A theory is presented of the near-resonant interaction of two-level atom with strong Gaussian fields. Analytical expressions are given for the fluorescence intensity and for the two-time first- and second-order correlation functions of the intensity in both cases of a chaotic field and a real Gaussian field. The usual three-peak structure in the fluorescence spectrum is found to disappear because the amplitude fluctuations of the driving light prevent the nutation process. The scattered intensity is shown no longer to present a Markovian character when the atom is driven by a Gaussian field. The atom gains a sort of memory because of the intensity fluctuations in the driving field.

A bistable absorber in the good-cavity limit and the Fokker-Planck equation describing the time evolution of the quasi-probability distribution for the transmitted field amplitude are considered. It is shown that the system evolves in two stages. The first consists of a local relaxation around the minimum (or minima) of a suitably defined potential. The second stage, which occurs over a much longer time scale, results from tunneling from one minimum to the other across the local maximum of the same potential. In this paper, only the first stage is analyzed. The time evolution of the expectation value of the transmitted field is described analytically. The accuracy of this approach is tested by numerical solutions of the Fokker-Planck equation.

The multiphoton excitation of a polyatomic molecule by two infrared laser pulses is discussed. The infrared-active vibrational mode of the molecule is modeled as an anharmonic oscillator driven by two near-resonant coherent fields. The effects of intramolecular vibrational coupling, and other internal relaxation mechanisms are represented by suitable irreversible decay terms in the master equation describing the evolution of the molecular degrees of freedom of interest. The results show that, if the parameters of the excitation pulses are chosen properly, significantly higher molecular excitation can be obtained as compared with the single-laser-excitation scheme. Comparison with some recent experimental results shows that these calculations predict trends which are consistent with the available data.

A Bayesian analysis of measurement process is formulated with respect to the physical quantity being subject to the master equation. The dynamical equation of the a posteriori probability distribution of the physical quantity is derived with the use of the system size expansion. The coupled nonlinear equations of the most probable path with the results of measurements taken into account and of the variance of the fluctuations around the path are derived, which are the extensions of those of the Kalman's filter in the linear control theory to the nonlinear physical system on the basis of a physically reasonable assumption, the "local nature of transition." The formula of the entropy reduction and the $H$ theorem which are associated with the measurement process are also derived.

Using a continuum theory, analytical expressions are derived for the van der Waals contribution to the surface and anchoring energies of nematic liquid crystals. The results depend on an effective molecular separation parameter $d$, whose experimental determination is critically discussed. For liquid crystals in contact with a solid of refractive index $n_s$, it is found that a planar alignment is favored for small $n_s$ and a hemeotropic alignment for large $n_s$. Between $n_s=1.35\mathrm{} \mathrm{and} 2.5$, the van der Waals contribution $\Delta \gamma$ to the anchoring energy changes by several erg/${\mathrm{cm}}^2$, which is of the same order as the total anchoring energy, so that the general trend is expected to survive in the presence of other than dispersive contributions. Around $n_s=1.5\mathrm{}$, $\Delta \gamma$ is small, and other than dispersive contributions are likely to dominate the anchoring energy.

The results of accurate variational calculations for the ground-state binding energies of several $N$-boson systems in two and three dimensions are reported; the systems specifically considered are small ($N=2, 3, 4, \mathrm{and} 5$) clusters of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms interacting through pairwise Lennard-Jones potentials. Evidence is found strongly suggesting each set of systems have a common functional relationship linking the binding energies and the De Boer quantum constant near threshold for self-binding. These calculations also confirm the existence of a unique threshold value of the interaction strength for all $N$-boson systems in $2D$. In addition, the linear relation between the binding energies of helium clusters present in $3D$ holds ture in $2D$ as well.

The diffusion coefficient associated with the order-parameter fluctuations near the critical point of fluids is expected to vanish asymptotically as $D=\frac{\mathrm{RkT}}{6\pi \eta \xi }$, where $\eta$ is the viscosity, $\xi$ the correlation length, and $R$ a universal constant. Our experiments, using 3-methylpentane-nitroethane, yield $R=1.02\mathrm{}\pm 0.06$, in agreement with the mode-coupling theory of critical fluctuations, but in disagreement with the value $\frac{R}{6\pi }=\frac{1.2}{6\pi }\simeq \frac{1}{5\pi }$, recently suggested by several investigators.

Accurate measurements are reported of both the correlation range and the Rayleigh linewidth of xenon near its critical point. The linewidth data agree quite well with the mode-mode coupling results, and disagree with renormalization-group dynamics calculations.

On the basis of a kinetic equation extensions have been obtained for the first time of the five hydrodynamic modes of a dense fluid from the hydrodynamic to the kinetic regime, until they cease to exist. An appreciable softening of the heat mode is found before it becomes a diffusionlike mode. These modes dominate the neutron scattering of fluids.

Nonequilibrium molecular dynamics calculations have been performed for soft disks. It was found that the strain-rate dependence $\gamma$ of shear viscosity $\eta$ and hydrostatic pressure $p$ could be described by the following functional forms: $\eta \left(\gamma \right)=A_{\eta }{log}_{10}\left|\gamma T_{\eta }\right|$ and $p\left(\gamma \right)=p\left(0\right)+A_p\left|\gamma T_p\right|{log}_{10}\left|\gamma T_p\right|$. These functional forms are the same as ones that have been predicted using asymptotic "long-time-tail" theories. The numerical values of the coefficients as determined from the simulations are several orders of magnitude greater than theory predicts. If the equation above for the effective shear viscosity is valid in the limit of small strain rates, then Navier-Stokes hydrodynamics does not exist in two-dimensional fluids.

One mechanism for the generation of heat in the laser-fusion process is via inverse bremsstrahlung. Since it is desirable to maximize the amount of heat produced, it is thus of interest to investigate possibilities for increasing the absorption rate via the inverse bremsstrahlung process. Here we show that, in the presence of a magnetic field, the absorption rate can be either increased or decreased depending on the relative orientations of the polarization vector of the laser beam, the incident direction, and the magnetic field direction, and depending on the relative magnitudes of the electron's average energy due to thermal and electric-field effects. In particular, for radiation incident along the magnetic direction, we show that the absorption of right-circularly-polarized radiation is decreased, whereas that of left-circularly-polarized radiation is increased, if we are in the weak-electric-field regime (thermal effects dominate). On the other hand, in the strong-electric-field regime, the rate of absorption of right-polarized radiation is greater than that of left. In addition, the right-polarized radiation will penetrate to a region of greater plasma density than the left, in the direction of the magnetic field.

At applied electric-field strengths below a threshold value ${\left(\frac{E}{n}\right)}_{\mathrm{thr}}$ the mobility of electrons is independent of field strength. At strengths just above the threshold the sign of the mobility-field coefficient $\frac{d\mu }{\mathrm{dE}}$ is positive in the low-density gas, negative in the high-density gas and low-density liquid [$1.0<\left(\frac{n}{{10}^{21}}\right)<\mathrm{}8\mathrm{}$], and positive in the normal liquid. The temperature coefficient of mobility $\frac{d\mu }{\mathrm{dT}}$ is positive at all densities. The change of sign of $\frac{d\mu }{\mathrm{dE}}$ going from $n=0.9\mathrm{} \mathrm{to} 1.0$ (${10}^{21}$ molecule/${\mathrm{cm}}^3$) is attributed to destructive interference of long-range attractive interactions, which tends to reduce the effective scattering cross sections at the lower velocities. The sign of $\frac{d\mu }{\mathrm{dT}}$ remains positive because quasilocalization of electrons occurs at density fluctuations of suitable magnitude in this region of $n$. Quasilocalization is characterized by large negative values of $\Delta H^{\prime }$ and $\Delta S^{\prime }$, with $\Delta G^{\prime }\simeq 0$. The density-normalized mobility $\mu n$ changes by less than a factor of 2 while increasing $n$ from the low-density gas to the liquid at $n=7\mathrm{}\times {10}^{21}$ molecule/${\mathrm{cm}}^3$. At $n>\mathrm{}8\mathrm{}\times {10}^{21}$ the value of $\mu n$ plunges, due to the formation of more stable localized states; field-assisted delocalization of the electrons causes $\frac{d\mu }{\mathrm{dE}}$ to become positive again. Electron behavior in fluid ethane is quite different from that in fluid methane. The differences are attributed to the difference in molecular shape; methane is much more spherelike than ethane. The less spherelike molecule has a lower average scattering cross section for electrons at 300 K in the low-density gas, a Ramsauer-Townsend minimum in ${\sigma }_{\nu }$ lying at a lower energy, a larger ratio of threshold drift velocity to the speed of sound $\frac{v_d^{\mathrm{thr}}}{c_o}$, and more stable localized states of electrons in the liquid.

High-resolution ($\frac{\Delta \lambda }{\lambda }\approx 0.015$) x-ray spectra in the wavelength region 3.7→6.2 Å (3.4→2.0 keV) have been collected from the Alcator tokamak using a flat crystal Bragg monochromator. Molybdenum $L$ lines and chlorine $K$ lines have been observed and charge-state identifications have been made by comparison with calculations.

Although the general invariance group is no larger than Galilean-scale dilitation, the Navier-Stokes equation for incompressible viscous flow admits time-dependent rotational invariance for solutions of special types, as shown by the analysis presented here.

We study the temperature dependence of the dark current and the quantum efficiency of photodetectors using our previous open-system detection model. We evaluate the joint contribution of these effects to the moments of the photoelectric current.

A previous calculation of the cesium $6s\to \epsilon p$ photoionization cross section is extended to include interchannel coupling to the $5p\to \epsilon d$ photoionization channels. For photon energies in the range $0.35 \mathrm{a}.\mathrm{u}.\mathrm{}\lesssim \hslash \omega \lesssim 1.12 \mathrm{a}.\mathrm{u}.\mathrm{}$, i.e., just above the near-threshold cross-section minimum, the $6s$ photoionization cross section is found to be dominated by $5p\to 5d$ resonance transitions. No experimental data exist for this energy region.

An earlier prescription for calculating states of hydrogen in arbitrary magnetic fields, which had been applied only to ground states of any given azimuthal symmetry, is now extended to states with longitudinal excitation. The numerical results are simple to obtain and compare favorably with an alternative calculation of such longitudinally excited states that has been presented recently. Adoption of the present procedure is recommended for calculating excited states, particularly at larger field strengths (≃ ${10}^{12}$ G).

The threshold law of the titled process, which is of the same form as that for electron-impact ionization of H, can experimentally be contaminated by simultaneous single photodetachment and Rydberg excitation of the residual atom, followed by ionization of the Rydberg atom by (room-temperature) blackbody photons. This gives a signal indistinguishable from direct double photodetachment. The correction is here calculated and shown to be finite at the double photodetachment threshold, and to have, possibly, an implication about the form of the threshold law itself. However, at room temperature its contribution is found to be small as it pertains to a recently initiated experiment at the Los Alamos Meson Physics Facility (LAMPF) for available energies greater than a millivolt above threshold. The temperature dependence of the correction is shown to be large ${\left(k_{B}T\right)}^{\frac{7}{2}}$; thus at higher temperatures the effect may be observable.

It is shown that the method of logarithmic perturbation expansion can be used to obtain the corrections to the energy and the logarithm of the wave function of the ground state of a hydrogen atom in a static multipole field, or a linear combination of static multipole fields to any order in perturbation theory.

A full description is given of an experiment in which the muonic helium atom, ${}_{}{}^4{}_{}{}^{}\mathrm{He}{\mu }^{-}{e}^{-}$, was first observed. The method of the experiment was to study the precession of the magnetic moment of negative muons stopped in 14-atm helium with a 2% Xe admixture. A signal at the characteristic Larmor frequency of the atom was observed with an amplitude which corresponds to a muon residual polarization of 5.0±0.7%. In pure helium at 14 atm, precession characteristic of free muons with a polarization of 5.8±1.3% was found. The hyperfine structure of ${}_{}{}^4{}_{}{}^{}\mathrm{He}{\mu }^{-}{e}^{-}$ and the various processes involved in the formation of this atom are also discussed.