We have made systematic measurements of the polarization of fluorescence from Sr-rare-gas systems excited off resonance by linearly polarized light. Measurements of this type give information about the evolution during a collision after the absorption of a photon. This information is important in obtaining and verifying aspects of the interaction potential that cannot be studied using conventional line-broadening experiments. We have also measured the collisional-alignment decay rates for Sr-rare-gas systems by observing the perturber-pressure dependence of the polarization of the fluorescence. To the best of our knowledge this is a new technique for measuring these decay rates.

In the resonant (charge) transfer and excitation (RTE) process, resonance states in the charge-changed projectile are formed due to Coulomb interaction with the target electrons. The calculated RTE cross section is proportional to the cross section for dielectronic recombination and its projectile energy dependence reflects the momentum distribution of the target electrons. Sample calculations for $K$ x-ray production in ${\mathrm{Si}}^{11+}$ + He, ${\mathrm{S}}^{13+}$ + Ar, and ${\mathrm{S}}^{13+}$ + He collisions are discussed. An estimate of the experimental background indicates that RTE is more pronounced in collisions with light targets.

The intensity and angular distribution of photoelectrons are calculated accurately for hydrogen molecules. The Hagstrom-Shull-type wave function is used for the initial state (i.e., the ground state of ${\mathrm{H}}_2$) and the final continuum wave function for $e+\mathrm{H}_2^{}{}_{}{}^{+}$ is obtained variationally. The two-center nature and the vibrational-rotational motion of the molecule are properly taken into account. The cross section and asymmetry parameter are calculated for each vibrational state of the residual ion. The result is in very good agreement with the recent measurement at 584 Å. The vibrationally unresolved cross section and asymmetry parameter are evaluated and compared with experiment at 736, 650, and 584 Å. Finally, a rather extensive study is made to clarify the effect of the accuracy of the initial- and final-state wave functions on the cross section.

As part of a continuing series of investigations, total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with CO and C${\mathrm{O}}_2$ using a beam-transmission technique. The projectile impact energies are in the range 1-500 eV for $e^{\pm }-\mathrm{CO}$, 30-500 eV for $e^{+}-\mathrm{C}{\mathrm{O}}_2$, and 100-500 eV for $e^{-}-\mathrm{C}{\mathrm{O}}_2$. An important aspect of our work is to compare the corresponding positron and electron total cross section ($Q_T$) curves for a given target gas. For both CO and C${\mathrm{O}}_2$ the electron $Q_T$ values are generally larger than the positron results. For both gases at low energies there are relatively narrow shape resonances for electron scattering and noticeable increases in $Q_T$ after the positronium-formation thresholds for positron scattering. At the highest energies investigated there are indications of a tendency toward merging of the positron and electron curves for each gas. A striking similarity is found between the present $e^{\pm }-\mathrm{CO} Q_T$ curves and the $e^{\pm }-{\mathrm{N}}_2 Q_T$ curves obtained by Hoffman et al. [Phys. Rev. A 25, 1393 (1982)], in that the corresponding shapes and magnitudes are very nearly the same. Estimates of potential experimental errors, as well as the experimental discrimination against projectiles scattered at small forward angles, are made.

A model based on Bethe's theory for inelastic scattering of fast charged particles is used to extrapolate single differential cross sections (SDCS) for ionization of helium, neon, and argon by protons with energy between 0.3 and 1.5 MeV to higher ion energies. Results predicted by the model show good agreement with measured total and SDCS and with ab initio calculations using the first Born approximation.

The production of free-free radiation in collisions of low-energy electrons with Ar atoms has been measured using the drift-tube technique. The excitation coefficients were obtained from measurements of the absolute intensity of continuum radiation at wavelengths of 500, 650, and 1300 nm. At 650 nm the electric-field—to—gas-density ratio $\frac{E}{N}$ was varied from 0.25×${10}^{-21\mathrm{}}$ to 10×${10}^{-21\mathrm{}}$ V ${\mathrm{m}}^2$ corresponding to mean electron energies from 1.2 to 5.4 eV. As expected, the emission was proportional to the argon density from 3×${10}^{24}$ to 15×${10}^{24}$ atoms/${\mathrm{m}}^3$. The experimental excitation coefficients are in good agreement with calculations using theoretical free-free emission cross sections and electron energy distributions and serve to demonstrate the usefulness of simple formulas relating the free-free emission cross section to measured momentum-transfer cross sections.

A semiclassical theory for Penning ionization is presented with suggested extensions to associative ionization. The theory makes use of classical nuclear dynamics and the adiabatic electronic representation. The advantages of the approach are threefold: (1) One can obtain single-channel information for all channels with just one equation of motion; (2) no explicit function $\epsilon \mathrm{}\left(R\right)\mathrm{}$ relating the electron energy to the internuclear distance need be imposed; and (3) no coupled discretized channels need be introduced. The shape function of the differential ionization cross section is derived analytically to lowest order in coupling strength, and then further simplified through the stationary-phase approximation. The equation of motion, however, is capable of a nonperturbative treatment via an approximation scheme which makes the nonadiabatic couplings separable in $\epsilon$. The stationary-phase result demonstrates the possibility of resonance and hence associative ionization.

The stopping powers of Ni, Ag, Au, and Pb for ∼7-MeV/N (where $N$ means nucleon) C ions and $\alpha$ particles have been measured. For the 7.00-MeV/N C ion, the stopping powers of Ni, Ag, Au, and Pb are found to be 1.353±0.016, 1.080±0.010, 0.832±0.006, and 0.825±0.006 MeV / mg ${\mathrm{cm}}^2$, respectively. From these results, the $Z_1^3$ deviation from the Bethe-Bloch formula is evaluated comparing with our experimental stopping power for $\alpha$ particles. In the course of analysis we employed appropriate effective-charge corrections. Pierce and Blann's effective-charge correction, e.g., gives a reasonable value (1.38) of the $b$ which appears in the Ritchie-Brandt theory, if a $\chi$ value of 1.29 in their theory is assumed.

We consider the time-dependent two-level problem of quantum mechanics, where the levels are coupled by a radio-frequency pulse with an arbitrary time-dependent envelope $V\left(t\right)\mathrm{}$. We derive an approximate solution for the system's transition amplitude $P\left(\infty \right)$ which is correct to the third order of perturbation theory, and which applies to all pulses $V\left(t\right)\mathrm{}$ with finite first and second moments which obey the following: $\lim t^{3}V\left(t\right)=0\mathrm{}$, as $t\to \infty$. Our form of solution for $P\left(\infty \right)$ provides a criterion for judging the validity of a solution previously conjectured by Rosen and Zener, and it is generally useful for providing line-shape details in many cases of practical interest.

We theoretically study intensity-saturation effects in resonant harmonic generation and multiphoton ionization. A first laser excites a two-photon resonance which is coupled by a second laser to an autoionizing state. Starting from an effective Hamiltonian for the three resonant atomic states, we derive a set of equations for the density matrix of the gaseous medium and the electromagnetic field within a semiclassical framework. We present and discuss analytical and numerical solutions of these equations which show a variety of line profiles depending critically on the intensities of the incident laser pulses.

Cross sections for the resonant collisional excitation $2s\to 2p(\Delta n=0)$ accompanied by the capture of the incident electron to a Rydberg state, and subsequent decay of this state by radiation emission, are estimated for the ${\mathrm{C}}^{3+}$ target ion. They are typically of the order of 1∼2×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ when averaged over an energy-bin size of $0.01\lesssim \Delta e_c\lesssim 0.03$ Ry, dominated by the high-Rydberg-state contribution.

For nonlinear short-pulse propagation in long optical fibers, the conventional static approximation of the nonlinear terms in the wave equation must be extended to include the derivative of the pulse envelope. As a result, an initially symmetric pulse will develop an asymmetric self-phase modulation and a self-steepening, which ultimately lead to shock formation unless balanced by dispersion. This effect may be responsible for the pulse asymmetries observed in recent experiments.

In a free-electron laser oscillator, both the amplitude and frequency of the optical field are free to evolve. We use a Lagrangian formalism to describe the dynamics of the interacting electron beam and optical wave in a single pass. The character of the evolution over many passes is controlled by the design of the undulating magnetic field in the free-electron laser interaction region. Several laser magnet designs are presented under the same formalism and compared: the undulator, the tapered undulator, the optical klystron, and transverse gradient ("gain-expansion") undulator. For each of these designs we numerically calculate the gain as a function of optical amplitude and frequency. These "gain surfaces" are used to infer a variety of properties of oscillator evolution and clearly demonstrate the relative merits of each magnet design.

We have studied the stimulated hyper-Raman scattering process in the presence of two strong electromagnetic fields whose frequency modes are initially populated and are near resonance with the two atomic transition frequencies. Using the Green's-function formalism, the excitation spectra are calculated in the limit of high photon densities of both laser fields. Expressions are derived for the relative intensities describing the following processes when (i) both pump fields are at resonance, (ii) the detunings take opposite values corresponding to the cooperative two-photon resonance, and (iii) both laser fields are off resonance. In processes (i) and (ii), the spectra consist of a single peak describing the stimulated hyper-Raman scattering and two pairs of sidebands induced by the laser fields. A new peak appears in process (iii) in addition to those appearing in processes (i) and (ii). The induced spectra are symmetric at resonance but asymmetric in the off-resonance processes, and the extent of the asymmetry depends strongly on the numerical values of the detunings. It is shown that when the two laser fields are treated classically, the derived results are identical to those obtained when the fields are quantized only when photon-photon correlations arising from each laser field are discarded. Numerical calculations for selected values of Rabi frequencies and detunings are graphically presented and discussed.

Superfluorescence emission profiles are computed using one-way coupled Maxwell-Bloch equations. Transverse effects are included in the full three-spatial-dimension case as well as in the cylindrical-symmetry case. Initiating quantum fluctuations are approximated by a random polarization source with a completely random phase and root-mean-square tipping angle of $\frac{2}{\sqrt{N}}$, where $N$ is the number of atoms in each volume element. These fluctuations reduce the tail of the output obtained with transverse effects alone. In fact, the fluctuations in output pulse shapes encompass the Cs data of Gibbs, Vrehen, and Hikspoors. The standard deviation for the delay time is found to be (12.5±4)% for Fresnel number of 0.8 compared with the value (10±2)% recently measured by Vrehen and der Weduwe, also for Cs. Inhomogeneous-broadening effects are also included in some simulations.

In this paper we show that a multistable laser is realizable by the use of a nonlinear medium as one of the resonant cavity mirrors. Using the semiclassical laser theory we derive a gain function with many maxima. The resulting gain equation has many real solutions. We discuss the possible application in switching by use of a suitable external perturbation.

The energy transport in a laser-heated thin foil is investigated with the use of a detailed radiation-hydrodynamic model. The calculation is performed for a long laser pulse (3 nsec full width at half maximum) at relatively low irradiance (${10}^{13}$ W/${\mathrm{cm}}^2$), incident on an 8-μm-thick aluminum foil, and results confirm earlier experimental hypotheses that the dense plasma at the rear side of the foil is heated predominantly by radiation. The model couples a one-dimensional, planar hydrodynamic calculation with a detailed description of the radiation-ionization dynamics in a totally self-consistent manner, thereby assuring that the dominant physical processes determining the energy transport are characterized accurately. The specific radiative mechanisms responsible for energy transfer from the front to the back surface of the foil are found to be quite complex and are described in detail. The front and rear spectral emission are presented and discussed from the point of view of both energetic and diagnostic considerations. Shifting absorption edges due to ionizing plasma are shown to be responsible for many interesting phenomena affecting the energy transport. Time integration of the $K$-shell spectral features is also studied and its effects on the temperature and density diagnostics are analyzed. Finally $K\alpha$ emission lines are shown to be a promising new diagnostic in determining the time development of the temperature near the foil ablation surface.

The de Gennes model is used to derive the energy of interacting dislocations in smectic-$A$ liquid crystals. This energy reduces to the energy of interacting vortices in type-II superconductors when the splay elastic constant $K_1$ is zero and to that derived from the Landau-Peierls elastic energy when spatial variations are slow on a scale of the bend and twist penetration depths. Furthermore, it has a well-defined $K_1\to \infty$ limit. The dislocation energy is used to study the nematic—to—smectic-$A$ transition as a function of $K_1$ in two dimensions and in $4-\epsilon$ dimensions. No evidence for the Nelson-Toner, anisotropic critical point is found in either case.

X-ray structure factors for heavy water were measured at temperatures of 40.0, 23.5, 15.5, 11.2, 7.1, 0.2, and -11.0°C. From these data isochoric temperature differentials (ITD) were constructed for pairs of temperatures $\Delta T=40.0-(-11.0\mathrm{})=51.0,23.5-0.2=23.3,\mathrm{and} 15.5-7.1=8.4$ °C around the maximum density point at 11.2°C. We show that the temperature dependence of the structural parameters can be obtained much more precisely from ITD than from the direct determination of the structure factor. A major structural rearrangement at the second-neighbor shell is identified from the interpretation of the ITD. We then interpret this rearrangement in terms of a possible increased molecular angular correlation of the tetrahedral structure at low temperatures.

Measurements of He-like x-ray line spectra for sulfur and chlorine impurity ions in the Alcator-$C$ tokamak plasma are reported. Results are presented on the relative line ratios of the resonance (w), intercombination (x and y), and forbidden (z) lines as well as dielectronic satellite lines for plasma conditions of $T_e=1.0-1.8\mathrm{}$ keV and $N_e=(1.5-7)\mathrm{}\times {10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The results are compared with atomic calculations employing accurate atomic data which take into consideration effects on the line ratios due to collisional coupling between the $n=2\mathrm{}$ states involved.