A generalized dressed-atom approach (GDAA) is developed to study the interaction of two or more strong fields with atomic and molecular systems. In the GDAA, part of the atom-field interaction is incorporated into the free-atom, free-field Hamiltonian and this expanded Hamiltonian is said to characterize a "dressed atom." The remaining atom-field interaction may then be thought to constitute a probe field interaction for this dressed atom. Provided that the dressed atom constitutes a reasonably good approximation to the true interaction, the GDAA should offer a relatively simple means for semiquantitatively predicting the positions of absorption resonances in quite complex atom-field systems. The GDAA is applied to the theory of a high-intensity, single-mode, standing-wave gas laser, the theory of Bloch-Siegert shifts, and the analysis of Autler-Townes or saturation spectroscopy experiments. The agreement between the GDAA and computer-determined values for laser resonance positions and Bloch-Siegert shifts is found to be surprisingly good over the entire range of applied field strengths. Some methods for using the GDAA to study complex spectroscopic systems are discussed.

Measurements of the long-range correlation length, the turbidity, and the susceptibility ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T$ have been made as functions of temperature and distance below the gas-liquid interface, in the range $1.25\times {10}^{-6\mathrm{}}\le \frac{(T_c-T)}{T_c}\le 6.15\times {10}^{-2\mathrm{}}$. The results are found to be accurately fit by the linear parametric equation of state; however, the best single exponent ${{\gamma }^{\prime }}_L$. describing the divergence of ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T$ is 1.189, which is significantly lower than the value determined in previous measurements for $T>\mathrm{}{T}_c$. A significantly better fit to the ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T$ data is obtained by using a modified power law of the form $A{\mathrm{}\left|t\right|\mathrm{}}^{-\gamma }(1+D{\mathrm{}\left|t\right|\mathrm{}}^x)$, with the leading exponent fixed at 1.223, the value observed for $T>\mathrm{}{T}_c$. This is taken to indicate that corrections to pure power-law behavior are more evident for $T<\mathrm{}{T}_c$ than for $T>\mathrm{}{T}_c$. The measured values of the correlation range are compared to values determined from measurements of the reflectivity of the interfacial layer and are found to show the same temperature dependence, but to be approximately one half of the latter in magnitude. The linear parametric equation of state is tested and found to describe the variation of ${\left(\frac{\partial \rho }{\partial \mu }\right)}_T$ with temperature and height in the cell quite accurately for $T>\mathrm{}{T}_c$, but it is not capable of simultaneously accounting for all of the data both above and below $T_c$.

Ultrasonic shear data are reported for the binary mixture nitrobenzene—$n$-hexane in the homogeneous phase above $t_c$. The observed dispersions at frequencies of 51 and 3 kHz have been compared with the predictions of a modified form of the Deutch and Zwanzig theory, together with data on the behavior of static viscosity.

The differential dilatometer technique is used in a volumetric study of the phase transitions in $p$-heptyloxyazoxybenzene. The pretransition volume changes associated with liquid-crystal order-parameter fluctuations are interpreted in terms of the Landau-de Gennes theory. This theory is consistent with the behavior in the isotropic-nematic transition region. For the nematic to smectic-$C$ pretransition the behavior is anomalous: As the smectic-$C$ transition point is approached the expansion in the nematic phase remains constant, rather than increasing as in the typical pretransition behavior.

We obtain in closed analytic form, estimates for the thermodynamic properties of classical fluids with pair potentials of Yukawa type, with special reference to dense fully ionized plasmas with Thomas-Fermi or Debye-Hückel screening. We further generalize the hard-sphere perturbative approach used for similarly screened two-component mixtures, and demonstrate phase separation in this simple model of a liquid mixture of metallic helium and hydrogen.

Microscopic studies on thermally induced undulation instabilities of the smectic $C$ phase of pentyloxybenzylideneheptylaniline ($\overline{5}\mathrm{BA}7$) and pentylphenyloctylthiobenzoate ($5\mathrm{PTB}\overline{8}$) are reported. The $A-C$ transitions of these compounds appear to be second order. The tilt angle in smectic $C$ is strongly temperature dependent. The biaxial order is small. Layer undulations can be produced with suitable samples by a stepwise reduction of the temperature. A few hundredths of a degree are sufficient. The preferred undulation mode has a wave vector parallel to the $C_2$ axis. A theoretical analysis and comparison with data from the smectic $A$ phase shows that the dilation modulus of smectic $C$ is about two orders of magnitude smaller than that of smectic $A$. The temperature dependence of the modulus near the transition to smectic $A$ is discussed theoretically on the basis of a Landau expansion.

The object of this paper is the beginning of a formulation of a method to find bounds to process functions, such as work and heat, for processes occurring in finite time. A general variational statement of the problem is given. Then model problems are solved, all but one of which are based on the "step-Carnot" cycle. This is similar to the reversible Carnot cycle but with the external pressure varying in finite steps. Such a system only needs to go through a finite number of equilibrium states during its cycle. The problems are the maximization of effectiveness of the step-Carnot cycle, the maximization of efficiency of the same cycle, the determination of optimal period for a step-Carnot cycle whose contact with the external reservoirs has finite heat conductance, and the determination of the maximum power and the rate at which maximum power is obtained, for a continuous Carnot cycle with finite heat conductance between system and thermostats.

Within the context of conventional time-independent thermodynamics, an algorithm is developed to construct potentials $\mathcal{P}$ that define the extremal values of work for processes with arbitrary constraints. An existence theorem is proved that demonstrates that such potentials $\mathcal{P}$ can be given for any quasistatic process. This theorem extends the capability of thermodynamics from reversible processes to one class of time-dependent processes. A corollary shows how such potentials can be constructed for systems whose time dependence is first order. A final theorem shows the equivalence of the extremal work derived by solution of an optimal control problem with the work derived as a change in the generalized potentials, $\Delta \mathcal{P}$. Examples are given to illustrate the constructions.

Experimental measurements of the diffusion coefficient of C${\mathrm{H}}_4$ in Kr at $T=220\mathrm{}°$K in the density range $0<\mathrm{}\rho \le 2{\rho }_c$ are reported. The experimental data are compared with self-diffusion for Kr and C${\mathrm{H}}_4$ and with the data for diffusion of Kr in C${\mathrm{H}}_4$ at about the same reduced temperature. The behavior of the mass effect on the diffusion process is shown clearly for the first time. At $\rho \le {\rho }_c$ the deviation from Enskog's predictions is already marked, whereas at $\rho \ge 1.5{\rho }_c$ the mass effect is of the same kind as for hard spheres.

We solve in closed form the boundary-value problem for the partial differential equations which describe the (azimuthal) rotation velocity and induced magnetic fields in a cylindrical plasma centrifuge with ring electrodes of different radii and an external, axial magnetic field. The electric field, current density, and velocity distributions are discussed in terms of the Hartmann number $H$ and the magnetic Reynolds number $R$. For small Hall coefficients, $\omega \tau \ll 1$, the induced magnetic field does not affect the plasma rotation. As a result of the Lorentz forces, the plasma rotates with speeds as high as ${10}^5$ cm/sec around its axis of symmetry at typical conditions, so that the lighter (heavier) ion and atom components are enriched at (off) the center of the discharge cylinder.

This note supplements a recent analysis of the bistable ballast resistor, by Ross and Litster. Their discussion of static phase boundaries is extended to the case of motion with uniform velocity. The existence of an effective potential, entirely analogous to equilibrium, and dependent on detailed balance, is questioned.

It is demonstrated that a $p$-polarized electromagnetic wave incident obliquely on a layer of inhomogeneous nontransparent plasma, whose density has a steep rise in the vicinity of the plasma resonance, can be totally absorbed. The effect occurs when the damping of the so-called leaking waves is fully compensated by the radiation inflow.

In a recent paper Lee, Rodgers, Das, and Sternheimer have reported on the evaluation of the fine and hyperfine structure of the $4{}_{}{}^2{}_{}{}^{}D$ state of the rubidium atom by considering the first-order polarization of the electron core. By means of more extensive calculations we have shown that the first-order results can be quite unreliable for such strongly perturbed systems. By including polarization effects to all orders and the most important correlation effects, we have been able to reproduce the experimental results for this state quite well. The good agreement obtained by Lee et al. for the spin-dipole parameter is in our opinion largely fortuitous.

Directional anisotropy around 90° of bremsstrahlung due to secondary electrons ejected by MeV-proton bombardments on a thin Al foil was measured. It is shown that this behavior can be explained by the relativistic retardation effect in bremsstrahlung production.

The stability conditions for the solutions of a two-unrestricted-determinant function (the half-projected Hartree-Fock function) are deduced from the necessary requirements for the minimization of the energy functional. The theory is applied to the case of the LiH ground state, in order to investigate the various solutions encountered in the variational problem. It is found that the two lowest solutions are local minima. The number of these solutions is estimated and their significance discussed as a function of the nuclear separation.

The theory of level splitting of certain vibration-rotation states (including the ground state) in spherical-top molecules is developed within the framework of the standard Racah-Wigner tensor algebra, thus demonstrating the direct applicability of these general techniques without making the usual modifications associated with the anomalous commutation relations of the body-fixed angular momentum components. The eigenvalue spectrum of the octahedrally invariant fourth-rank tensor operator is examined in detail, and the unexpected periodic symmetry and asymptotic degeneracy features of the spectrum are described.

Electron-impact scaled collisions strengths $Z^2\Omega$ for a large number of transitions involving configuration-mixed initial and/or final states are obtained for berylliumlike ions in the limit $Z\to \infty$. These results are compared with the corresponding non-configuration-mixed results. It is found that the effect of configuration mixing is usually small (≲30%) and nearly independent of impact electron energy, but in a very few cases it is large, resulting in a change in $\Omega$ by about a factor of 2. The results are expected to be of interest for applications to high-temperature plasmas containing moderately heavy elements.