Efficiency of the second-harmonic generation by a $p$-polarized electromagnetic wave that is obliquely incident upon an inhomogeneous plasma is analytically investigated considering the plasma-density gradients characteristic for the laser-produced plasmas. The results obtained are in excellent agreement with earlier numerical calculations.

The paper discusses the connection between the deterministic and stochastic description of nonlinear, generally nonequilibrium systems. The fluctuations are treated in terms of a Markov process (master equation or Fokker-Planck equation). For processes obeying the symmetry of generalized detailed balance (GDB), the deterministic flow is cast into a form exhibiting the maximum amount of information about the stochastic dynamics. The deterministic flow contains information about Kramers-Moyal moments of order $n\ge 2$. A semipositive definite, symmetric transport matrix is introduced which satisfies generalized Onsager relations. In terms of this transport matrix the deterministic flow of processes obeying GDB can be cast into the standard form of thermodynamics. Some of the results are elucidated using a nonlinear birth and death master equation with nearest-neighbor transitions. Given the deterministic flow, the focus is on the problem of reconstruction of the original stochastic dynamics. The information contained in the deterministic flow of processes obeying GDB is not sufficient for a reconstruction of the stochastic dynamics. Given only the information of both, the stationary probability and deterministic flow, we identify a class of Fokker-Planck processes for which the stochastic dynamics can be uniquely reconstructed.

It is shown that the interplay between chemical reactions and thermodynamic stability gives rise to some novel phenomena manifested in a slowing down of the chemical reaction and in changes in the critical behavior of transport processes. In an $n$-component fluid, when all the components participate in the reaction, the rate vanishes near the critical point $T_c$ as ${\left[\frac{(T-T_c)}{T_c}\right]}^{\gamma }$, where $\gamma \sim 1.25$. When one of the components does not participate in the reaction, the rate vanishes, in general, as ${\left[\frac{(T-T_c)}{T_c}\right]}^a$, where $a\sim 0.12$. If more than one component is nonreactive the rate of the reaction is not sensitive to the approach to $T_c$. Reactive binary mixtures are treated in detail on the basis of a mode-coupling theory. In contrast to nonreactive mixtures, the shear viscosity has no divergence near the critical point. In addition, the diffusion coefficient has a different temperature and wave-vector dependence.

We report new measurements of the Rayleigh-Brillouin spectrum of a critical binary mixture and review the existing data in this field. We used a double-pass Fabry-Pérot interferometer and a weakly scattering system (nitroethane-isooctane) so that the Brillouin lines were not affected by the wings of the Rayleigh line, even in the immediate vicinity of the critical point. No anomaly was detected in the shift or width or intensity of the Brillouin lines. From the ratio of the intensities of the Rayleigh and Brillouin lines we deduced the ratio of specific heats ${\gamma }_0$, the correlation length ${\xi }_0$, and the susceptibility amplitude ${\left(\frac{\partial c}{\partial \mu }\right)}_{\mathrm{pT}}^0$, without any hypothesis about light-scattering amplitudes. These values lead to an amplitude ratio $R_{\xi }^{+}{R}_c^{-\frac{1}{3}}$ in full agreement with the theoretical values.

The amplification of millimeter electromagnetic radiation within the cathode region of a cold-cathode glow discharge is correlated with the variations in the current-voltage characteristic of the glow discharge. The transfer of energy from the fast electrons to the electromagnetic wave amplifies the incident radiation and also causes a decrease in discharge current. The latter is called the "negative response." This negative response is superimposed on a "positive response"; i.e., an increase in current and decrease in voltage, which follows from the reduction of electron-ion recombination rate by the incident electromagnetic radiation. The negative response was extracted from the measured current-voltage variations, and the measured amplification is shown to be proportional to the negative-response voltage multiplied by the electron density. Divergence of the incident electromagnetic wave by the plasma (whose refraction index is smaller than one) was measured and compared with the theory. The effect of this divergence on the measurement of amplification is discussed. The theory of stimulated emission of bremsstrahlung has been developed to explain the above experiments. Two methods of calculations are compared: one based on differences in populations of levels, as is conventional in laser physics, and the second method which exploits the dependence of the scattering cross section on the incident electron energy. Possible improvements of the theory are discussed.

We consider a two-level system where a pair of discrete quantum states of arbitrary widths and separation are coupled by a general time-dependent external field with a pulselike matrix element, $V\left(t\right)\mathrm{}$. Using a transformation suggested by Robinson [Phys. Rev. A 24, 2239 (1981)] and applying the WKB approximation, a quasiadiabatic solution is constructed for the system's transition amplitude. This solution is useful in estimating transition amplitudes in the asymptotic region for a wide variety of smoothly varying pulses $V\left(t\right)\mathrm{}$.

Close-coupling calculations are presented for $e-\mathrm{H}$ inelastic scattering involving the exact contribution of $n\le 3$ states for energies between 1 and 4 Ry. These calculations were performed with an extension of the noniterative integral equation method of Smith and Henry. Comparison with experiment shows a good agreement and permits us to estimate the quality of earlier approximations.

The apparent and direct electron excitation cross sections of both the $1s_2$ and $1s_4$ levels of Ne are measured using laser-induced fluorescence. The experiment utilizes radiation trapping to maintain appreciable level populations. The direct excitation cross section for the $1s_2$ level has a peak value of 1.0×${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ at 65 eV; the direct excitation cross section for the $1s_4$ level has a peak value of 8.8×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ at 65 eV.

We describe our observations of the effect of stimulated and four-wave mixing processes on the kinetics of atomic states dressed by an intense laser field in the presence of collisions. The effects are observed via the forward- and side-scattered light. We suggest that the effects observed by Harter et al. are produced by amplification of background laser fluorescence by these processes.

Doppler-tuned x-ray spectra of 1- to 2-MeV C ions taken 6 ns after foil excitation contain three lines between 282 and 291 eV, identified as $2p\to 1s$ forbidden transitions in C III $(1s2\mathrm{}{p}^{3)}{}_{}{}^5{}_{}{}^{}S_{}^0{}_{}{}^{}\to (1\mathrm{}{s}^{2}2p^2) {}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$, 289.4 eV and $(1s2s2\mathrm{}{p}^{2)} {}_{}{}^5{}_{}{}^{}P_{}^e{}_{}{}^{}\to (1\mathrm{}{s}^{2}2s2p) {}_{}{}^3{}_{}{}^{}P_{}^o{}_{}{}^{}$, 287.3 eV, and C II $\left(1s2\mathrm{}{s}^{2}2p^2\right) {}_{}{}^4{}_{}{}^{}P_{}^e{}_{}{}^{}\to \left(1\mathrm{}{s}^{2}2s^{2}2p\right) {}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}$, 284.5 eV. This is the first observation of the ${}_{}{}^5{}_{}{}^{}S\to {}_{}{}^3{}_{}{}^{}P$ and the ${}_{}{}^4{}_{}{}^{}P\to {}_{}{}^2{}_{}{}^{}P$ transition in any ion. The energies agree with measured energies of Auger electrons from these upper terms, but disagree with recent $\frac{1}{Z}$ expansion calculations.

Using the renormalization-group method of Derrida, Gervois, and Pomeau, we have extended the calculation of the Feigenbaum universal bifurcation ratio to third order. The calculated values for the limit point and the bifurcation ratio are, respectively: $a^{*}=1.401\mathrm{}1487$ and $\delta =4.675\mathrm{}3244$, which agree extremely well with the exact values: $a^{*}=1.401\mathrm{}1551$ and $\delta =4.669\mathrm{}2011$.

A previously developed extension of the quasistatic approximation for the broadening of the Lyman-$\alpha$ line is applied to Lyman $\beta$. This extension allows for slow variations of the total field acting on a radiating atom. The corresponding low-frequency Stark broadening depends on the radiator velocity and is therefore correlated with the thermal Doppler broadening. With the low-frequency field correction, a major disagreement with experiment near the center of the Lyman-$\beta$ line is removed. Corresponding corrections for lines from high-$Z$ hydrogenic ions are also estimated.

The single-configuration Dirac-Fock treatment of doublet splittings in alkalilike spectra is examined, and it is shown that the nonrelativistic limit is not the nonrelativistic Hartree-Fock, but a multiconfiguration, core-polarization approximation. The latter is the mechanism which has conventionally been invoked to account for the anomalous doublet splittings of nonpenetrating states in alkalilike atomic systems. Both approaches should thus be capable of representing the doublet anomalies.

The mean spherical model with continuous pair functions is solved exactly, in the limit of strong coupling, for a $\nu$-component classical plasma with the $D$-dimensional Coulomb interaction. Exact lower bounds for the one-component plasma correlation energy, which are very effective for arbitrary coupling, are derived.

A simple rigorous proof is given of the complementarity theorem introduced by Lowke, Parker, and Hall (LPH) to relate electron back diffusion to forward diffusion. It provides additional insight from which we conclude (i) that the theorem does not strictly apply to the situation discussed by LPH, and (ii) that for certain types of Monte Carlo simulations of electron-swarm behavior, the relaxation distance of the injected electrons can be minimized by injecting them into the forward hemisphere with a mean energy substantially higher than the anticipated equilibrium value. The Monte Carlo simulation data of Braglia and Lowke provide an interesting example of the effects discussed, and an improved version of this type of simulation is proposed. It employs an ideal source, having the same velocity distribution as the electrons arriving at an absorbing anode. In the steady-state distribution generated by such a source all the electrons carry useful information, even when the source is placed arbitrarily close to an absorbing anode.

Chantry's gedanken experiment leads to the conclusion that the relaxation distance of electrons emitted from a source can be eliminated if the electrons have the same velocity distribution as the electrons arriving at an absorbing anode. In this paper Monte Carlo results are reported that confirm this conclusion.

de Schepper and Cohen have shown recently that the hydrodynamic modes for a hardsphere fluid may be extended to quite large wave vectors, and that at high densities the hydrodynamic part of the dynamical-structure factors dominates over a wide range of wave vectors. The objective here is to describe the results of a similar analysis of two simple models of the self-structure factor. The simplicity of the models allows a scaling of the frequency and wave vectors such that a universal form independent of potential model, density, and temperature is obtained. The diffusion mode and hydrodynamic part of the self-structure factor are calculated, leading to a behavior similar to that observed by de Schepper and Cohen.

It is pointed out that a previously proposed kinetic model of the generalized Enskog equation does not describe properly the hydrodynamic spectrum of density fluctuations because of the neglect of the momentum state corresponding to the longitudinal stress tensor in the representation of the memory function. It is shown that inclusion of this state leads to a kinetic model which gives results in agreement with light-scattering data.

The first observation of nonresonant, stimulated anti-Stokes Raman emission is reported. A metastable $\mathrm{Tl}\left(6p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^o{}_{}{}^{}\right)$ inversion is created by selective photodissociation of TlCl. Raman scattering from the Tl metastable state to ground using 532- and 355-nm pump lasers resulted in stimulated emission at 376 and 278 nm, respectively. Conversion efficiencies up to 10% are reported.