We report on a quantitative experimental investigation of velocity-changing collisions by means of velocity-selective optical pumping (VSOP). We have calculated the VSOP line shape for an atom with hyperfine structure with the use of two phenomenological kernels for the collision effects: the Keilson-Storer kernel, and a two-term kernel consisting of a broad Keilson-Storer part and a narrower Gaussian component. Corrections were included to account for the finite absorption in the sample and the backward reflection of the pumping beam. The experiments were carried out in sodium vapor with neon as the perturber gas. The $D_1$ line of sodium was used for optical pumping, and the orientation of the ground state was detected. Free parameters of the theory were determined by fitting the predicted line shapes to experimental curves. The Keilson-Storer kernel proved unsatisfactory, but the two-term kernel reproduced well the observed line shapes over the entire collision profiles in the neon pressure range 0-57 mtorr. In an independent experiment using rapidly modulated VSOP we also measured directly the cross section of velocity-changing collisions: $\sigma =(1.13\mathrm{}\pm 0.10)\times {10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$. The large weight obtained for the narrow Gaussian from the fits, as well as the collision cross section which is three times as large as the cross section deduced from tabulated gas kinetic radii, may indicate the presence of collisions with relatively small velocity changes in addition to hard-sphere encounters.

A new technique for the construction of $N$-body Coulomb scattering amplitudes is proposed, suggested by the simplest case of $N=2\mathrm{}$: Calculate the scattering amplitude in eikonal approximation, discard the infinite phase factors which appear upon taking the limit of a Coulomb potential, and treat the remainder as an amplitude whose absolute value squared produces the exact, Coulomb differential cross section. The method easily generalizes to the $N$-body Coulomb problem for elastic scattering, and for inelastic rearrangement scattering of Coulomb bound states. We give explicit results for $N=3\mathrm{} \mathrm{and} 4$; in the $N=3\mathrm{}$ case we extract amplitudes for the processes (12)+3→1+2+3 (breakup), (12)+3→1+(23) (rearrangement), and (12)+3→(12)′+3 (inelastic scattering) as residues at the appropriate poles in the free-free amplitude. The method produces scattering amplitudes $f_N$ given in terms of explicit quadratures over ${\mathrm{}(N-2\mathrm{})\mathrm{}}^2$ distinct integrands.

We have performed calculations for the excitation of the $2p_m$ states of the hydrogen atom by electron impact for $\lambda$, $\chi$, and $\frac{d\sigma }{d\Omega }$ within our recently proposed asymptotic Green's function approximation, and the results are compared with the experimental observations and other theoretical predictions. Agreement between our calculated results and experimental data is reasonably good.

The line shapes of the $6sns$, ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-6\mathrm{}{P}_{\frac{1}{2}}\mathrm{ns}$ transitions in Ba as a function of the laser intensity are modeled with the use of two-channel multichannel quantum-defect theory and the isolated-core-excitation model. An analytical expression for the line shape is derived and used to obtain a value of $\left|〈g\right|r\left|e〉\right|=4.1\mathrm{}$ a.u. for the dipole transition moment of the ${\mathrm{Ba}}^{+}$ ionic $6s-6P_{\frac{1}{2}}$ transition, in agreement with a recent theoretical calculation.

The zero-core—contribution method is extended to polyatomic anions and applied to the calculation of the photodetachment cross section of N${\mathrm{O}}_2^{-}$. Good agreement with experiment is found except near the threshold. The results near threshold are sensitive to the approximations used in the calculations.

${\mathrm{N}}_2^{2+}$ ions, produced by electron impact of ${\mathrm{N}}_2$, are observed to predissociate into ${\mathrm{N}}^{+}$+${\mathrm{N}}^{+}$ when irradiated by a dye laser at photon energies between 14900 and 19500 ${\mathrm{cm}}^{-1\mathrm{}}$. Five structured bands in the spectrum are associated with absorptions from $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ ($v=0,1,2\mathrm{}$) into three predissociated levels of the $1{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state. The two highest levels of the $1{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ are believed to dissociate by tunneling through the Coulombic barrier in its potential-energy curve, whereas the lowest level is predissociated by the $1{}_{}{}^1{}_{}{}^{}\Sigma _u^{-}{}_{}{}^{}$ state. Kinetic-energy analysis of the ${\mathrm{N}}^{+}$ photofragments establishes the energy of $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ ($v=0\mathrm{}$) at 4.8±0.2 eV above the ${\mathrm{N}}^{+}$+${\mathrm{N}}^{+}$ dissociation asymptote and yields an adiabatic appearance potential of 43.6±0.2 eV. Molecular constants for the $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ and $1{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ states are determined.

An experimental and theoretical investigation of the 594.5-nm optogalvanic effect in the Ne positive column is described. The effect is a decrease in discharge conductance due to laser-induced depletion of metastable atoms. Absolute measurements of the effect per unit of absorbed laser power are reported for a wide range of discharge conditions. Postive-column discharges with radius-pressure products of 0.1-1.0 cm Torr and with sustaining direct currents of 1-16 mA are studied. The effect is modeled in this regime by applying perturbation theory to key rate equations that describe the discharge. The model predictions are in agreement with the experimental measurements. Absolute densities of atoms excited to the $2p^{5}3s$ levels are also reported. The regime studied covers the transition from a discharge sustained primarily by single-step electron-impact ionization to a discharge sustained primarily by two-step ionization via the $2p^{5}3s$ metastable levels. The global power balance of the discharge is dominated by wall losses of atoms excited to the $2p^{5}3s$ levels at all pressures and currents studied.

A general theoretical model for the treatment of multiphoton ionization is presented. In this treatment, we have explicitly included coherent effects resulting from harmonic generation. The predictions of this model are in agreement with experimental observations that we present in this paper. In addition, the model is contrasted with much earlier theoretical treatments of multiphoton ionization and accounts for the anomalous multiphoton-ionization behavior reported by others in the literature.

Intense vacuum-ultraviolet stimulated emission in molecular hydrogen, on both the Lyman and Werner bands, following excitation by two-quantum absorption at 193 nm on the $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\to E$,$F{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ transition, has been observed. The shortest wavelength seen in the stimulated-emission spectrum was 117.6 nm corresponding to the $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\mathrm{}(2-5)\mathrm{}$ $Q\left(2\right)\mathrm{}$ transition. The $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state appears to be populated with a mechanism involving electron collisions. The radiative cascade mechanism found to lead to the vacuum-ultraviolet emissions on the Lyman band also causes strong infrared stimulated emission to occur on the $E$,$F{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\to B{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ band. Two entirely separate radiative excitation channels are observed to play important roles in the state-selective molecular population of the $E$,$F{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ level. One involves two 193-nm quanta in the $X\leftarrow E$,$F$ amplitude while the other process combines a 193-nm quantum with a first Stokes-shifted photon in ${\mathrm{H}}_2$. The optical Stark effect was seen to play a significant role in the excitation process with shifts of molecular resonances as large as ∼45 ${\mathrm{cm}}^{-1\mathrm{}}$. Substantial deviations from Born-Oppenheimer behavior, resulting in a dramatic shift of the stimulated spectrum depending upon the excited-state rotational quantum number, were clearly observed for molecular levels close to the potential maximum separating the inner and outer wells of the $E$,$F{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state. The maximum energy observed in the strongest stimulated line was ∼ 100 μJ, a value corresponding to an energy conversion efficiency of ∼ 0.5%. The pulse duration of the stimulated emission is estimated from collisional data to be ∼ 10 ps, a figure indicating a maximum converted vacuum-ultraviolet power of ∼ 10 MW.

Experimental data of the cross section and of the asymmetry parameter $\beta$ in combination with photoelectron-spin-polarization results of Ar $3p^6$ and Kr $4p^6$ subshell photoionization were used to determine the bound-free matrix elements and phase-shift differences of the continuum wave functions in a photon-energy region from threshold to 41 eV. The paper includes a discussion of the first autoionization region between the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ thresholds in the framework of the angular-momentum-transfer formalism. The experimental results are compared with semiempirical and ab initio calculations with the use of line positions and oscillator strengths in the discrete part extrapolated to the continuum in application of the multichannel quantum-defect theory.

A computational study is made of mode-selective multiphoton excitation in a model nonseparable anharmonic-oscillator system. The time evolution of quantal wave packets on the Hénon-Heiles potential surface is treated via Floquet theory; periodicity of the classical laser field, $E_{0}cos\omega t$, is utilized to formulate the propagator for the state amplitudes. In a previous study by Hose and Taylor, a criterion was developed by which the quantum states corresponding to the classical quasiperiodic motion are identified. Two types of quasiperiodic states exist in the Hénon-Heiles system: $Q^{\mathrm{I}}$, the normal mode and $Q^{\mathrm{II}}$, a local (bond) mode; highly mode-mixed states are designated as $N$ (nonquasi-periodic). In the present study it is found that efficient multiphoton excitation into a subset of the $Q^{\mathrm{I}}$ states lying between the classical critical energy and the dissociation barrier is obtained, provided that the field strength is not too large and the frequency of the laser is tuned to the fundamental of the $Q^{\mathrm{I}}$ ladder. Implications for mode-selective excitation in real systems are discussed.

The radiative properties of very complex atomic configurations in highly ionized species can be determined easily by means of statistical moments, computed ab initio from relativistic radial orbitals. A case of current interest, the spectrum of Gdxii, is studied as an example. The locations and the widths of the zones of the upper configurations responsible for the bulk of the spontaneous emission are determined formally and numerically. The fact that these zones may sometimes lie well above the center of gravity of the configuration can be interpreted by considering the relevant $G^1(nl,n^{\prime }l-1)$ Slater integral and its angular coefficients. Moreover, it is shown that two transition arrays involving the same upper configuration may actually originate from disconnected zones of the latter. A linear relationship is established formally between the coefficients of $G^1(nl,n^{\prime }l-1)$ and the $J$-file sums of Condon and Shortley. This gives us the opportunity to evaluate the sum of the line strengths of all the transitions originating from a given upper level and, therefore, a method for checking oscillator-strength angular calculations.

A model of continuum-continuum transitions induced by very intense laser beams is solved. The energy distribution of the photoelectrons is found and it exhibits the broadening and the splitting induced by the laser field. Similar effects are known for bound-bound transitions, but our model made it possible to study them systematically in the continuum.

The first detailed observation of the perturbations by an electric field of the Rydberg autoionizing resonances of a rare-gas atom is reported. The Stark spectroscopy of autoionizing Rydberg states converging to the second ionization limit $4p^5{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^o{}_{}{}^{}$ in Kr I is performed in an atomic beam with the use of a two-step laser excitation from the $4p^{5}5s{\left[\frac{3}{2}\right]}_2^0$ metastable state. In addition to the verification of the ${\left(n^{*}\right)}^{-3\mathrm{}}$ dependence of the zero-field width of two autoionizing series and of the linear dependence of the Stark-manifold splittings with the electric field, we have observed an ${\mathrm{}\left(n\right)\mathrm{}}^{0.7}$ law for the dependence of the Stark-manifold splitting along the series, an important narrowing and dissymmetrization of the $n{d}^{\prime }$ resonances, and a nonlinear-field behavior of Stark-manifold components in the vicinity of the first anticrossing. Discussions are carried out with the use of the recent work on Rydberg-Stark spectroscopy and on autoionizing states in two-electron atoms.

Extensive new data on the bound even-parity spectrum of neutral barium have been provided recently by two-step optogalvanic spectroscopy. Highly excited members of the Rydberg series $6sns{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$, $6snd{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$, and $6sng$ as well as perturbing levels of these series belonging to $5dnd$ configurations have been observed. By means of multichannel quantumdefect theory (MQDT), we have performed the analysis of all available energy data relevant to the $J=1,3,4,5\mathrm{}$ even-parity bound spectra of barium. The behavior of $6sns{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$, $6snd{}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$ series perturbed by levels pertaining to the $5dns \mathrm{}(n=7,8\mathrm{})\mathrm{}$ and $5dnd \mathrm{}(n=6,7\mathrm{})\mathrm{}$ configurations is clearly interpreted. The MQDT analyses of $J=1\mathrm{} \mathrm{and} 3$ spectra complementary to that previously performed on the $J=2\mathrm{}$ spectrum offer the possibility of studying the anomalous fine structures of highly excited $6snd$ configurations. The MQDT analyses of $J=3-5\mathrm{}$ spectra explain the observed energies of $6sng{}_{}{}^3{}_{}{}^{}G_{3,4,5}^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}G_4^{}{}_{}{}^{}$ Rydberg levels; the fine structures of the $6sng$ series are only partially resolved in the vicinity of $5d7d$ $J=3\mathrm{} \mathrm{to} 5$ perturbing levels. Since the $J=0\mathrm{}$ and $J=2\mathrm{}$ bound even-parity spectra of Ba have already been analyzed by MQDT, we have now a full MQDT description of the bound even-parity spectrum for $J$ values ranging from 0 to 5.

The widths of the $L$ x-ray lines $L{\alpha }_1$, $L{\alpha }_2$, $L{\beta }_1$, and $L{\gamma }_1$ of tungsten have been measured to an accuracy of 4-6% with a high-resolution 90-cm-radius bent crystal Cauchois spectrograph, with the use of the photographic-photometric method. The measured values have been compared with those of earlier workers as well as with those obtained from theoretical self-consistent-field calculations. Our values of the $L{\alpha }_1$ and $L{\alpha }_2$ widths are in good agreement with those deduced from the recent Dirac-Hartree-Slater estimates of Chen et al. for the $L$ and $M$ subshells. Contrary to results of earlier workers, the width in eV of the $L{\beta }_1$ line is found to be larger than that of the $L{\alpha }_1$ line, a result which is in conformity with theoretical predictions. On the other hand, our measured value of the width of the $L{\gamma }_1\left(L_2{N}_4\right)$ line does not agree with the results of earlier workers, but is in fair agreement with the nonrelativistic estimate of McGuire. The need for a relativistic calculation of the $N$ subshell widths therefore seems to be indicated.

A theory for selective-vibrational-dephasing experiments using transient stimulated Raman scattering in high laser depletion is presented. The transient stimulated Raman scattering equations are numerically solved including laser depletion for homogeneously and inhomogeneously broadened vibrational line shapes. A detailed account of the effects of low and high laser depletion on the vibrational amplitudes is given. An interpretation for excite-and-probe vibrational-dephasing experiments undertaken in the low- and high-laser-depletion regimes is developed. For a collinear laser probe pulse, high depletion of the excitation laser pulse leads to enhanced coherent Stokes probe scattering from one vibrational subgroup in the inhomogeneously broadened line shape. This selective scattering is greatly dependent upon the excitation laser pulse width and the off-resonance frequency of the vibrational subgroup. The enhanced selectivity in high laser depletion allows the homogeneous dephasing time $T_2$ to be measured to a high level of accuracy.

We show that two distinct realizations of the SU(3) symmetry—the Gell-Mann SU(3) symmetry in quark physics and the Elliott SU(3) symmetry in nuclear physics—can be applied to the same dynamical system in intense-field electrodynamics under different experimental conditions. We also present a set of simultaneous soliton solutions in the Elliott SU(3)-symmetry scheme which has not been derived previously.

We consider a "chaotic Raman attractor" as an example of the type of chaotic behavior that may occur in nonlinear optics.

Chaotic emission is observed from a 3.39-μm He-Ne laser. We find different sequences leading from coherent to chaotic states depending on the laser frequency. In particular, all three theoretically known sequences—"period doubling," "Ruelle-Takens," and "intermittency"—are observed.