Collisional and radiative processes in a glow discharge leading to quenching and excitation transfer in the $n=4\mathrm{}$ helium sublevels are investigated by means of a laser-perturbation method. Laser-induced population perturbations are solutions of coupled rate equations, the coefficients of which are determined by an accurate numerical method of data analysis (the identification method) previously developed, so as to minimize the difference between experimental curves and those calculated from the model. In the pressure and current-intensity ranges investigated, only the radiative and atom-atom collision processes contribute to quenching and excitation transfer in the $n=4\mathrm{}$ sublevels. Numerical identification of the $n=4\mathrm{}$ experiments provides a nearly complete set of thermally averaged cross sections and in particular shows that the singlet-triplet transfers are mainly due to stepwise collisional processes via the $4F$ state.

Measurements of the $n=2\mathrm{}$ density operator and the probability for $n=2\mathrm{}$ production are reported for hydrogen atoms produced by passing 20 to 1000 keV protons through thin carbon targets. Measurements of the dependence of $2p$ production on the number of protons in the incident projectile and on the time this proton cluster spends within the foil are also discussed.

Liquid lead has been studied at 623 K by thermal neutron scattering technique. The range of momentum transfers is from 1.0 to 6.8 ${\mathrm{\AA }}^{-1\mathrm{}}$. Up to 2.6 ${\mathrm{\AA }}^{-1\mathrm{}}$ the highest energy transfer is 12 meV and above 2.6 ${\mathrm{\AA }}^{-1\mathrm{}}$ it is 18 meV. The measurement is corrected for experimental effects including multiple scattering. No deconvolution of the resolution function has been performed. Normalization to the structure factor, obtained from an independent diffraction measurement has been done. The result is presented in the form of constant momentum transfers. The first and second moments have been compared with theoretical values. Intersections of the dynamic scattering function for constant momentum transfers between 1.0 and 1.4 ${\mathrm{\AA }}^{-1\mathrm{}}$ show inelastic peaks originating from collective excitations in the liquid.

Dirac-Fock (DF) and Dirac-Slater (DS) calculations of the photoionization of Hg $5d$ and Cd $4d$ have been performed in an effort to assess the utility of such calculations for the prediction of subshell cross sections and branching ratios as well as to attempt to elucidate the effects of relativistic (especially spin-orbit) interactions. Our results indicate that in both cases, the subshell cross section is predicted rather well by the DF results, while the DS is too large (by a factor of ∼2) and peaks too close to threshold. For the Hg $5d$ branching ratio, the DF values are in excellent agreement with experiment and the DS results are in pretty fair agreement as well. For Cd $4d$, on the other hand, neither calculation is entirely satisfactory; away from threshold the DF result, however, is reasonably good. The possible origin and implications of these results are discussed.

We have developed a simple physical model of laser ionization based on resonance saturation that involves the most important collisional and radiative interactions, yet lends itself to analytical solutions that enable the time history of the free electron density to be evaluated. We have been able to demonstrate that in the case of sodium the predictions of this simple model are within 15% of the values calculated by our extensive LIBORS computer code. The model has also been used to estimate the ionization time for each of the alkali metals over a wide range of conditions. These results are found to be consistent with several experimental observations.

The linearized theory of a free-electron-laser amplifier consisting of a relativistic electron beam transported along the axis of a helical wiggler in the presence of an axial guide field is solved exactly. With suitable re-identification of parameters, the theory also applies to the case where the wiggler is replaced by a circularly polarized subluminous radio-frequency pump. The dispersion relation is derived and numerical examples of solutions are presented. These indicate (a) that the use of an axial field permits operation of a laser of given high frequency and undulatory transverse velocity of the unperturbed electron beam at lower values of the pump field, (b) that the gain can be enhanced by approaching the condition of resonance between the effective frequency of the pump and the cyclotron frequency, and (c) that the breadth in frequency of the region corresponding to spatially exponentially growing operation can be much extended.

Extended-x-ray-absorption-fine-structure (EXAFS) spectra of aluminum and magnesium have been measured with single nanosecond pulses of soft x rays generated by laser-produced plasmas. This technique provides a practical alternative to synchrotron radiation for the acquisition of EXAFS data for elements having atomic numbers up to 40. It also provides a unique capability for the analysis of molecular structure in highly transient chemical species.

The contribution of dielectronic recombination to the population of radiative He I states ($3\le n\le 8$) in a He dc hollow-cathode discharge of 4-cm length and 2-cm diameter is determined in the discharge pressure range 0.6-4.0 Torr and at discharge current values of 10, 13, 16, 19, 22, and 25 mA by radial sampling of the emitted line radiation and the ion densities. In addition, the afterglow phase of the same discharge is explored as to the radial and time dependence of line radiation intensity and ion density. In the active phase at low density (2 Torr) and at higher pressure but at too low a discharge current (below 12 mA) all the radiation is due to electron impact excitation of the He states. Only at a pressure of about 4 Torr and a discharge current above 20 mA does recombination play a significant role in populating radiative He I states on the axis of the discharge. The radial dependence of the He II radiation shows the geometric density increase of the very fast unscattered electrons having the full sheath energy. Both results are relevant for hollow-cathode-laser development.

A unified quantum treatment is presented for bidirectional two-mode ring lasers using the approach of Haken and co-workers. Equations of motion for the reduced density matrix of the field have been derived and used to study the fluctuation properties of the radiation field in different kinds of active media. The steady-state photon number distribution for an inhomogeneously broadened gas ring laser is derived. It is shown that for equal losses the relative fluctuations and normalized cross correlation of the photon numbers approach values $\frac{1}{3}$ and $-\frac{1}{3}$, respectively, for large excitations. For unequal losses the laser may emit typical single-mode laser radiation in the direction of the favored mode and typical equilibrium blackbody radiation in the direction of the weaker mode. Our results generally agree with the predictions of earlier treatments when the losses are equal. However, for unequal losses we predict different behavior for large excitations. Effects of detuning, spatial, and temporal variation of atomic inversion on the fluctuation properties have been investigated in the coherent-state diagonal representation of the density matrix, and steady-state intensity distributions have been derived for various kinds of active atoms. It is shown that for inhomogeneously broadened solid-state ring lasers spatial inhomogeneities may couple the modes strongly and the statistical properties of such lasers, characterized by a double-peaked intensity distribution and large relative-intensity fluctuations, may be similar to those of a homogeneously broadened ring laser. Curves are presented to illustrate the behavior.

Resonance-fluorescence power spectrum of two-level atoms driven by a strong monochromatic laser field is discussed, taking into account effects due to the dipole-dipole interaction between the atoms. For two atoms confined to a region smaller than the wavelength of the driving field, we predict significant deviations of the power spectrum from the single-atom case. The two-atom spectrum is nonsymmetric, the wing peaks are shifted from their Rabi-frequency positions, and additional maxima may appear.

Light scattering was used to determine the structure factor $S(k,\tau )$ of two binary mixtures which were quenched through the critical point. The experiments, which were carried out in isobutyric acid-water and 2,6-lutidine-water, show that the normalized structure factor has the scaling form $\overline{S}(k,\tau )=K^{-3\mathrm{}}F\left(\frac{k}{K}\right)$ where $K^{-1\mathrm{}}$ is the size of a growing domain and $F\left(x\right)\mathrm{}$ is time independent. Our findings are in approximate agreement with those of Marro et al., who obtained $\overline{S}(k,\tau )$ from a computer simulation of a quenched $A-B$ Ising lattice. The computer experiments yield an initial time dependence in $F\left(x\right)\mathrm{}$, whereas the light-scattering measurements reveal no evidence of this transient effect over the (dimensionless) time interval $6\lesssim \tau \lesssim 1000$. The experiments spanned the temperature range $3\times {10}^{-6\mathrm{}}\le \epsilon \le 2\times {10}^{-5\mathrm{}}$, where $\epsilon \equiv |\frac{T_f}{T_c}-1|$. In this interval the integrated scattering cross section at every instant of time is proportional to ${\epsilon }^{2\lambda }$ with $0.22\le \lambda \le 0.33$.

A general renormalized statistical theory of Vlasov turbulence is given which proceeds directly from the Vlasov equation and does not assume prior knowledge of sophisticated field-theoretic techniques. Quasiparticles are the linear excitations of the turbulent system away from its instantaneous mean (ensemble-averaged) state or background; the properties of this background state "dress" or renormalize the quasiparticle responses. It is shown that all two-point responses (including the dielectric) and all two-point correlation functions can be completely described by the mean distribution function and three fundamental quantities. Two of these are the quasiparticle responses—the propagator and the potential source—which measure, respectively, the separate responses of the mean distribution function and the mean electrostatic potential to functional changes in an external phase-space source added to Vlasov's equation. The third quantity is the two-point correlation function of the incoherent part of the phase-space density which acts as a self-consistent source of quasiparticle and potential fluctuations. This theory explicitly takes into account the self-consistent nature of the electrostatic-field fluctuations which introduces new effects not found in the usual "test-particle" theories. Explicit equations for the fundamental quantities are derived in the direct interaction approximation. Special attention is paid to the two-point correlations and the relation to theories of phase-space granulation.

The average degree of ionization $Z^{*}$ for a high-density plasma is examined in Thomas-Fermi and Debye-Huckel-Thomas-Fermi statistical models. To clarify ambiguities in defining $Z^{*}$, predictions from several versions of these models are compared for the case of a pure hydrogen plasma for ion number density $N_1={10}^{23}$ ${\mathrm{cm}}^{-3\mathrm{}}$, at $kT=1-1000\mathrm{}$ eV. The results offer some insight regarding the validity of the statistical models.

Energy deposition (preheat) by suprathermal electrons in laser-irradiated targets is described using a model based on Spencer's electron transport calculation. Approximate analytical, as well as numerical, solutions are found for the specific energy deposition ${\mathcal{E}}_{\mathrm{dep}}\left(\frac{J}{g}\right)$ and fraction $f$ of energy transmitted beyond a given depth, measured in curvilinear ranges. Example calculations are presented for homogeneous targets of gold, aluminum, and carbon. The application of the model to treat multilayer composite targets is briefly considered. The parametric dependence of these results reveal that the most significant parameter by far is the source temperature $T_h$ of the hot or suprathermal electrons. The predictions of the preheat model are shown to be in good agreement with experimental data on gold and aluminum disks irradiated by Nd-glass laser pulses at the ${10}^{15}$ W/${\mathrm{cm}}^2$ level. Included in the theory-data comparison are predictions for these experiments based on several other simple models and large code simulations of the experiments. Future aluminum disk experiments in which $T_h$ is accurately measured could provide a sensitive test for the theories.

Fatty-acid side chains are simulated by the randon-walk path of a particle in three dimensions. Three independent Markov states are defined and made to correspond to trans and gauche states of real chains, by analogy to their packing properties. The stochastic matrix of transition probabilities is computed, and from it, the probability density for the occurrence of each Markov state is derived. We calculate the partition function for all configurations of a chain in the field of four nearest neighbors. The attractive interaction is of the form $\frac{1}{r^6}$ and depends upon the depth from the initial position. The phase transition experimentally observed in phospholipids is reasonably fitted by one free parameter in this model. The transition temperature, width, change in fraction of trans-to-total bonds, initial and final fractions—before and after the transition—, and enthalphy of transition, are calculated in good agreement with experimental results and models based on mean-field calculations. Applications of this approach to more complex lipid molecules are discussed.

A generalized Saha equation which explicitly includes excited atomic states and their interactions is used for calculating the degree of ionization and the effective ionization potential for a dense classical partially ionized hydrogen plasma. The effective ionization potential for each atomic state, characterized by the principal quantum number $n$, is calculated as is the change $\Delta I_n$ of the ionization potential due to different types of particle interactions, i.e., atom-atom, atom-ion, atom-electron, and ion-electron interactions. The effective ionization potential for each state is used for determining $n^{*}$, the maximum admissible principal quantum number for a hydrogen atom in a plasma. It is found that the strongest influence on the effective ionization potential comes from the familiar electron-ion interactions (Debye shielding), and from the less effective but not negligible induced-dipole interactions between atom-ion and atom-electron pairs. The van der Waals interactions of the dispersion-type turn out to be negligible in agreement with earlier work. The changes $\Delta I_n$ are shown to produce corresponding changes in the population of the atomic states which are of the order of ten percent; however, the degree of ionization of the total atom is found to be very insensitive to the $\Delta I_n$, and to vary only one tenth of a percent by order of magnitude.

It is shown that the Boltzmann equation for isotropic scattering of Maxwell molecules, considered by Bobylev, Krook, and Wu (BKW), can be written in the form of an energy-space kinetic equation. A transformation leads to a new class of kinetic models, possessing exact solutions of the BKW type.

In a hydrogenic plasma, the effective potentials $V_{\mathrm{ij}}\mathrm{}\left(r\right)\mathrm{}$ of electron-proton, electron-positron, and electron-electron systems are considered. The inclusion of quantum-mechanical effects leads to a finite value of $V_{\mathrm{ij}}\mathrm{}\left(0\right)\mathrm{}$ at the origin. For each system simple and analytical expressions are obtained at high temperature and small interparticle distance. Barker's numerical computations are extended to the high-temperature domain, small-$r$ region, and electron-positron system. The symmetry effects are included for the electron-electron system. Various simple approximate formulas are obtained which describe $V_{\mathrm{ij}}\mathrm{}\left(r\right)\mathrm{}$ at all distances. Some of them are in excellent agreement with the numerical results.

The convective instabilities occurring in cholesteric and smectic-$A$ liquid crystals are investigated by linear stability analysis. We find oscillatory instabilities not predicted previously and we present a thorough examination of the stationary instabilities which have been discussed previously in a simplified version by Dubois-Violette and Parsons. The differences in the convective instabilities in cholesterics and smectics $A$ due to the differences in the underlying hydrodynamic equations and to the different orders of magnitude of the corresponding transport coefficients and static susceptibilities are considered in some detail. We discuss the mechanisms of the instabilities which are different from those of nematics and we obtain the threshold conditions for cholesterics and smectics $A$ for the critical temperature difference and for the frequency by energy-balance considerations. The influence of a temperature-dependent pitch is investigated in a separate section.

We use a method based on Fourier analysis to obtain wave-train solutions for monatomic as well as diatomic lattices with exponential nearest-neighbor interaction. The monatomic solution obtained by our method agrees with the solutions of Toda. The nonlinear dispersion relation found by us for the diatomic chain goes over to known results in the appropriate limits. The effect of nonlinearity on the dispersion curves is illustrated for various mass ratios.