The ionization cross section for 3s electrons in argon has been determined for impact energies between the threshold and 1 keV, by observation of the vacuum-ultraviolet emission lines originating from the 3s3${\mathit{p}}^6$ ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$→3${\mathit{s}}^2$3${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$${,}_{3/2}$ transitions in argon ions produced by electron impact. The cross section of the argon 1067-Å resonance line, measured by electron-energy-loss spectroscopy, is adopted as the normalization standard. The maximum of the 3s-ionization cross section is 6.27×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 46 eV impact energy. A double-peaked structure is demonstrated very clearly in the ionization-function curve. The error in the cross section is estimated to be 21%, including the error in the normalization standard. Comparisons of the present results are made with previous measurements and with theoretical results.

Intermolecular-interaction potentials depend on the vibrational coordinates of the molecules involved. We study the effect of this v dependence (as we will call it for brevity) on the symmetry of line shapes of rotovibrational collision-induced-absorption (RVCIA) spectra of collisional complexes such as ${\mathrm{H}}_2$-He or ${\mathrm{H}}_2$-${\mathrm{H}}_2$. If the v dependence is ignored, individual line shapes Γ(ω) of CIA spectra satisfy the widely used ‘‘detailed balance’’ relationship Γ(-ω)=$e^{\mathrm{-}ħ\omega /\mathrm{kT}}$Γ(ω), where ω designates the frequency shift relative to the molecular transition frequency, and T the temperature. However, if one accounts for the v dependence, the symmetry of a computed profile is modified significantly, the more so the higher the temperature and the higher the vibrational overtones one considers. These differing symmetries are described in quantitative terms that are of interest in modeling or analyzing RVCIA spectra. This paper may be considered the second, concluding part of our theoretical study of the influence of the v dependence on RVCIA spectra; the previous part [Phys. Rev. A 36, 4700 (1987)] deals with the influence of the v dependence on the integrated intensities (spectral moments) of RVCIA spectra.

Projectile K-shell ionization and excitation cross sections were measured for 8.6-MeV/amu ${\mathrm{Ca}}^{19+}$,18+ beams incident on various gas targets. The ionization and excitation cross sections are compared with the Glauber approximation and with the plane-wave Born approximation. Various factors that may affect the ionization and the excitation cross sections in this intermediate-velocity regime, such as screening by the target electrons, polarization, and binding, are examined in order to establish their relative importance.

Electron-capture cross sections by 8.6-MeV/amu ${\mathrm{Ca}}^{18+}$, ${\mathrm{Ca}}^{19+}$, and ${\mathrm{Ca}}^{20+}$ beams were measured in gas targets from ${\mathrm{H}}_2$ to Xe. Measurements were made of total electron capture as well as of electron capture in coincidence with projectile K x rays. Comparison with the eikonal approximation shows a reasonable agreement with the experimental data. The effect of various metastable states of the heliumlike ${\mathrm{Ca}}^{18+}$ ion created by electron capture for ${\mathrm{Ca}}^{19+}$ projectiles is analyzed, showing that these states play an important role in the detected x-ray yields.

Diffusion of muonic hydrogen atoms in gaseous hydrogen is studied. Scattering kernels are derived from the kinematics of an inelastic binary collision. The effect of rotations of the hydrogen molecules is treated by defining and computing an effective inelastic energy transfer $Q_{\mathrm{eff}}$. The Doppler effect is taken into account by averaging the cross sections over the Maxwellian velocity distribution of the target molecules.

Total cross sections for single ionization and charge transfer have been calculated using the classical-trajectory Monte Carlo (CTMC) technique for collisions of both positrons and protons with helium. Analysis of the classical trajectories has helped to explain the differences in the collision mechanisms responsible for the observed relative magnitudes of the positron and proton electron-removal cross sections. In the intermediate collision velocity range (1.5 a.u.<v<4.5 a.u.) it is found that because the positron is much smaller in mass than the proton, two dynamical effects occur leading to differences in their efficiency in electron removal. First, positrons are less likely to singly ionize helium than are protons since they possess less kinetic energy above the ionization threshold and accordingly there is a lower probability for ionization. Second, positrons are more likely to remove an electron from helium by charge transfer than are protons since they may be deflected by the target to large positive or negative scattering angles and be accelerated or decelerated to more readily momentum-vector match with an orbital electron. In the large-velocity regime (v>4.5 a.u.) positrons and protons are found to be equally likely to singly ionize helium, but positrons remain at least half an order of magnitude more likely to remove an electron by charge transfer.

The semiclassical, optical potential, and quantal distorted wave theories of collision-induced radiative quenching are discussed and the relationships between them are presented. The three procedures are applied to the calculation of the cross sections at thermal collision energies of the process He(2 ${}_{}{}^1{}_{}{}^{}\mathit{S}$)+He(1 ${}_{}{}^1{}_{}{}^{}$S)→He(1 ${}_{}{}^1{}_{}{}^{}$S)+He(1 ${}_{}{}^1{}_{}{}^{}$S)+hν. Several choices of the potential energy curves of the excited A ${}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state of the ${}_{}{}^4{}_{}{}^{}$${\mathrm{He}}_2$ molecule are employed. The accuracy of the semiclassical and optical potential methods is assessed by comparison with the quantum-mechanical theory. It is shown that the cross sections are sensitive to the barrier in the A ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ potential but the major uncertainty in the predicted cross sections lies in the adopted transition dipole moment.

We develop a simple and general framework for understanding the relationships between absorption and emission in multilevel systems (molecules, semiconductors, condensed matter, etc.) in and out of equilibrium. Conditions under which the absorption and emission spectra display mirror image symmetry are determined. The relationship between the absorption and emission cross sections at frequency ω and temperature T, previously derived by Neporent and McCumber for thermally equilibrated systems, is generalized and a new derivation is presented which extends its range of application to systems out of equilibrium. This derivation requires only microscopic reversibility and that the optically coupled manifold of levels belonging to both initial and final electronic states are each in internal thermal equilibrium at the same temperature T. The initial and final (e.g., ground and excited electronic) states need not be in equilibrium. If more than one excited-electronic-state manifold participates in the dynamics and these excited-electronic-state manifolds are not themselves in equilibrium, a somewhat more complicated relationship between absorption and emission can be obtained, provided that the internal degrees of freedom within the electronic-state manifolds are each in equilibrium at the same temperature.

Effects of inhomogeneous broadening are examined. Good agreement is found between the Neporent-McCumber relationship and experimental absorption and emission spectra for the laser dyes rhodamine-6G and 2,5-biphenyloxazol PPO, and the solid-state laser crystal alexandrite. A new relationship between absorption and emission in direct-band-gap semiconductor materials is derived. This relationship allows us to calculate the emission from a direct-band-gap semiconductor provided the dependence of the band-gap energy on excitation level is not large. These relationships hold even when pumping conditions differ for emission and absorption experiments; the dependence of emission on excitation level can thereby be calculated. This permits the gain spectra of semiconductor diode lasers to be determined from their absorption spectra. In the limit that both the absorption and emission are measured for a thermally equilibrated nondegenerate direct-band-gap semiconductor system with identical pumping conditions for the absorption and emission experiments, our new relationship reduces to previous results.

This is the first of two papers that develop the theory of oscillatory spectra. When an atom is placed in a magnetic field, and the absorption spectrum into states close to the ionization threshold is measured at finite resolution, so that individual energy levels are not resolved, it is found that the absorption as a function of energy is a superposition of sinusoidal oscillations. These papers present a quantitative theory of this phenomenon. In this first paper, we describe the physical ideas underlying the theory in the simplest possible way, and we present our first calculations based upon the theory. In the second paper, the theory is developed in full detail, proofs of all of the assertions are given, and we describe the algorithm that was used to make the calculations.

A formula is derived for oscillations in the near-threshold absorption spectrum of an atom in a magnetic field. Three approximations are used. (1) Near the atomic nucleus, the diamagnetic field is negligible. (2) Far from the nucleus, the waves propagate semiclassically. (3) Returning waves are similar to (cylindrically modified) Coulomb-scattering waves. With use of these approximations, together with the physical picture described in the accompanying paper, an algorithm is specified for calculation of the spectrum.

The double-modulation electron-spin-resonance (DOMESR) phenomenon is thoroughly studied from both the experimental and theoretical points of view. The extension of the previous two-wave model to a more comprehensive interpretation in the frame of the ‘‘dressed-atom’’ formalism directly accounting for the effects of both modulations allows a detailed modeling of the actual experiments. The very narrow peaks experimentally observed are interpreted point by point in terms of the multiple resonances occurring when the many-sideband structure is accounted for. The full explanation of the effects of different experimental parameters is reported. The effectiveness of DOMESR spectroscopy in very slow motion studies is proven by the present analysis in that the conditions found for the maximum enhancement of the signal and the direct connection of the linewidth to the relaxation parameters of the single spin-packet ensure virtual independence of this spectroscopy from the inhomogeneous broadening.

The fraction of the total line strength of one of the core-hole spin-orbit-split manifolds in x-ray absorption is proved to be related to the expectation value of the valence-band spin-orbit operator. The relation obtained, by angular momentum algebra, is valid if the total angular momentum of the core hole is a good quantum number, which is true for deep core levels. Therefore, the branching ratio of deep-core-hole manifolds is probably the most direct probe to measure the valence-band spin-orbit interactions, especially in transition-metal compounds. The branching ratio measures the angular part of the spin-orbit operator and is complementary to magnetic measurements which determine the Landé g factor.

The influence of saturation and Stark shifting on the collisionless coherent anti-Stokes Raman scattering (CARS) spectra of Q-branch vibrational transitions has been studied theoretically and experimentally. These processes can significantly affect the magnitude and the spectral distribution of the CARS signal at the input power levels required to obtain detectable signals in typical low-pressure CARS experiments. The theory is developed with use of the Placzek approximation and rotation-vibration Bloch equations are obtained. The Doppler effect, nonresonant susceptibility, and spatial variation of power density in the probe volume are neglected. The Rabi frequency is found to increase with the square root of the final-state vibrational quantum number while the Stark frequency is nearly independent of this quantity. In order to understand the nature of some of the effects observed in scanning CARS, approximate analytic solutions are given for simple cases and CARS output spectra are obtained as a function of detuning. A Rabi-split response spectrum is found for all J with a Stark-induced asymmetry favoring the lower-frequency peak. Although the orientational degeneracy is lifted by both the Rabi and Stark terms, only a very weak dependence on the rotational quantum number is found in the spectral distribution; this allows use of the unsaturated rotational-quantum-number scaling in the saturated regime. It is apparent that the deviations due to saturation are weak for rotational temperature measurements while they are important for vibrational temperature measurements. Experimental results for low-pressure (2–5 mbar) nitrogen are also discussed. Appreciable deviations due to saturation and Stark effects are found for pump laser and Stokes power density products greater than 30 (GW/${\mathrm{cm}}^2$${)}^2$. Semiquantitative agreement with theory is obtained for the observed dependences on power and on rotational and vibrational quantum numbers. A method allowing quick correction of saturated experimental data is proposed.

A detailed theoretical discussion is presented of the effect of colored noise on nonlinear dynamical systems. The ideas arising from it are applied to a particular example of such a system: a model of dispersive optical bistability, considered in the contexts of both additive and multiplicative forcing. Analogue experiments and digital simulation techniques are used to explore the applicability and range of validity of theories proposed to date. The physical phenomenon of bimodality induced by the finite bandwidth (alone) of additive forcing is demonstrated in detail for the first time. The three existing theories capable of accounting for this phenomenon can each be categorized in terms of a single (constant) value of a characteristic parameter $P_{\mathrm{expt}}$; but it is shown that, in reality, $P_{\mathrm{expt}}$ varies weakly with the correlation time τ of the noise. The variation of $P_{\mathrm{expt}}$ with τ is investigated in the currently accessible range of 0.1≤τ≤∞. .AE

An analytical function is derived which can serve as a criterion of the instability boundary of both optical-bistable (OB) systems and laser systems with an injected coherent signal (LIS). By means of the criterion, we analyze, in various asymptotic limits, the instability regions of OB and LIS in the parameter space. In both good- and bad-cavity conditions of LIS a full hysteresis loop between coexisting stable states is observed for the first time.

Density-matrix formalism is applied to construct a model for third- and fifth-harmonic generation in a four-photon resonant medium. The transition rates and coherent superposition of states associated with the resonant levels are treated rigorously, while the adiabatic following approximation is used to calculate the contribution of all nonresonant levels to the interaction. Expressions for the quadratic and quartic Stark shift and multiphoton ionization emerge naturally from the theory. At the power level required for appreciable four-photon absorption, the intensity-dependent Stark shift and ionization become overwhelming and limit the efficiency of harmonic generation. Because of the Stark shift, an intense pulse suffers a ‘‘self-detuning’’ effect and propagates through the resonant medium with minimal absorption. The interference of the phase-dependent Raman scattering with the multiphoton transition is found to be responsible for the vanishing of ionization in the presence of the generated third-harmonic field, an effect which was observed in earlier experiments.

We show that a previously derived modulational instability formula is universally applicable to nonlinear optical feedback systems.

We present the first mixed-type solutions to the coupled nonlinear Schrödinger equations which govern optical pulse propagation in a birefringent fiber. These represent polarization-modulated pulses which, apart from the absolute phase, propagate unchanged in form. It is shown they are bound states of two solitary waves which separately have constant and uniform orthogonal linear polarizations. Furthermore, there exists a minimum-energy threshold for the formation of these bound states.

The interplay between linear birefringence, nonlinear polarization changes, and chromatic dispersion may lead to phase-matched parametric four-photon mixing. As a result, new modulational polarization instabilities are predicted in single-mode fibers: Instability of an intense light beam may occur even when the wave is coupled to a spatially stable eigenmode, and propagates in the normal-dispersion regime.

We propose a density-functional theory for the isotropic-nematic transition of hard ellipsoids which is in fair quantitative agreement with the recent computer simulations of this system and which improves considerably upon the earlier theoretical attempts. The theory has an explicit oblate-prolate symmetry and leads to simple analytic expressions, e.g., for the equation of state of the isotropic phase. When the free energy of the nematic phase is expanded with respect to the Maier-Saupe quadrupole order parameter, an explicit Landau theory is produced, which is shown to underscore considerably the strength and the width of the transition. A virial expansion of the free energy produces in turn an Onsager theory for finite elongations whose results are shown to tend only very slowly to their Onsager limiting value. We also propose a Lindemann rule for orientational freezing.