We present a theoretical interpretation of the experimental results on three-photon-resonant four-photon ionization of Kr reported by Landen, Perry, and Campbell [Phys. Rev. Lett. 59, 2558 (1987)] and Perry and Landen [Phys. Rev. A 38, 2815 (1988)]. Our calculations are based on multichannel quantum-defect theory combined with a density-matrix formalism describing the spatiotemporal development of the process. We obtain good agreement with the data, which even at intensities as high as ${10}^{14}$ W/${\mathrm{cm}}^2$ show the imprint of the underlying atomic structure.

Photoelectron energy spectra following (2+1)-multiphoton ionization of ${\mathrm{H}}_2$ via the double-well E,${\mathit{F}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ state have been measured using a magnetic-bottle electron spectrometer. The range of vibrational levels studied, $v_E$,F=3–9, includes states localized at the bottom of the E well, and also states that span both wells. The branching among the vibrational ionization channels is governed by the strong R dependence of the electronic wave function and the degree of localization of the vibrational wave function in the E or the F well. Theoretical analysis confirms that at least two mechanisms contribute to the observations: a direct process involving only the E,F state and the ${\mathrm{H}}_2^{+}$ ionization continuum, and an indirect process involving the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ autoionizing state. Mass spectroscopic measurements carried out for $v_E$,F=9, J=1 show that about 25% of the ions produced were ${\mathrm{H}}^{+}$. Qualitative arguments suggest that most of these protons arise from dissociative ionization to the ${\mathrm{H}}_2^{+}$ continuum.

The present paper combines a Langevin quantum-regression method with a density-operator approach to derive the master equation for the quantum theory of multiwave mixing in a very efficient way. The approach is quite general and is particularly valuable for analyzing complicated media such as semiconductors. It is used in the present paper to derive the quantum multiwave-mixing equations in a squeezed vacuum. We find improved formulas for resonance fluorescence in a squeezed vacuum as well as the squeezing coefficients in a squeezed vacuum. Comparing squeezing spectra in squeezed and ordinary vacua, we find significantly enhanced squeezing for the appropriate pump-vacuum relative phase.

The propagation and interaction of light in the two-dimensional optical superlattice, a two-dimensional periodically modulated medium, were investigated by the dynamical theory with the four-wave approximation, which is based on an incident beam that satisfies Bragg conditions in two orthogonal directions. The excited fields and the intensities of exiting waves in the two-dimensional optical superlattice were calculated. The relation between relative intensities of the exiting waves and angular deviation of the incident beam as well as the index-modulated parameters of the superlattice was obtained. Also discussed are possible applications of the superlattices in optical communications.

The three-dimensional time-dependent set of Maxwell-Bloch equations for amplified spontaneous emission in diffraction-dominated (low Fresnel number) and small-gain systems is solved. The explicit form of the modes of radiation for gain and density profiles which are typical of hot-plasma x-ray-laser experiments is derived. The behavior of the excitation strengths of the modes is analyzed. It is found that, due to the combined effects of diffraction, refraction, and gain, the excitation strength is a more rapidly decreasing function of the mode number in systems with smaller Fresnel numbers. The behavior of coherence as a function of propagation length in systems with various Fresnel numbers is computed and analyzed.

A reduced equation of motion that describes the excited-state dynamics of interacting two-level impurity molecules in a dielectric host crystal is derived starting from a microscopic model for the total system. Our theory generalizes the derivation of the conventional superradiance master equation for molecules in vacuum; the role of photons in the conventional theory is played by polaritons (mixed crystal-radiation excitations) in our approach. Our final equation thus contains dispersive and superradiant polariton-mediated intermolecular interactions. The effect of the dielectric host is completely contained within a rescaling of these interactions with the transverse dielectric function ε(ω) of the crystal taken at the impurity’s transition frequency. Our theory yields all local field and screening factors for both the dispersive and the dissipative couplings from a single, unified starting point. Known scaling laws for the spontaneous-emission rate and the instantaneous dipole-dipole interaction are extended to the frequency region where the dispersion of ε(ω) is important.

The TE modes of a partially-dielectric-loaded waveguide can be excited by an electron beam via an explosive instability. The second harmonic of the TE mode drives density oscillations that are in resonance with the beam. The density oscillations couple with the TE mode to produce a nonlinear current driving the TE mode. The TE mode and the longitudinal second-harmonic wave amplitudes grow explosively with time on a time scale ${\tau }_{\infty }$∝‖ɛ̃(0)${\Vert }^{\mathrm{-}1}$ where ‖ɛ̃(0)‖ is the normalized initial TE wave amplitude. Allowing a low-density plasma in the device decreases the explosion time.

In a three-level ssv-type atom, which has two excited levels and a ground level, the fluorescence exhibits quantum beats due to a correlation between the two excited levels. However, a measurement of the atomic level yields information about the atomic state but completely suppresses the quantum beats. Here we discuss a modification of the atomic-level measurement to allow for a continuous transition from perfect quantum beats to complete information of the atomic level. The complementary relation between the beats and knowledge of the atomic level is quantitatively shown to be a proportionality between the beat visibility and the probability of making an incorrect measurement of the atomic level. We discuss the realization of the scheme in the context of current experimental proposals involving atomic-level measurements via Fock number states in micromaser cavities. Imperfect photodetector efficiency and micromaser cavity damping are incorporated into the formalism.

We present a simple model that allows a description for the correlation of counterpropagating waves and the formation of spatial patterns in superfluorescent emission. No separation between left- and right-running waves is performed, and retardation effects are fully included. The results indicate qualitative agreement between our calculations and experiments in KCl:${\mathrm{O}}_2^{\mathrm{-}}$. .AE

The effects of radiation guiding on an electron-beam channeling x-ray laser are investigated. An essential feature of this laser is distributed feedback induced by Bragg reflections. Using the Maxwell-Bloch scheme including the transverse Laplacian it is shown that reflection guiding and absorption guiding can avoid the diffraction of the amplified radiation out of the amplification medium for a beam radius in the range of 100–1000 Å. This opens the possibility of considering a channeling x-ray laser with high current density ${(10\mathrm{}}^5$ A/${\mathrm{cm}}^2$), low emittance, but with very small total current (10 μA), similar to the beam used in scanning electron microscopy. On the other hand, gain guiding can avoid radiation diffraction in such small beam systems, but only with a further increase in the requirements on the beam current density. It is noted that planar superlattice crystals with a periodic structure of the order of 100 Å transverse to the beam propagation can be useful for radiation guiding.

Instabilities are predicted for absorptive optical bistability in a nonideal Fabry-Pérot cavity on the basis of Maxwell-Bloch theory. We show that the stability problem of the Maxwell-Bloch equations can be formulated in terms of a Riccati differential equation with boundary conditions. We have integrated this equation numerically. Our results crucially depend on the value of the transmission coefficient of the mirrors. For finite values of this quantity we find that the nearest side mode is responsible for two disconnected instability domains in the plane spanned by the output intensity and the medium response time. One of these domains can generate instabilities in the upper and the lower branch of the steady-state curve if the cooperation parameter is sufficiently large. Higher side modes can give rise to positive-slope instabilities as well. From our findings it can be understood why a recent experimental search for instabilities in absorptive optical bistability has led to a negative result. We finally demonstrate that the instability spectra of a Fabry-Pérot cavity and a so-called equivalent ring cavity differ considerably.

The k-photon Jaynes-Cummings model is studied with respect to the properties of the radiation mode. In particular, the generation of coherence and squeezing in the one- and two-photon model is examined for the case that the field is initially in the vacuum state and the atom is prepared in a coherent superposition of states. Coherence is found in the one-photon model, and squeezing appears in both the one- and the two-photon model, the maximum noise reduction being 25% and 45%, respectively. In the one-photon model coherent squeezed radiation is generated, whereas the two-photon model yields a squeezed vacuum. For an initial atomic preparation close to the ground state, this squeezed vacuum may be interpreted as the result of two-photon interaction between the quantized cavity field and a classical atomic current.

We consider the problem of reducing the contributions of pump and spontaneous-emission noise to the output noise of a laser field. We consider two models based on an atomic medium, one analytic, the other involving numerical simulation of a sequence of pump cycles. The predictions of both models are shown to agree. For regular pump excitation the intensity fluctuations of a laser well above threshold may be reduced to zero at the cavity resonance frequency if the ratio of the lifetimes of the upper and lower lasing levels is sufficiently large that spontaneous emission is suppressed. When the atomic lifetimes are comparable, spontaneous emission reduces the amount of noise reduction. For example, for equal atomic lifetimes regular pump excitation will result in a maximum noise reduction in the output field of one-half the shot-noise limit.

The nonlinear, stationary solution of the kinetic equations for a one-dimensional plasma with a small collision term is presented; we analyze the problem of the plasma interaction with a negative ion moving with small constant velocity through a one-component (electronic) plasma. A self-consistent two-scale analysis is performed. The solution is compared with the results of the corresponding linearized equations that are commonly used. It is shown that the theory based on the linearized Vlasov equation without collision term cannot give an adequate description of either potential profile or stopping power, at least in the regime of small projectile velocities.

The quenching of Einstein A coefficients was observed by measuring the branching ratio of visible and extreme-ultraviolet line intensities for C iv, C iii, and N v ions.

New measurements are reported on the red shift of the He ii $P_{\alpha }$ line emitted from a gas-liner pinch plasma with electron densities in the range of (1.0–2.4)×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and electron temperatures between 5 and 7 eV. Line radiation was observed from a homogeneous and optically thin core region of the pinch discharge, free of impurity elements that could distort the line shape. The experimental shifts vary linearly with electron density and are in good agreement with calculations that we have performed on the basis of the classical-path and impact approximations for electron perturbers, according to the theory developed by Griem [Phys. Rev. A 27, 2566 (1983); 38, 2943 (1988)]. An important role is assigned in this work to the Δn=0 interactions. Our calculations are also in good agreement with earlier measurements by Pittman and co-workers [Pittman et al., Phys. Rev. Lett. 45, 723 (1980)] and Fleurier and le Gall [J. Phys. B. 17, 4311 (1984)]. The measured profiles of this line are entirely symmetrical within experimental error and show no evidence of an asymmetry favoring the blue wing.

Diffusion theory of electrons in a drift tube is extended to satisfy approximate energy balance as the electrons migrate radially and axially in a steady, uniform electric field. Previously, it was assumed that the characteristic energy is independent of position in the drift region, leading to the well-known linear diffusion equation for the spatial variation of electron density. This assumption is not made in the present paper. Results for the spatial variation of average energy are close to those obtained from exact solution to the linearized Boltzmann equation. In addition, it is shown that diffusion theory with energy balance gives a relation between characteristic energy and drift-tube geometry which is in close agreement with the empirical relation that has been used for years to analyze steady drift-tube measurements, without invoking the assumption of anisotropic diffusion.

The nonlinear Bhatnagar, Gross, and Krook [Phys. Rev. 94, 511 (1954)] kinetic equation is solved for boundary conditions leading to planar Couette flow and heat transport. In the limit of zero Knudsen number but arbitrary uniformity parameter (shear rate), an exact ‘‘normal’’ solution is obtained. The velocity distribution function is illustrated explicitly for states far from equilibrium. At finite Knudsen number, the distribution function is obtained from numerical solution of a set of nonlinear, singular integral equations. Results are presented for a range of values of the Knudsen number and uniformity parameter. The approach to a normal state is studied with a determination of the hydrodynamic profiles, velocity slip, shear viscosity, and a nonlinear Burnett transport coefficient.

The onset of convection in binary mixtures enclosed between two horizontal, isothermal boundaries and driven by an externally applied vertical concentration gradient is investigated. It is shown that internal temperature variations, which are driven by the Dufour effect, can have significant influence on the stability boundaries of the quiescent state, depending on the strength of the Dufour coupling.

The evolution of a one-dimensional array of reactive lamellae with distributed striation thickness is studied by means of simulations, scaling analysis, and space-averaged kinetics. An infinitely fast, diffusion-controlled reaction A+B→2P occurs at the interfaces between striations. As time increases, thin striations are eaten by thicker neighbors resulting in a modification of the striation thickness distribution (STD). Scaling analysis suggests that the STD evolves into a universal form and that the behavior of the system at short and long times is characterized by two different kinetic regimes. These predictions are confirmed by means of a novel numerical algorithm.