We present and discuss a theory for hydrodynamic effects in forced-Rayleigh-scattering experiments. The efficiency of phase-conjugate wave-front generation by forced Rayleigh scattering varies significantly with the nature of the solvent and the scattering angle. Acoustic effects become important for small scattering angles and times shorter than 0.5 ns under which conditions the simple thermal-diffusion theory fails. The phase-conjugate reflection efficiency predicted by our theory at short times is much smaller than that predicted by the thermal-diffusion theory, and thus need not obscure other quantum electronic processes.

The nonlinear evolution of free-electron lasers in the presence of an axial guide field is investigated numerically. A set of coupled nonlinear differential equations is derived which governs the self-consistent evolution of the wave fields and particle trajectories in an amplifier configuration. The nonlinear currents which mediate the interaction are computed by means of an average over particle phases, and the inclusion of fluctuating space-charge fields in the formulation permits the investigation of both the stimulated Raman and Compton scattering regimes. The initial conditions are chosen to describe the injection of a cold, axially propagating electron beam into the interaction region which consists of a uniform axial guide field and a helical wiggler field which increases to a constant level adiabatically over a distance of ten wiggler periods. After an initial transient phase, the results show a region of exponential growth of the radiation field which is in excellent agreement with linear theory. Saturation occurs by means of particle trapping. The efficiency of the interaction has been studied for a wide range of axial guide fields, and substantial enhancements have been found relative to the zero-guide-field limit.

A solution of the scalar wave equation in a rotationally symmetric focused Gaussian amplifying medium is constructed from coupled Gaussian-Laguerre solutions of the free-space wave equation and is formally exact. The solution allows for the construction of the characteristic eigenmodes of the system and has direct application to Raman lasers, dye lasers, and the free-electron laser. The eigenvalues corresponding to the eigenmodes are directly related to mode growth rates and phase velocities. Numerical results for single- and multiple-pass growth are evaluated and exhibit an unpredicted gain enhancement that can be related to the phenomenon of "gain focusing" first described for uniform parabolic gain distributions. The solution for a focused parabolic gain distribution is also presented. Output intensity distributions are found for initial conditions corresponding to spontaneous emission and to a specified initial field. The same method of solution can be applied to a medium with a focused Gaussian refractive index.

The effect of phase and amplitude fluctuations of the pump mode on the quantum-statistical properties of the signal mode is considered. It is shown that these fluctuations in the laser field tend to decrease the squeezing of the signal field.

The class of large-amplitude traveling-wave solutions to the nonlinear Vlasov-Maxwell equations is investigated in which the wave pattern is stationary in a frame of reference moving with the pondermotive phase velocity $v_p=\frac{\omega }{(k+k_0)}$. Here, ${\lambda }_0=\frac{2\pi }{k_0}$ is the wavelength of the transverse helical wiggler field, and ($\omega ,k$) are, respectively, the frequency and wave number of the saturated radiation field which is assumed to be monochromatic and circularly polarized. The conservation of (average) density, momentum, and energy are imposed as additional exact constraint equations that connect the final (saturated) and initial states of the combined electron-beam-radiation-field-wiggler-field system. These constraint equations reduce the generality of the nonlinear equilibrium Bernstein-Greene-Kruskal solutions and allow estimates to be made of the saturated field amplitude in terms of initial properties of the beam-wiggler system. As a simple example that is analytically tractable, we consider the case where the initial distribution $F_0\left(\gamma \right)$ and the saturated untrapped distribution $F_u\left({\gamma }^{\prime }\right)$ are prescribed by rectangular distribution functions centered around axial velocity $v_z=\frac{\omega }{(k+k_0)}$, assuming a moderate field amplitude with $b_T=\frac{e\delta {\hat{B}}_T}{m{c}^{2}k}<1\mathrm{}$ and small fractional energy spread in the beam electrons. For a tenuous beam with $\omega \simeq \mathrm{kc}$ and $k\simeq (1+\frac{v_p}{c}){\gamma }_p^2{k}_0$ where ${\gamma }_p={(1-\frac{v_p^2}{c^2})}^{-\frac{1}{2}}$, it is found that the saturated amplitude of the radiation field is given approximately by $\delta {\hat{B}}_T=\frac{{\Delta }_L}{10{(1+b_w^2)}^{\frac{1}{2}}}\frac{{\omega }_p^2}{c^2{k}_0^2}\left[1+\frac{v_p}{c}\right]{\hat{B}}_w$ where $b_w=\frac{e{\hat{B}}_w}{m{c}^2{k}_0}$, ${\Delta }_{L}m{c}^2$ is the characteristic half-width energy spread in the laboratory frame, and ${\omega }_p^2=\frac{4\pi n_b{e}^2}{m}$ is the nonrelativistic plasma frequency squared.

We discuss the theory of photoexcitation of an autoionizing resonance. We solve a set of coupled stochastic integrodifferential equations which are based on the Fano model for autoionization, but which include the effects of weak elastic collisions, weak or strong laser excitation, and finite laser bandwidth. We determine the exact photoelectron spectrum and give formulas for spectral peak positions and widths. Redistributive scattering is evident, as are various effects normally associated only with transitions between discrete levels. Under certain conditions symmetries and fixed points of the electron spectrum can be predicted.

The operation of maser devices using highly excited Rydberg atoms as active medium is investigated. These systems, working in a transient regime reminiscent of super-radiance generate few-hundred-ns-long bursts of mm-wave radiation. They are characterized by extremely low inversion density thresholds ($N_0\sim {10}^4$ atoms typically) and very small peak power outputs (in the ${10}^{-11\mathrm{}}$- to ${10}^{-13\mathrm{}}$-watt range). This study is interesting in the context of new tests of super-radiance and electrodynamics theory. It also opens new possibilities in the important domain of mm-wave amplification and detection. In this paper, we present a simple semiclassical theory of transient Rydberg maser operation and we report the experimental observation of a large number of maser emission lines on Na atoms, spanning a wide frequency interval ($60<\mathrm{}\nu <950\mathrm{}$ GHz). The technique used to monitor the emission is the indirect field ionization procedure which reveals fast radiative transfers occurring between Rydberg levels. Another method involving the direct detection of the maser radiation by a heterodyne receiver is described in a subsequent paper, along with a report on the investigation of various effects related to the triggering of the transient maser emission by external radiation.

The "triggering" of transient maser systems using excited Rydberg atoms as active medium is investigated theoretically and experimentally. We show how the maser pulse characteristics are modified by the interaction of the active medium with small amounts of resonant mm-wave radiation. The phase, polarization, and emission delay of the radiated field are modified by this triggering: this can be used to detect very small powers of mm-or sub-mm-wave radiation. A complete discussion of the quantum and thermal noise of these detectors is presented. A demonstration of the feasibility of this technique is reported. Triggering signals corresponding to a detectivity of ${3\times 10}^{-17\mathrm{}}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$ at 108 GHz ($\lambda =2.8\mathrm{}$ mm) have been measured at room temperature. The quantum-noise limit—corresponding to ${6\times 10}^{-19\mathrm{}}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$—should be reached with liquid-helium-temperature-cooled detectors.

Stationary oblique shock waves produced by a laser-produced plasma (carbon and lithium) expanding over a wedge and related phenomena have been investigated experimentally. Such a plasma is characterized as an ionization-frozen multicomponent flow which consists of a number of different charge-state groups with different flow velocities. Well-defined multiple shock fronts which are formed around a wedge are differentiated using a spectroheliographic method. Data obtained from an ion-charge collector and from spectroheliographs indicate that these shock fronts (with different inclinations) result from different flow components. The plasma behind the shock is predominantly recombining rather than ionizing. In addition, the innermost shock is found to be more like a transverse shock where the flow particles are transported along the shock plane. This is also evidenced by a large charged-particle cumulation (and collimation) which occurs when two such oblique shock waves are brought to intersect each other. The measured shock inclinations as a function of wedge angle indicate a considerable departure from a simple gasdynamic flow model. It is believed that the self-generated (reversed) magnetic field associated with the laser-produced-plasma shock greatly influences the shock behavior.

The energy transfer between donor and acceptor molecules has been studied by steady-state and picosecond time-resolved spectroscopies in solutions with a given concentration of donor molecules and different concentrations of acceptor molecules. The rise and decay times of the fluorescence from donor and acceptor molecules were measured. The decrease of the decay times of the donors and the rise time of the acceptors with an increase of the concentration of acceptors observed in the mixed solution indicates a faster energy-transfer rate between the donors and acceptors. Theoretically calculated fluorescence profiles and efficiencies of energy transfer are in agreement with the experimental results.

Experimental observations of the high-energy ions (> 10 keV/Z), expanding from a laser-produced plasma, have shown strong evidence of coronal electrostatic fields. In this paper, it is shown by a linear analysis that a multispecies streaming plasma can be subject to a two-stream ion-ion instability leading to density modulations. The effect of the charge-to-mass ratio on growth rate for different species accelerated by the same background electric field has been investigated. The theoretical model is consistent with experimental observations.

The applicability of the high-frequency Stark effect in the diagnostics of turbulent plasmas is demonstrated by measurements of He I line profiles ($4{}_{}{}^1{}_{}{}^{}D$-$2{}_{}{}^1{}_{}{}^{}P$) in a turbulent linear discharge. The existence of electron plasma waves with field strengths of 6-9 kV/cm is confirmed spectroscopically. Competing effects—e.g., the emission of molecular lines—are discussed in detail and are shown to be negligible under our experimental conditions.

We present in this paper a soliton theory for the open states in deoxyribonucleic acid (DNA) and synthetic polynucleotide double helices. Kink and antikink solutions for the equation of motion of the sine-Gordon form correspond to the open states with positive and negative helicities. The energy of open form and the length of the open configuration which are theoretically estimated are in the same order with the values inferred from kinetic experimental data.

A microscopic theory suitable for the derivation of nonlinear hydrodynamic equations in systems with a broken symmetry is presented. The theory is based on a recently introduced projection operator technique which yields results correct to any order in the deviation from equilibrium and rests on a gradient expansion around "local-equilibrium" ensembles. The usefulness of the method is demonstrated by deriving nonlinear nematodynamics. The reversible as well as the dissipative nonlinearities are obtained in what seems to be a transparent fashion. Comparison with phenomenological derivations is presented.

Experiments are reported in which a critical mixture of 3-methylpentane plus nitroethane has been subjected to varying pressures. By varying the static pressure we have determined the dependence of the critical temperature on pressure with considerable accuracy. By sudden depressurization we have cooled the sample adiabatically leading to phase separation via spinodal decomposition; from these experiments we have determined the adiabatic thermal pressure coefficient near the critical point. By combining our data with the experimental density data of Greer and Hocken, we are able to determine the asymptotic critical behavior of the thermal-expansion coefficient and of the specific heat, consistent with two-scale-factor universality. We also obtain a value for the adiabatic coupling constant which appears in the dynamic scaling theory for the frequency dependence of the critical sound attenuation. Our thermodynamic data appear to be in good agreement with the critical ultrasonic attenuation data obtained by Ishida and Harada for 3-methylpentane plus nitroethane.

An extensive computer simulation investigation of the time dependence of spinodal decomposition in a two-dimensional, one-component fluid is reported. This investigation includes constant-temperature and constant-energy simulations, both of which are followed to very long times. We observe the detailed density morphology, finding different growth regions for the average size of the formed liquid clusters. The late-time growth law for the average cluster size is found to be $t^{\frac{1}{2}}$ for the isothermal and $t^{\frac{1}{3}}$ for the constant-energy simulation, respectively. The physical origin of these results is explained by asymptotic analysis of the governing equations of cluster growth. A linear hydrodynamic theory for the early stages of the separation process is also presented.

Experimental studies of the x-ray generation from laser plasmas are performed by using a picosecond high-power laser. The intensity of emitted x rays shows a periodic variation as a function of the atomic number of target materials. Calculations based on the transient collisional-radiative model, rather than the conventional steady-state coronal model, are carried out. Experimental and computational results show good agreement with each other, and indicate the importance of the transient nature of multiple-ionization processes. The dependence of hot-electron temperature on the atomic number is briefly discussed.

Equilibrium polymerization is very different in one dimension than in higher dimensionality. The transition that occurs in the limit of vanishing initiation equilibrium constant (which is a critical point in higher dimensionality) becomes a first-order transition at non-vanishing temperature in one dimension. A simple model of equilibrium polymerization that has been discussed recently for higher dimensionality is solved exactly by the transfer-matrix method in one dimension. The equivalent $n\to 0$ vector model of magnetism is also solved exactly for all fields and temperatures by transfer-matrix methods and is analyzed by an exact renormalization-group transformation. The renormalization-group analysis contains several interesting features including the fact that the parameter space of the Hamiltonian must be enlarged to six dimensions, yet remains finite. The connection of the model and transition treated here with the Zimm-Bragg model of the helix-coil transition and with the one-dimensional Ising model of magnetism is discussed.

The characteristic exponents of the normal modes for light in the Grandjean texture of a cholesteric film were calculated with values for the dielectric constants that are typical for common cholesteryl derivatives. The reflection properties are discussed in terms of the penetration depth given by the real part of the characteristic exponent. The results are presented in graphs which give the structure and the reflection coefficients as a function of wavelength and angle of incidence. It was found that there are unpolarized bands that are missing in a stability chart published earlier.

Expressions are derived which relate the averages of a dynamical variable in the molecular-dynamics ensemble and in the generalized canonical ensemble. Fluctuations in the two ensembles are also related. These results make the full equilibrium theory of classical statistical mechanics accessible to evaluation by molecular-dynamics computations.