phase diagrams occur for the three-dimensional model. The phases involved are magnetic and nonmagnetic solids, magnetic and nonmagnetic liquids, and nonmagnetic gases. Liquid ferromagnetism occurs if the exchange interaction is sufficiently strong, $R>\mathrm{}0.15\mathrm{}$.

In an investigation of ray velocities (phase velocities in the direction of the group velocity) with an electrostatic dispersion relation in a hot magnetized plasma, the lower hybrid wave and the ion acoustic wave were found to coexist. Using a small launcher in a magnetized plasma near the lower hybrid frequency, we could observe the lower hybrid wave for low neutral-gas pressure [$P\simeq \mathrm{}(1-2)\mathrm{}\times {10}^{-4\mathrm{}}$ Torr] and the ion acoustic wave for high neutral-gas pressure [$P\simeq \mathrm{}(7-8)\mathrm{}\times {10}^{-4\mathrm{}}$ Torr]. The observed phase velocities for three dimensions were found to be in accord with the theoretical ray velocities. The wave fronts of the lower hybrid wave and the ion acoustic wave are parallel and circular to the magnetic field, respectively.