The authors compare the spectrum of the regularly spaced photoabsorption resonances which appear in the vicinity of the ionization threshold for atoms in a strong magnetic field with predictions of semiclassical strong-mixing models. Quasi-Landau resonance positions are shown to agree well with a two-dimensional WKB semiclassical hydrogenic model which depicts the resonances as bound states of the electron in the combined Coulomb and magnetic fields and which reduces to the Rydberg levels at low excitation. Simpler treatments based on a modified Bohr atomic model are also discussed. The data include observations in the $M=0\mathrm{}$ even-parity channels of Bai and Sri with $B=25-40\mathrm{}$ kG and the $M=-1\mathrm{}$ odd-parity channel of Bai at $B=47\mathrm{}$ kG. A scaling procedure is used to allow field-independent comparison among the various spectra and models. Although fairly successful in predicting energy positions, none of the models attempts to explain the complex resonance profiles or superposed fine structure, thus indicating the need for a comprehensive quantum-mechanical theory of the diamagnetic strong-mixing regime. The appearance of the quasi-Landau resonances in $M=0\mathrm{}$ even-parity spectra is consistent with the requirement that the states have even parity with respect to reflection along the direction of the magnetic field in order for the resonances to occur.

The authors investigate resonant two-step laser excitation in a cascade three-level system using an atomic-uranium beam. Both single-photon and two-photon processes are studied by varying the time delay between the two different excitation pulses. The experimental data are compared to the results of simple theoretical models. The calculations include the effects of the magnetic sublevels. The major theoretical prediction of a two-photon resonance is verified experimentally by using simultaneous irradiation from two lasers. The agreement between theory and experiment is found to be satisfactory.

This paper is concerned with broad-scale studies of self-generated magnetic fields by use of the magnetic-tape method. A refined technique which enables a sensible and fine-structure observation is described. In a large-cone-angle experiment parity existence is verified by using convergent and divergent laser beams. In a small-cone-angle experiment it is shown that negative and positive magnetic fields cross the resonance region; the existence of parity is demonstrated by reversing the sign of the incident angle. These observations strongly support resonance absorption as the mechanism of field generation.

Large-amplitude oscillations are predicted in the free-induction decay (FID) of atomic or spin two-level quantum systems that are coherently prepared by a resonant electromagnetic field in the form of a square amplitude pulse of duration $T$. The effect should be detectable in nuclear magnetic resonance (NMR) or in optical resonance experiments when the transition line shape is inhomogeneously broadened ($T_2^{*}\ll T_2$) and the applied field is intense enough that the pulse area $\chi T\ge 2\pi$, where $\chi$ is the Rabi frequency. Physically, the effect is due to Rabi oscillations which are generated in the preparative stage and are then reproduced because of atomic memory in the radiative period which follows. The atomic polarization derived from the Bloch equations assumes the form of an integral over the inhomogeneous line shape, and can be evaluated numerically for arbitrary linewidth $\sigma$ or analytically when $\sigma \to \infty$. The characteristics of the oscillation are unusual since the oscillation frequency, which is of order $\chi$, increases with time while the oscillation envelope vanishes identically for times $t\ge 2T$. Coherent emission is therefore confined to one pulse width $T$ immediately following the pulse. Previous NMR and infrared FID experiments with small-area pulses ($\chi T\le 4\pi$), interpreted erroneously in terms of an edge echo effect, support the oscillatory FID behavior described here. For the case $\chi T>\mathrm{}4\mathrm{}\pi$, an experimental test would be even more decisive, as the oscillations are more pronounced.

The spectral width of the amplified spontaneous emission emitted by a xenon 3.51-μm laser amplifier was measured as a function of discharge length using optical heterodyne techniques. Measurements were made for ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$ at a pressure of 190 mTorr and also for mixtures of ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}$ at a partial pressure of 190 mTorr with helium at partial pressures of 1.3, 2.5, and 4 Torr. Spectral narrowing and, usually, saturation-induced rebroadening were observed for each set of pressures used. The spectral-narrowing data in the regime of unsaturated amplifier gain are in reasonable agreement with existing spectral-narrowing theories which assume a random phase among excited atoms. The observed saturation-induced spectral-rebroadening rates are lower than those predicted by such theories. This behavior indicates that a more complex theoretical treatment of the influence of saturation on a broadband radiation field may be required.

Time-dependent spectra of light emitted after two-photon excitation with nonresonant intermediate state are studied by the perturbation method. Two components are distinguished: a slow one $I_s$ connected with the excited-state population, and a fast one $I_f$ connected with the double-quantum optical coherence. For large detuning of the two-photon frequency from the excited-ground-state energy separation, the fluorescent light is found to be $I_s-\left|I_f\right|$, and the Raman-scattered light is $I_f$. The influence of statistics of light, laser linewidth, spectral bandwidth of the detector, and transfer of coherence from the ground to the excited state are also taken into consideration.

The output characteristics of a high-intensity laser oscillator are examined by means of a semiclassical density-matrix approach. Unlike previous investigations the time derivatives of the field and material quantities are retained, and a basic instability in the semiclassical laser equations is revealed. This instability accounts for the recently reported spontaneous pulsations in xenon laser oscillators. The pulsations can be interpreted as a consequence of spectral holeburning in inhomogeneously broadened lasers, and stability criteria are derived for both standing-wave and traveling-wave oscillators. In simplest terms the pulsations should occur in any inhomogeneously broadened laser where the product of the homogeneous linewidth and the cavity lifetime is less than unity.

Four observed spectrum lines in titanium-containing tokamak discharges have been identified as follows: Tixiv $2s^{2}2p^5{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ at 2115.3 Å, Tixv $2s^{2}2p^4{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ at 2544.8 Å, Tixvii $2s^{2}2p^2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ at 3834.4 Å and ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\to {}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ at 3371.5 Å. The identifications are based on observed time behavior and correlation with intensities of resonance lines of other titanium ions, and on general agreement with predicted wavelengths and intensities.

Sound absorption and dispersion near the critical point in a critical binary mixture are studied numerically within the framework of the Kawasaki mode-coupling theory. The critical absorption shows a marked decrease near the critical point, in contrast to the usual critical behavior. It is shown here that the existing mode-coupling approach breaks down over a wide range of frequencies and temperatures.

The high densities obtained in recent laser fusion experiments have created a need for additional plasma density diagnostics. The ratio of two heliumlike satellite lines, the $1s2p{}_{}{}^3{}_{}{}^{}P\to 2p^2{}_{}{}^3{}_{}{}^{}P$ and $1s2s{}_{}{}^3{}_{}{}^{}S\to 2s2p{}_{}{}^3{}_{}{}^{}P$ transitions, has shown promise as a spectral diagnostic when laser-imploded microballoons are seeded with medium-atomic-weight gases. In this study results are presented from a collisional-radiative ionization-dynamics model with photoexcitation processes included that indicate that the emission from these satellite lines is strongly affected by opacity in density and temperature regimes common to plasmas to which this diagnostic could be applied effectively. The radiation emission attenuated by photon reabsorption is presented for neon, aluminum, and argon plasmas and compared with results predicted when the calculation is undertaken in an optically-thin-plasma approximation. The opacity effects are seen to cause multivaluedness in the line ratio at several temperatures, and an overall loss of sensitivity of the ratio with density is predicted.

In this paper the authors investigate the nonlinear scattering of upper hybrid laser radiation by electron Bernstein modes in a homogeneous plasma. The kinetic-model approach has been used to obtain the nonlinear electron-density perturbation of magnetized electron Bernstein modes and nonlinear current density of unmagnetized scattered laser radiation. The threshold for the scattering is found to be rather low. It is found that the decay of upper hybrid laser radiation in a plane perpendicular to the magnetic field into electron Bernstein waves and scattered laser radiation in the ordinary mode is not possible. The analysis presented here for ${\mathrm{Co}}_2$-laser-produced plasma is valid marginally, whereas, for Nd:glass-laser-produced plasma, the validity of this investigation is described very well. The growth rate for the scattering by fundamental electron Bernstein modes in Nd:glass-laser(power density = ${10}^{12}$ W/${\mathrm{cm}}^2$)-produced plasma having a temperature of a few keV, is ≃ 6.0×${10}^{11}$ rad ${\sec }^{-1\mathrm{}}$.

A simple set of rate equations is proposed to describe nucleation and growth of droplets in metastable, near-critical fluids. These equations are used in conjunction with steady-state nucleation theory to compute completion times for phase separation in binary mixtures. Reexamination of available experimental data provides little if any evidence for the major failure of conventional nucleation theory that has been postulated on the basis of these data.

Two types of approximate theory for nonuniform fluids are applied in the calculation of properties of nonuniform one-dimensional fluids whose particles interact via a Lennard-Jones (6-12) potential. The nonuniformity is produced by an external potential well which creates a local high-density "phase" in equilibrium with a low-density phase exterior to the well. The density and size of the local phase are varied by changing the depth and width of the well. The first type of theory invokes the density-functional (DF) approach. The authors examine a sequence of density functionals, all of which use the Percus-Yevick (PY) approximation for the interparticle interaction. Density profiles and free energies are found by means of the DF theories and compared with each other as well as with results obtained from Monte Carlo calculations for the same systems. The other type of theory, the linearized Percus-Yevick (LPY) treats the particle-well interaction in the Percus-Yevick approximation. This theory is used to obtain density profiles only. It is found that the results of the DF theories generally agree quite well with the numerical experiments while those of the LPY do not. The reason for the failure of the LPY is discussed in terms of an effective particle-well potential. Finally, important implications are derived concerning the reliability of the various DF theories for more realistic calculations in three dimensions.

A model for calculating x-ray line radiation from a cylindrical plasma is described and applied to transitions in AlIX-XIII. A new numerical technique for radiation transport calculations is described. Calculations are presented for a cylindrically symmetric plasma with a hot, dense core surrounded by a low-density corona. It is shown how both the total radiant flux and the line profiles vary with the plasma parameters, and it is indicated how such results can be used to deduce information about the radiating plasma.

An expression which includes the effects of departures from local equilibrium due to gradients i the plasma is derived for the energy loss of a beam of particles passing through an inhomogeneous plasma. A combination of the test-particle approximation with the Lenard-Balescu collision term and the the Bhatnagar-Gross-Krook (BGK) approximation for the plasma kinetic equations is used. The occurrence of microinstabilities in the BGK model is discussed, and it is found that they do not contribute to the plasma stopping power for the cases of interest here. Calculations for an $\alpha$-particle beam with energies from 0.5 to 4.5 MeV in a fully ionized hydrogen plasma are presented. The effects of flow-velocity gradients are small, whereas those due to temperature gradients may produce a sizeable change in the plasma stopping power. The largest changes are found at high temperatures (≳ 1 keV) and at low energies (≲ 1 MeV). These nonequilibrium contributions can be directly observed if the energy loss of a probe beam is measured at various angles with respect to the plasma gradients.

The rate coefficients for the dissociative ion-electron recombination of ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ and ${\mathrm{D}}_3$${\mathrm{O}}^{+}$ have been measured in shock-heated afterglow plasmas. A low-pressure, pressure-driven shock-tube apparatus enabled gas temperatures in the range $800\le T_g\le 3000$ K to be investigated. A focused microwave beam which intersected the shock-heated plasma permitted the separate, controlled variation of the electron temperature over the range $800\le T_e\le 9000$ K. The recombination coefficient for ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ was found to vary approximately as $T^{-\frac{1}{2}}$ over both the ranges of electron and gas temperature covered, with the power-law exponent increasing slightly with temperature. The coefficients for ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ and ${\mathrm{D}}_3$${\mathrm{O}}^{+}$ were found to be equal within the experimental error. The results indicate that the effect of vibrational excitation on the recombination coefficient is small for these ions. The present results for ${\mathrm{H}}_3$${\mathrm{O}}^{+}$ extrapolated to 540 K agree reasonably well with a previous measurement by the well-established pulsed-afterglow microwave-cavity technique. The absolute magnitude of the measured coefficient decreased from a value of 7.5±1.0×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ at $T_g=T_e=800\mathrm{}$ K to 4.2±0.3×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ at $T_g=T_e=3000\mathrm{}$ K for ${\mathrm{H}}_3$${\mathrm{O}}^{+}$.

In dilute solutions of poly-$\gamma$-benzyl-L-glutamate (PBLG), the only observable transition is intramolecular helix-coil transition. At higher concentrations, intermolecular correlations become important under suitable conditions resulting in transitions of the system to liquid-crystalline state. We present here a mean-field analysis of combined helix-coil and isotropic-nematic (cholesteric) transitions, taking into account the effect of "induced rigidity" as proposed by de Gennes and Pincus. Coupled self-consistent equations for the orientation and helix-fraction order parameters are derived and solved, a universal phase diagram as well as temperature and density dependence of order parameters calculated, and results compared to actual experimental data. We find qualitative agreement but a poor quantitative fit for PBLG in pure solvents. In solvents containing denaturing agents, we predict an isotropic-nematic-isotropic reentrance phenomenon, and propose experimental conditions under which it can be observed.

Ultrasonic absorption and velocity have been measured in several liquid mixtures of S${\mathrm{O}}_2$ + C${\mathrm{Cl}}_4$. Special emphasis was given to the homogeneous-solution region near the critical mixing point. Near the critical point it was found that high absorption and strong frequency dependence were characteristic of the ultrasound, unlike its behavior outside of the critical region. The frequency dependence of the absorption measurements near the critical point and along the critical isochore can be satisfactorily fit with the Kawasaki multiple-relaxation calculation by the adjustment of three fit parameters. Near the critical point the background absorption was found to be independent of frequency over the range of investigation (2 to 66 MHz). Including an adjustable single-relaxation term in the absorption fit indicated that the best fit was obtained as the amplitude of the single-relaxation term went to zero. The characteristic-frequency and amplitude parameters, taken from fits of the Kawasaki calculation to the data nearest the critical point, were scaled in reduced temperature. The characteristic-frequency data are consistent with a scaling exponent of 1.89±1 if a critical temperature is chosen for the scaling that is slightly lower than the observed phase separation temperature, i.e., if the presence of a pseudospindodal curve is assumed. This is the exponent value that is predicted by the mode-mode coupling and Ising lattice-gas theories. Neither dispersion nor temperature-dependent anomalies were observed for the ultrasonic velocity.

The ground and several excited states of the hydrogen molecular ion in a homogeneous magnetic field aligned along the molecular axis are studied. A pair of self-consistent equations is derived in prolate spheroidal coordinates and solved numerically. The results are exact at the limit of vanishing magnetic field and more accurate than previous results up to about ${10}^9$ G. The equilibrium internuclear separation and binding energy for a number of electronic states are reported as functions of the magnetic field strength. The dissociation energy is deduced from the study of the hydrogen atom in the same coordinate system.

The authors propose a novel "supermultiplet" classification of doubly excited states for two electrons in the same shell, constructed from more limited $O\left(4\right)\mathrm{}$ multiplets of earlier classification schemes. The supermultiplets are found to give a much more efficient classification, and bring to light new features of the spectrum, including accidental near-degeneracies of certain levels having different values of the angular momentum. When viewed with these supermultiplets, the intrashell spectrum is found to bear a striking similarity to a cutoff rotation-vibration spectrum of a linear triatomic molecule. This structure is interpreted with rotations and bending-mode vibrations for a strongly coupled electron pair; energies for these apparent excitations are found to have approximate separability.