Transitions between energetically close-lying Rydberg states are characterized by large electric dipole matrix elements and frequencies low enough that at room temperature the photon occupation number is ≃ 10. Consequently the absorption of blackbody radiation and the stimulated emission produced by it lead to an efficient redistribution of population among nearby levels. In some cases dramatic effects are observed, for example, radiative lifetimes that are shortened to a small fraction of their $T=0\mathrm{}$ values. In addition, the 300-K blackbody field of ∼10 V/cm produces ac Stark shifts of the Rydberg levels. Calculations of the population redistribution effects and ac Stark shifts are presented. Examples are given to illustrate that care must be taken in experiments with highly excited atoms to ensure that only one state is being observed rather than a distribution.

Spontaneous and stimulated emissions are modified when the emitting system is placed inside a guiding structure. This effect is investigated theoretically for Raman scattering taking place in optical fibers, and its role in the behavior of a single-mode fiber Raman laser discussed.

The authors describe an analytical approach to nonlinear-optics calculations which is well adapted to obtaining accurate and efficient numerical results for steady-state nonlinear interactions including finite transverse beam profiles, beam-diffraction effects, and power-depletion effects. This general approach is applied to obtain new results for optical second-harmonic generation with both Gaussian and unstable-resonator transverse beam profiles, under large-signal conditions where pump depletion as well as diffraction effects are important.

Collisional and photoabsorption properties of electronically excited molecular hydrogen are studied by means of selective excitation of the ${\mathrm{H}}_2$ ($E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$) double minimum state. The $v=2\mathrm{}$ level of the inner well of the $E$,$F$ state is populated by two-photon absorption of ArF* laser radiation at 193 nm. Intracavity prisms are used to narrow the laser linewidth and tune the laser to excite single rotational levels selectively. Both ${\mathrm{H}}_2$ and HD have been excited in this manner, but the ${\mathrm{D}}_2$ absorption lines are outside the laser tuning range. The population densities of the $E$,$F$ rovibrational levels are measured by monitoring the near-infrared $E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}\to B{{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ fluorescent emission. The $E$,$F$-state radiative lifetime, electronic and rotational collisional relaxation rates, and photoionization cross section at 193 nm are measured. The large electronic quenching cross section (∼100 ${\mathrm{\AA }}^2$) observed is compared to a Born approximation calculation of inelastic scattering in the ${\mathrm{H}}_2$ ($E$,$F{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$) system and is found to be due to collisional population of the $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state. Observations of vacuum-ultraviolet $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to X{{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$ emission support this conclusion. The rotational relaxation cross sections are ≲0.2 Å for ${\mathrm{H}}_2$, but are much larger in HD (∼10 ${\mathrm{\AA }}^2$).

Fine-structure-changing collisions within the $3{}_{}{}^3{}_{}{}^{}D$ state of atomic helium are investigated by a laser perturbation method in a helium low-pressure glow discharge. Measurements, performed for different experimental situations, lead to the determination of thermally averaged values for the excitation transfer cross sections: ${\overline{\sigma }}_{3{}_{}{}^3{}_{}{}^{}D\mathrm{}(J=1\mathrm{})\mathrm{}\to 3{}_{}{}^3{}_{}{}^{}D\mathrm{}(J=2,3\mathrm{})\mathrm{}}=180\mathrm{}\pm 60$ ${\mathrm{\AA }}^2$ and ${\overline{\sigma }}_{3{}_{}{}^3{}_{}{}^{}D\mathrm{}(J=2,3\mathrm{})\mathrm{}\to 3{}_{}{}^3{}_{}{}^{}D\mathrm{}(J=1\mathrm{})\mathrm{}}=42\mathrm{}\pm 5$ ${\mathrm{\AA }}^2$.

A sensitive electron-impact spectrometer has been used to study vibrational excitation of the ground state of ${\mathrm{N}}_2$ through the first excited doublet state $A{}_{}{}^2{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ of ${\mathrm{N}}_2^{-}$. Oscillations are observed in both the cross sections of the parent state $A{{}_{}{}^3{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ and the ground state. The absolute values of these cross sections have been measured. The observations are readily accounted for by calculations which use the "boomerang model." The resonant cross sections for excitation of the vibrational levels of the parent state vary in the same way as the direct Franck-Condon factors between the $A{{}_{}{}^3{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ and the ground states of the neutral molecule, whereas the observed intensities of the vibrational levels of the ground state are almost constant.

The author presents a theory for $N$-photon ionization in which two intermediate states can be in resonance with the ground state over a narrow range of photon frequencies. The theory is applied to the three-photon $6F$ resonance-state spin-orbit doublet in the four-photon ionization of Cs. The amplitude for the process is the sum of two components, each containing a complex line-shape function. In the limit of zero splitting, the previously studied isolated resonance result, in which the line shape is Lorentzian is recovered. For the splitting in Cs ($6F$), the absorption profile is studied as a function of the splitting parameters of the state, the $6S\to 6F$ three-photon Rabi rate, and the field-induced shift and width for the $6S\to 6F$ transition. This profile depends in a complicated way on the relative magnitudes of the shift and the spacing between the fine-structure levels.

The present paper investigates the self-focusing and self-trapping of a Gaussian laser beam propagating transverse to the static magnetic field in a plasma. In order to accomplish this, moment theory has been adopted, as opposed to the paraxial-ray approximation, which has so far been the basis for most of the analyses of self-focusing in the recent past. An equation for the beam width has been set up and solved numerically; the condition for self-trapping of the beam has also been obtained.

Negative-muon capture ratios have been determined at three different concentrations for each of the following solid solutions: Cu-Al, Ag-Zn, NaCl-NaBr, KCl-KBr. Over a concentration ratio range of approximately 50 for the alloy systems and 400 for the alkali halide solutions, the data show that the peratom capture ratios are essentially independent of atomic concentration, in agreement with some recent theoretical models.

Monte Carlo results are found for a simple fluid with a pair potential consisting of a hard-sphere core and a Lennard-Jones attractive tail. They are compared with several of the most promising recent theoretical treatments of simple fluids, all of which involve the decomposition of the pair potential into a hard-sphere-core term and an attractive-tail term. This direct comparison avoids the use of a second perturbation scheme associated with softening the core, which would introduce an ambiguity in the significance of the differences found between the theoretical and Monte Carlo results. The study includes the optimized random-phase approximation (ORPA) and exponential (EXP) approximations of Andersen and Chandler, an extension of the latter approximation to nodal order three (the N3 approximation), the linear-plus-square (LIN + SQ) approximation of Høye and Stell, the renormalized hypernetted chain (RHNC) approximation of Lado, and the quadratic (QUAD) approximation suggested by second-order self-consistent $\Gamma$ ordering, the lowest order of which is identical to the ORPA. As anticipated on the basis of earlier studies, it is found that the EXP approximation yields radial distribution functions and structure factors of excellent overall accuracy in the liquid state, where the RHNC results are also excellent and the EXP, QUAD, and LIN + SQ results prove to be virtually indistinguishable from one another. For all the approximations, however, the thermodynamics from the compressibility relation are poor and the virial-theorem results are not uniformly reliable. Somewhat more surprisingly, it is found that the EXP results yield a negative structure factor $S\left(k\right)\mathrm{}$ for very small $k$ in the liquid state and poor radial distribution functions at low densities. The RHNC results are nowhere worse than the EXP results and in some states (e.g., at low densities) much better. In contrast, the N3 results are better in some respects than the EXP results but worse in others. The authors briefly comment on the RHNC and EXP approximations applied to the full Lennard-Jones potential, for which the EXP approximation appears somewhat improved in the liquid state as a result of the softening of the potential core.

The authors present the first x-ray measurement of the liquid crystal smectic $A$ phase layer spacing at high pressure. It is found that at relatively high temperatures, when the smectic $A$ phase is compressed toward the reentrant nematic phase, there is no change in the layer spacing. On the other hand, at lower temperatures, where the smectic $A$ phase is being compressed toward the stable solid phase, the layer spacing decreases. A systematic increase in the layer spacing as a function of time is also noted.

The absorption of laser radiation by a spherical plasma is considered via geometric optics. In addition to the rays, the amplitude of the electric field is found, and absorption by inverse bremsstrahlung is modeled. The authors find resonance absorption in a domain outside the region where geometric optics is valid, assuming there a locally one-dimensional plasma-density variation and matching the geometric-optics solution with the solution of the wave equation. The total absorption coefficient obtained from the ray-tracing method is compared with the numerical results based on an exact solution of Maxwell's equation, and excellent agreement is found.

The general theory of nuclear-spin relaxation induced by translational self-diffusion in liquid crystals developed in a previous paper is applied to the smectic $A$ phase. The inter- and intralayer molecular jumps, the spin distribution, and the elongated shape of the molecules are taken into account. Results are obtained for the frequency and angular dependence of $T_1$ and presented graphically for a variety of parameters. The results for the smectic $A$ phase are compared with those for the nematic and isotropic phase. The theory reproduces the proton relaxation rate in terephtal-bis-butylaniline very well.

A hierarchy of generalized Nernst-Einstein relations is derived that gives relations among the higher-order transport coefficients that describe nonlinear mobility and non-Fickian diffusion. The results are valid for the case where the transported species is present in only trace concentration, but are otherwise general. In particular, the relations are independent of the density of the medium and of the form of the molecular interactions. Both a phenomenological derivation and a more rigorous one based on a linear kinetic equation are given. Some additional relations are also obtained for the special case of gaseous ions interacting according to the Maxwell model, which may be approximately valid for gaseous ions having more general interactions.

The assertion by Chen and Lee that known finite-depth-soliton solutions do not smoothly tend to Benjamin-Ono solitons in the deep-water limit is incorrect.

Quadrupole polarizabilities ${\alpha }_q$ have been calculated for ${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{4+}$, ${\mathrm{Ni}}^{5+}$, and ${\mathrm{Cu}}^{6+}$ (in $3d^5$ configuration) and ${\mathrm{Ru}}^{3+}$, ${\mathrm{Rh}}^{4+}$, ${\mathrm{Pd}}^{5+}$, and ${\mathrm{Ag}}^{6+}$ (in $4d^5$ configuration) by means of the Hartree-Fock-Slater wave functions and the Sternheimer differential-equation procedure. In the cases of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Ru}}^{3+}$, ${\alpha }_q$ values of 0.14 and 0.52 ${\mathrm{\AA }}^5$, respectively, are obtained.

This paper is a study of the topological relationship between two scalar fields of a molecular system, the electronic charge density and the Coulombic field generated by the atomic nuclei—the nuclear potential. Because of the essential observation that the only local maxima of ground-state charge distributions occur at the positions of the nuclei, the nuclei are the point attractors of the gradient vector fields derived from the charge density and from the nuclear potential. The basins associated with the set of point attractors in either field partition a molecular system into nucleus-dominated regions. The common boundary of any two such neighboring regions contains a particular critical point which generates a pair of gradient paths linking the neighboring attractors. The union of this pair of gradient paths and their end points is called an interaction line. The network of interaction lines defines an elementary graph of the molecular system which identifies the dominant physical interactions in both the charge density and the nuclear potential. Having defined a unique elementary graph for either scalar field for any molecular geometry, the authors partition the total nuclear-configuration space into a finite number of regions. Each region is associated with a particular structure defined as an equivalence class of elementary graphs. The representation of this structural partitioning of nuclear-configuration space is called a structure diagram, which is analogous to a thermodynamic phase diagram. Bader, Nguyen-Dang, and Tal have previously shown that chemical concepts like bonds and molecular structure can be rigorously defined through such a topological analysis of the electronic charge distribution in a molecule. In this paper the authors trace the fundamental role of the nuclear potential in determining the topological properties of this charge distribution. Through a detailed study it is demonstrated that the structure diagrams of the charge density and of the nuclear potential are homeomorphic for the ${\mathrm{H}}_2$O system. It is conjectured that this homeomorphism exists in general for any molecular system.

The authors calculate the integrated vibrational excitation cross section for $e$-${\mathrm{N}}_2$ scattering in the interval 0-50 eV using the continuum multiple-scattering model with the Hara exchange approximation. Resonant enhancement is observed at 2.4 eV owing to the well-known ${\pi }_g$ shape resonance. In addition, however, enhanced vibrational excitation is found centered at ∼26 eV, arising from a broad shape resonance in the ${\sigma }_u$ channel. The authors propose this one-electron feature as the main source of the enhanced vibrational excitation observed by Pavlovic et al. in the 15-35 eV region.

The Glauber eikonal series method proposed by Yates has been used to analyze the elastic scattering and excitation of the $2{}_{}{}^1{}_{}{}^{}S$ state of the helium atom by electrons of energy 200 and 400 eV. Results for the elastic scattering are in good agreement with the Glauber calculations. For the excitation of the $2 {}_{}{}^1{}_{}{}^{}S$ state of helium, the present calculation underestimates the differential cross section at higher scattering angles and overestimates at small angles in comparison with Glauber calculations of Byron and Joachain.

The authors present a method for applying the Schwinger variational principle to the scattering of low-energy electrons by molecular ions. Numerical procedures for its application in the static-exchange approximation are specifically discussed. As examples, the $s$- and $p$-wave phase shifts for $e$-${\mathrm{He}}^{+}$ are obtained. The procedure also provides the electron continuum wave functions which can be used to obtain photoionization cross sections. The resulting cross sections for photoionization of the ground and metastable states of the He are seen to be accurate.