High-precision measurements of the ${\mathrm{I}}_2$ $B-X(43,0)\mathrm{}$ $R\mathrm{}\left(15\right)\mathrm{}$ hyperfine spectrum are reported. The $R\mathrm{}\left(15\right)\mathrm{}$ and previously measured (43,0) $P\left(13\right)\mathrm{}$ spectra were fitted to 2.6 and 5.4 kHz, respectively. A previous fit of the $P\left(13\right)\mathrm{}$ which resulted in a nonzero value for the magnetic octupole coupling constant was apparently in error. Extension of such measurements to other rotational vibrational levels in ${\mathrm{I}}_2$ using an optically pumped laser and two-step excitation is discussed.

It is shown that in a recent work the program proposed cannot be carried out in curved space, while in flat space the result is a consequence of the restriction to differentiable paths. The use made of Fourier series is also shown to require modifications.

The decay rate of thermalized positrons in helium is calculated as a function of the fluid density, incorporating the multiple scattering of the positron off the atoms. The obtained nonlinearity agrees with experimental data outside the self-trapping region.

The Glauber approximation with allowance for Coulomb effects is used in the study of the rotational excitation of the C${\mathrm{O}}^{+}$, HC${\mathrm{O}}^{+}$, and ${\mathrm{N}}_2$${\mathrm{H}}^{+}$ heteronuclear molecular ions from the $J=0\mathrm{}\to 1$ state by electron impact.

The total photon interaction cross sections for eight elements in the range $20\le z\le 60$ at photon energies of 52.4, 60, 72.2, and 84.4 keV have been measured. The photoelectric cross sections at these photon energies have been deduced by substracting the scattering cross sections from the measured values of total interaction cross sections. The results have been compared with corresponding theoretical values available in the literature.

The observation of oscillatory structure in the forward peak from fast-projectile electron loss observed by M. Suter et al. is interpreted as being due to autoionization of highly stripped ions.

Correlation functions $〈\mathrm{Tr}\overleftrightarrow{a}\mathrm{}\left(t\right)\mathrm{}\overleftrightarrow{a}\mathrm{}\left(o\right)\mathrm{}〉$ associated with Cartesian tensors of the second rank are calculated for a classical system of free asymmetric rotors. The corresponding Euler equations are solved analytically and the average over a canonical ensemble is calculated by combining analytical and numerical methods; the long-time behavior of these functions is also investigated. This calculation does not involve any approximation. The results are discussed in detail and a number of physically interesting cases are presented. The overall complexity of the free-asymmetric-rotor problem exceeds that of the symmetric-, spheric-, or linear-rotor problem by an order of magnitude.

The partial quenching of the fluctuations of orientation of nematic layers by very large magnetic fields predicted by de Gennes and leading to increased birefringence has received a first experimental verification by Poggi and Filippini. In this paper a detailed account of this effect is given: the progressive disappearance of the induced-excess-birefringence (IEB) effect near a second-order smectic transition is connected quantitatively with the divergence of the bend elastic constant. As the isotropic phase is approached from below, the increase of IEB is connected with the decrease of the elastic constants. The IEB varies with field as ${\mathrm{}\left|H\right|\mathrm{}}^u$ with $u=1\mathrm{}\pm 0.02$ over the whole nematic range. This critical-exponent determination is closely connected with the problem of the divergence of the longitudinal susceptibility in magnetic systems where both "semiclassical" and renormalization-group approaches predict a divergence in $H^{-\frac{1}{2}}$ in three dimensions. As no data seem to be available for the magnetic problem, the linear form obtained here provides a first experimental test of the validity of both approaches based on the quenching of transverse magnetization fluctuations by a field. Departures from the de Gennes model observed near $T_{\mathrm{NI}}$, as well as deviations of the induced birefringence, observed in the same experiments, from the mean-field behavior in the isotropic phase above the nematic transition, emphasize the need for new theoretical description of this weakly first-order phase transition.

A solution of the linearized quantum Liouville equation for a plasma in a uniform magnetic field is obtained in the coherent-state basis and the linear response of such a system to general electromagnetic perturbations studied. The method is further extended to the situation where the magnetic field is inhomogeneous.

In the general framework of the system size expansion of Van Kampen and Kubo, we consider the Fokker-Planck equation for a model of absorptive bistability in the bad-cavity limit. The physical system is described by the reduced atomic density operators after adiabatic elimination of the cavity field variables. Mapping of the master equation into $c$-number form according to the normal-ordering mapping scheme yields known results for the atomic fluctuations and correlation functions; however, it also leads to a Fokker-Planck equation with a non-positive-definite diffusion matrix. The symmetrical-order-mapping scheme eliminates this difficulty. The leading contribution to the system size expansion yields a Fokker-Planck equation for the symmetrical-ordered density function having a positive-definite diffusion matrix. The atomic expectation values and fluctuations previously derived from the quantum Langevin equations emerge naturally from this Fokker-Planck equation.

A nonlinear diffusion approximation for a previously derived master equation describing an inhomogeneous Boltzmann gas in a lumped phase space is proposed. A fluctuating kinetic equation is obtained which differs from the usual Langevin equations in three essential properties: the drift and random force are nonlinear, the random noise obeys a generalized fluctuation-dissipation theorem, and there is no reference to equilibrium. Relations with other approaches to hydrodynamic fluctuations are discussed.

The authors present here the $K$-shell ionization cross sections for elements between $Z=48\mathrm{} \mathrm{and} 104$ by 2-MeV electrons in the relativistic-plane-wave Born approximation using the Møller interaction between the two electrons under consideration; these are then compared with experimental data.

The effects of reactant-ion electronic states on charge transfer cross sections involving 0.6- to 3.0-keV ${\mathrm{C}}^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{O}}^{+}$ ions with ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ molecules are illustrated. Total cross sections, computed from state-selected cross sections and weighted according to reactant-ion-beam state distributions, are compared with the recently published data of Lockwood et al., which were obtained at higher energies.

It is shown that the infinite-depth limit of internal-wave solitons in finite-depth stratified fluids agrees with the previously known soliton solutions for infinite depth, in agreement with a result of Joseph but in sharp disagreement with a recent letter by Chen and Lee. Moreover, a second class of alleged solitons found by Chen and Lee do not solve the equation.

Recent papers have presented data on the high Rydberg states of rubidium in external electric fields. We present an analysis which accounts for the energy positions and symmetry aspects of the resonances seen in these experiments. Our analysis places these new data in the framework of previous studies of such resonances.

The theory developed by Band to treat the dependence on foil tilt angle of the polarization of emitted light is applied to the special case of $2s-3p$ transitions in neutral helium.

The NMR data for methyl-deuterated $p$-azoxyanisole are reexamined in view of the correct $T^{†}$ given by $T^{†}-T_c=\frac{1}{8}(T_c-T^{*})$ and an effective critical exponent for the coherence length of the nematic order-parameter fluctuations in the Landau-de Gennes mean-field theory.

The identity recently derived by Hiller, Sucher, and Feinberg between a $\delta$-function expectation value and the expectation value of a smoothly varying operator is generalized. A discussion of the relative accuracy of the two expressions for approximate wave functions follows.

It is shown that angular momentum operator identities between ladder operators and rotation operators can be used to study the trace problem. Using an explicit representation of the rotation matrices, the author evaluates the trace of the operator $J_{-}^k{J}_{+}^k$ in closed form. The identities are found to be particularly useful in providing checks for the sum of the traces.