Experimentally determined potentials for the $X {}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ and $A {}_{}{}^2{}_{}{}^{}\Pi$ states of the NaNe van der Waals molecule are presented. The potentials are generated from the temperature dependence, in the range 140-330 K, of the intensity in the far-red wing of the Na resonance line perturbed by Ne gas. The NaNe emission spectrum is observed in the wavelength range 585-650 nm. From Morse potential fits to the $A {}_{}{}^2{}_{}{}^{}\Pi$-state data we obtained $D_e=160\mathrm{}\left(20\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ at $R_e=5.1\mathrm{}\left(1\right)\mathrm{}{a}_0$. The $X{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ ground state is purely repulsive within the $4-7a_0$ range of our experiment.

A precision determination of the $4{}_{}{}^3{}_{}{}^{}S-{}_{}{}^3{}_{}{}^{}P$ interval in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is accomplished by observing resonances between Zeeman-tuned He ${}_{}{}^3{}_{}{}^{}\mathrm{S}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{P}$ sublevels with four different C${\mathrm{O}}_2$ laser frequencies. This value is compared with the previous best experimental and theoretical values.

The two-photon excitation parameter for the ground state to $6p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{} \mathrm{}(J=0\mathrm{})\mathrm{}$ excited-state transition in xenon is calculated in both the Dirac-Fock and Hartree-Fock approximations. We make a direct summation over the bound and continuum intermediate states. Our results are compared with calculations based on the average-energy closure method and the results of a recent experiment by Chen et al.

We report precision measurements and calculations of several parameters of the $A {}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ state of the OH free radical. Data from quantum beats in the fluorescence of laser-excited molecules and direct optical spectroscopy have been combined to yield the following values for the parameters of our model Hamiltonian: $\gamma =6862\mathrm{}\left(43\right)\mathrm{}$ MHz, $b+\frac{c}{3}=774.1\mathrm{}\left(0.4\right)\mathrm{}$ MHz, $c=168.9\mathrm{}\left(0.8\right)\mathrm{}$ MHz, $g_r=0.00248\mathrm{}\left(0.6\right)\mathrm{}{\mu }_B$, and $g_A=0.0242\mathrm{}\left(11\right)\mathrm{}{\mu }_B$. Calculations that include coupling to the ground $X {}_{}{}^2{}_{}{}^{}\Pi$ states through rotational or spin-orbit interactions produce estimates of these parameters that agree very well for all except $g_A$.

A Vlasov-type equation derived from the many-particle Schrödinger equation is used to study the linear response of atomic electrons to an external time-varying field. A remarkably simple expression for the photoabsorption cross section, valid over a wide range of incident photon energies, is obtained. Application of this expression to the Xe atom yields cross-sectional values which are then compared against experimental data. The results are found to be encouraging.

Free-electron lasers, except for those using helical wigglers, are predicted in most cases to generate higher harmonics of the fundamental optical frequency. The basic equations describing this process are derived and applied in particular to the linearly polarized wiggler.

Calculations are presented for the asymptotic change in the energy momentum of a free charged particle that is swept over by an intense laser pulse propagating in the direction of a constant magnetic field that is also present. Relevant features of the classical orbit and these asymptotic changes are obtained as a function of pulse intensity, duration, and polarization. Significant changes occur if a stringent resonance condition is satisfied which requires a very large constant magnetic field, but these changes are reduced precipitously off resonance.

The dynamical behavior of a Fabry-Perot cavity filled with atoms having Zeeman sub-levels in the ground state is investigated. Such a system is known to exhibit optical tristability, where the atomic spins are oriented parallel or antiparallel to the optical axis even for the linearly polarized incident light. It is found that, when a static magnetic field is applied transversely to the optical axis, self-sustained precession of the spin polarization occurs. Correspondingly, the ${\sigma }_{+}$ and ${\sigma }_{-}$ components of the transmitted light are modulated at about the Larmor frequency. We also show that the Bloch equation describing the motion of the spin polarization in the cavity can be reduced to van der Pol's equation.

Investigations of the production of Hg${\mathrm{Cd}}^{*}$ in an $e$-beam preionized quasistable discharge have theoretically predicted and experimentally demonstrated that fluorescence efficiencies of ∼50% can be achieved. Spectra of the emission for a discharge pumped mixture of Cd-Hg-Ne show the Hg${\mathrm{Cd}}^{*}$ continuum peaking at 470 nm and absolute fluence measurements indicate [Hg${\mathrm{Cd}}^{*}$] ∼ ${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Measurements of the gain and/or absorption of this mixed metal exciplex show absorption of the Hg${\mathrm{Cd}}^{*}$ excited state across the entire band. The absorption peaked at the blue end of the continuum (457 nm) and decreased to ∼ 1% in 40 cm at the red end of the continuum (517 nm).

Transmission characteristics of a nonlinear Fabry-Perot interferometer with two independent optical beams are considered under steady-state conditions. The sum frequency of two beams is taken to be near a two-photon resonance of the nonlinear medium. Two beams are coupled through the intensity-dependent susceptibility and influence propagation of each other in a mutual way. A set of two coupled-state equations is derived and it incorporates saturated absorption and dispersion as well as the effect of optical Stark shift. The system is found to have as many as seven stationary states, and at most, three are stable among them. The degenerate case of a single beam in two-photon resonance with the medium is also considered with particular emphasis on the effect of the optical Stark shift on optical bistability.

It is proved that the spectral analysis of Dekker's covariant path integral for continuous Markov and quantum processes in curved spaces leads to a lattice expression that is exactly equal to Graham's fixed-point propagator. The proof proceeds in Riemannian normal coordinates. In addition, the well-known difference between the scalar potentials in the formal path integrals is explained.

We report the first measurements of the susceptibility of a pure fluid near its critical point made with sufficient accuracy to directly reveal the existence of corrections to scaling. The measurements were made on the critical isochore for $T>\mathrm{}{T}_c$, in the range $9.6\times {10}^{-5\mathrm{}}\le t\le {10}^{-1\mathrm{}}$, and are described to within 0.11% rms deviation by the expression $\chi ={\Gamma }^{+}{t}^{-\gamma }(1+a_1{t}^{\Delta }+a_2{t}^{2\Delta }+a_3{t}^{3\Delta })$, with $\Delta$ fixed at 0.496, $\gamma =1.246\mathrm{}\pm 0.010$, and $a_1=1.6\mathrm{}\pm 0.3$. The measurements are consistent with either the high-temperature series result $\gamma =1.250\mathrm{}$, or the renormalization-group result $\gamma =1.241\mathrm{}$. With $\gamma$ fixed at 1.241, the value assumed by $a_1$ is 1.3±0.2. The effects of revised scaling are considered using a parametric model, and are found not to affect the results for ${\Gamma }^{+}$, $a_1$, or $\gamma$.

The phenomenon of cross-field runaway electrons can occur in cases where large dc electric fields with steep gradients exist perpendicular to a magnetic field. A derivation of the circumstances which permit this condition to exist is presented.

The growth of droplets in a binary fluid mixture undergoing phase separation has been studied by light scattering. The measurements have focused on the dependence of growth rate on the overall composition of the mixture. For mixtures of the critical composition, there is a crossover: $k_m$, the wave number corresponding to the maximum in the scattering, decreases initially as $t^{-\frac{1}{3}}$ and at long times as $t^{-1\mathrm{}}$. In accordance with hydro-dynamic arguments of Siggia [Phys. Rev. A 20, 595 (1979)], the existence of the crossover can be related to the volume fraction of the new phase $v$. Only for $v\gtrsim 0.10$ are cross-overs observed. In more dilute mixtures we find $\left|k_m\right|=A{t}^{\frac{-1\mathrm{}}{3}}$ with $A$ proportional to $v$ for $0.10\gtrsim v\gtrsim 0.02$. Lifshitz-Slyozov growth, for which $A$ is independent of $v$, is found for $v<\mathrm{}0.02\mathrm{}$.

The generalized Enskog equation is used to describe the dynamic structure factor $S(k,\omega )$ for a hard-sphere gas. The problem of constructing kinetic models for the calculation of $S(k,\omega )$ is considered and the minimum set of matrix elements of the exact collision operator required by hydrodynamics is identified. The source of existing discrepancies between kinetic-model calculations and light-scattering experiments is also found and removed. Sensitivity of $S(k,\omega )$ to the parameters of kinetic models is discussed and a simple model proposed. A preliminary comparison of the hard-sphere $S(k,\omega )$ calculated from this model with neutron-scattering data from gaseous krypton is given.

This paper gives a method for the study of relaxation at instabilities described by Langevin equations. The method uses backward-diffusion equations which are conditioned on the initial value of the dynamical variables, instead of the Fokker-Planck (forward) equation. By using the backward-diffusion equation, we obtain simple characterizations of relaxation properties such as the mean time to leave an observation region about the instability, the probability of reaching a preferred steady state, the mean time to reach it, and the rate of reaching it. Ensemble effects are included by averaging over the initial distribution. This kind of approach complements previous studies which were based on the Fokker-Planck equation. The interaction of noise, strength of the instability, and size of the observation region is studied. A number of examples are given (lasers, ferromagnets, classical molecular scattering, spontaneous optical resolution) and the methods developed here are applied to calculate properties of spontaneous optical resolution.

We present here results for the fourth frequency moments of current-correlation functions for liquid rubidium obtained by simulating a system of 108 particles interacting through a potential due to Price et al. These are compared with the results obtained using superposition approximation for the static triplet correlation function involved in the molecular expressions for these moments. It is found that the superposition approximation is reliable as far as the calculation of moments is concerned.

The ${\mathrm{H}}^{-}$ ion is known to have a metastable $2p^2{}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$ triplet excited state. To see if an analog of this state is present in the positronium negative ion ${\mathrm{Ps}}^{-}$ the Coulomb binding energy $E_b$ of the lowest-energy even-parity $L=1\mathrm{}$ configuration of two identical-charge $-e$ fermions of mass $m_1$ plus one spinless particle of mass $m_2$ and charge $+e$ is calculated. We find a value of $E_b$ below the $n=2\mathrm{}$ level of the neutral atom for $\frac{0\le m_2}{M\le 0.17}$ and $\frac{0.90<\mathrm{}{m}_2}{M\le 1}$, where $M=2\mathrm{}{m}_1+m_2$ is the total mass of the ion. However, we cannot say that the ${}_{}{}^3{}_{}{}^{}P_{}^e{}_{}{}^{}$ state exists for ${\mathrm{Ps}}^{-}$, where $\frac{m_2}{M}=\frac{1}{3}$.

The relativistic dynamics of fast current carriers are rich in intriguing phenomena not appearing in the Newtonian limit. It is shown that because of these dynamics there is a bound to the efficiency of driving current in a tokamak either by waves or other means. Analytic techniques uncover the current-drive scheme which yields the maximum attainable efficiency.

The large spin-orbit interaction in Rb and Cs strongly couples states of different $\left|m_l\right|$ during the passage from low to high field during the electric field ionization of excited states. The result of this is that the technically accessible rise times which yield very selective ionization of Na yield much lower selectivity for Rb and Cs.