The autoionization of the helium atom has been experimentally studied in ${\mathrm{He}}^{+}$ + He collisions between 3 and 140 keV by electron spectrometry. The excitation of the two collision partners has been considered. Above 10 keV, the shapes and excitation cross sections of the $2s^2{}_{}{}^1{}_{}{}^{}S$, $2s2p{}_{}{}^3{}_{}{}^{}P$, $2p^2{}_{}{}^1{}_{}{}^{}D$, and $2s2p{}_{}{}^1{}_{}{}^{}P$ are determined by a numerical fitting procedure which is reported in detail; below 10 keV the (${}_{}{}^1{}_{}{}^{}D+{}_{}{}^1{}_{}{}^{}P$), and $2p^2{}_{}{}^1{}_{}{}^{}S$ line intensities are obtained by planimetry since important post-collision effects are observed. From the angular distributions measured below 15 keV, the relative sublevel populations are deduced for the $2p^2{}_{}{}^1{}_{}{}^{}D$ and $2s2p^{1}P$ levels and are compared with those obtained by other authors in a coincidence experiment; the excitation processes are then discussed within the quasimolecular-excitation model. For the highest collision energies, the asymmetry of the angular distributions with respect to 90° as well as the line shapes above 100 keV are interpreted by the occurrence of sudden electronic transitions to the continuum. A comparison of the differential cross sections for emission of electrons by autoionization of the fast and slow particles permits us to show that the quasimolecule model cannot explain what is observed above a collision velocity of about 0.5 a.u. The dependence of the total cross sections against the collision energy is also discussed in terms of an evolution of the excitation mechanism from a quasimolecular to an atomic one; the specific variation of the $2s2p{}_{}{}^3{}_{}{}^{}P$ cross section strengthens this interpretation. These total cross sections are compared with those estimated from earlier ${\mathrm{H}}^{+}$ + He data published by us; similar autoionization cross-section values are expected for the two systems at high collision velocity.

For many years it has been commonly believed that one of the few things known for certain about the Coulomb three-body problem is the asymptotic form of the longest-range term in the elastic polarization potential. The form $r^{-4\mathrm{}}$ is derived initially from the adiabatic approximation and persists at intermediate energies if the approximation used for the local potential is completely regular in momentum space. The more realistic equivalent local approximation is examined. The momentum-space potential has discontinuities in its second derivative, which lead to a severe modification of the $r^{-4\mathrm{}}$ form. This is true for electron-atom scattering in general.

Lifetimes of twenty-three nonresonant levels belonging to the $3p^5\mathrm{nd} \mathrm{}(n=4,5,6,7\mathrm{})\mathrm{}$ configurations of Ar I have been measured. The single-photon delayed-coincidence method has been used to obtain the lifetime values given in the present work. As far as we know, no previous experimental results exist for sixteen of these levels. The experimental error was in the range from 1 to 10%. Our results have been compared with other experimental values and with our calculations performed in $\mathrm{jK}$ coupling and Coulomb approximation and with the intermediate-coupling results given in the literature.

Spectra of the elements barium ($Z=56\mathrm{}$) through Nd ($Z=60\mathrm{}$) observed in laser-produced plasmas from pure-metal targets in the wavelength region 70- 130 Å are presented. This completes our survey of the spectra of elements from cesium to hafnium in two regimes of optical thickness. The two sets of spectra show significant differences. In order to interpret the whole range of results, we need to know the ground-state configurations of ions in stages up to XVI for $Z=55-71\mathrm{}$. From the limited-term analyses available, a table of plausible ground configurations is proposed. In the preparation of the table, the effect of increasing ionization, for a fixed $Z$, on wave-function contraction is studied, and it is inferred that the $4f$ electron becomes more tightly bound than $5p$ between the sixth and seventh spectra of the elements from cerium to lutetium; similarly the $4f$ electron attains a higher binding energy than does the $5s$ between the fourteenth and fifteenth spectra. Consequently, for a considerable range of ion stages, particularly in the rare earths, the ground configurations have a partially filled $4f$ shell. Furthermore, in the same ion stages, the $4f$, $5p$, and $5s$ energies stay relatively close together so that configurations of the type $5p^{l}4f^{n-l}$ lie very near the nominal ground configuration. A corresponding manifold of overlapping excited configurations of the type $4d^{9}4f^{n+1\mathrm{}}$, $4d^{9}5p^{l}4f^{n-l+1\mathrm{}}$ is also expected and the observed resonances arise from all of the $4d-4f$ transitions permitted between the upper and lower manifolds of configurations. The numbers of transitions are calculated and in the majority of cases are very great. Because the principal quantum number does not change, the transitions will tend to overlap for different ion stages of any one element. The widths and varying complexity of the resonances can be explained in a satisfactory, if qualitative, way by means of available theory. The very considerable difference between the compound and metal spectra can be explained in terms of the effect of number density of the active species on both emission and absorption processes in the two types of plasma.

Photoelectron spectroscopy has allowed us recently to observe both giant resonance enhancement of primary lines and two-electron resonant Auger satellites corresponding to the resonant behavior of photoemission cross sections relative, respectively, to autoionization and Auger decay channels. Using multichannel scattering theory and configuration-interaction formalism, I have established general formulas to parametrize the photoemission cross sections in the neighborhood of nonradiatively decaying excited states located near inner-shell ionization thresholds. Considering both cases of one isolated resonance and $n$-resonant states decaying into $N$ continua, I have shown why the two types of decay (autoionization and Auger) cannot be treated independently and have stressed the importance of coupling autoionization channels with Auger ones to account for the asymmetry profile of Auger partial cross sections and the presence of extra structures due to neighboring resonances in Auger channels. Some examples are discussed to point out that the relative importance of primary and satellite lines may vary strongly with the element and the energy range considered. The determination of the parameter ${\alpha }_n\left(\mu E\right)$ typical of a resonance $n$ in each channel $\mu$ provides us with many types of information, such as the importance of interchannel coupling, spin-orbit effects, and the existence of shake-up satellites.

Rotationally quasibound levels (shape resonances) in the $A{}_{}{}^1{}_{}{}^{}\Pi$ state of C${\mathrm{H}}^{+}$ are investigated with the use of a coaxial fast-ion-beam — laser photofragment spectrometer. Individual shape-resonance levels are excited with laser radiation from high rotational levels of the $X{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ ground state of C${\mathrm{H}}^{+}$. The dissociative decay of the quasibound levels into ${\mathrm{C}}^{+}\left({}_{}{}^2{}_{}{}^{}P\right)+\mathrm{H}\left({}_{}{}^2{}_{}{}^{}S\right)$ is monitored by observation of the charged photofragment and measurement of its kinetic energy. Analysis of the excitation wavelength spectrum, the photofragment kinetic energies, and in some cases, the width of the shape resonances allows the assignment of the quantum numbers which characterize the quasibound levels. An improved potential-energy curve for the $A{}_{}{}^1{}_{}{}^{}\Pi$ state is determined by the comparison of the experimental levels with those calculated for trial potentials. Thirty-two quasibound levels have been identified experimentally as grouping into $\Lambda$-doublet components for rotational quantum numbers $12\le J^{\prime }\le 35$ and vibrational quantum numbers $0\le v^{\prime }\le 10$. Spectroscopic constants for the $X{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ and $A{}_{}{}^1{}_{}{}^{}\Pi$ states are obtained from a correlated fit of the excitation spectra for the shape-resonance levels and previous spectroscopic observations of the bound levels. Our study also allowed the first direct experimental determination of the dissociation energy of C${\mathrm{H}}^{+}$: $D_0^0=4.080\mathrm{}\pm 0.003$ eV.

Experimental line profiles and integrated intensities for backscattered and forward-scattered resonance radiation using white-light excitation are compared with theory for Na densities between ${10}^{19}$ and ${10}^{22}$ atoms/${\mathrm{m}}^3$. Absorption measurements are used to verify linewidth and line-wing asymmetry parameters and Na-vapor densities. The scattered spectral intensities are normalized to the incident flux using calibrated diffusers. Measurements of scattered intensities were also made as ${\mathrm{N}}_2$ was added to the Na. The line profiles and integrated intensities were calculated using a model which assumes complete redistribution of the radiation scattered by an atom and takes into account both radiative and nonradiative transport of the resonance excitation. The theory includes the effects of quenching by ${\mathrm{Na}}_2$ and by admixed ${\mathrm{N}}_2$. With the white-light excitation used in these experiments the radiative transport is found to dominate the nonradiative transport except at wavelengths very near line center. The agreement between experimental and theoretical line profiles is good at high sodium densities (∼ 5 × ${10}^{21}$ atoms/${\mathrm{m}}^3$) where instrumental resolution is not a serious limitation. The agreement between experimental and theoretical integrated intensities is within experimental error for Na densities of (1-10) × ${10}^{19}$ and (2-5) × ${10}^{21}$ atoms/${\mathrm{m}}^3$. At the intermediate densities the approximately 20% discrepancy may be caused by the incomplete redistribution of radiation in the scattering process.

We have studied the evolution of excited states in an atomic-hydrogen beam undergoing radiative cascade after formation by charge exchange in a gas cell. A simple one-parameter model of the charge-exchange excitation probability is used to estimate the initial population of long-lived Rydberg states. Special attention is paid to the $n=2\mathrm{}$ state populations at long times (several microseconds). The persistent Lyman-$\alpha$ radiation from the $2P$ state can be a significant source of background for sensitive experiments using the metastable $2S$ state. Numerical and graphical results are presented.

Theoretical photoionization cross sections, angular distributions, and spin-polarization parameters for outer $\mathrm{ns}$ and $\mathrm{}(n-1\mathrm{})d$ subshells of the group-$\mathrm{II}B$ elements zinc, cadmium, and mercury are determined above their respective $\mathrm{}(n-1\mathrm{})d$ thresholds. Account is taken of electron-electron correlation and of the spin-orbit interaction by means of the relativistic random-phase approximation. Comparisons are made with previous theoretical work and with available experimental measurements.

Associative ionization in $\mathrm{Na} 3s-\mathrm{Na} 4d$ and $\mathrm{Na} 3s-\mathrm{Na} 5s$ collisions has been studied using two dye lasers to produce excited sodium atoms. The rate constants for these processes are reported.

Laser (light-pressure) cooling of two-level ions stored in a Penning trap is treated theoretically, in the limit that the frequencies of motion of the ions are much smaller than the natural linewidth of the optical transition. Rate equations for the mean-squared amplitudes of motion are derived from a semiclassical analysis, which is confirmed by quantum-mechanical perturbation theory. Simultaneous cooling of all three modes of motion is shown to require a spatially nonuniform laser-beam profile, unlike the case of the harmonic trap. Comparison is made with recent experiments. Also, the three-dimensional harmonic trap is treated by two simple methods. One is based on the energy rate equations and the other on Langevin-type equations. Identical results are obtained by the two methods for the steady-state energies. These results are compared with the works of others.

The dynamical behavior of the dipole interaction of an equally spaced multilevel atom is analyzed. The atom has a spinlike distribution of dipole transition moments and interacts with a single-mode radiation field. Detailed results for three-level ($J=1\mathrm{}$) and four-level ($J=\frac{3}{2}$) systems are presented.

General theory and analytic results are given for calculating the fragment yield and the rate constant of bound-to-continuum decompositions of atoms or molecules induced by a multiphoton effect called the resonant photon-as-catalyst effect (PCE). In this effect the rate constant of the reaction is increased due to the presence of the photons while the total number $N$ of photons in a given mode of the electromagnetic field is conserved. The decomposition processes include (1) photon-catalyzed (PC) predissociation from excited states of molecules in general, as well as from ground states in some cases of polyatomic molecules: $AB+N\hslash \omega \to A+B+N\hslash \omega$; and (2) PC autoionization of atoms or molecules: $A+N\hslash \omega \to A^{+}+e^{-}+N\hslash \omega$. Both pulsed and continuous-wave laser irradiation are studied. Optimization of the yield and the rate constant by the lifetimes of the molecular states are studied for the first time. Resonant enhancement [Lau, Phys. Rev. Lett. 43, 1009 (1979)] and coherent saturation [Lau, Phys. Rev. A 22, 614 (1980)] in this effect are further elaborated. Laser-stimulated decays and optical Stark shifts due to the continua and other nonresonant discrete states are included in the theory. The rotational line strengths for symmetric top molecules predissociated by resonant PCE using linear-polarized laser radiation are also given.

The rate of blackbody-radiation-induced transfer from the $19s$ state of sodium to the $18p$ and $19p$ states has been measured over the temperature range 6 to 210 K. The rate, which ranges from 2 × ${10}^3$ to 20 × ${10}^3$${\mathrm{s}}^{-1\mathrm{}}$, is sensitive to several transfer coefficients and spontaneous transition rates as well as to the precise form of the radiative distribution. The results are in good agreement with theory.

The reflection and transmission of electromagnetic waves by a dielectric slab which performs a given accelerated motion is investigated theoretically. Rigorous expressions for the reflected and transmitted waves are obtained by making use of the covariant properties of Maxwell's equations under the arbitrary coordinate transformations. It is found that there exists a small Doppler shift in frequency for the transmitted wave due to the drag effect of the moving medium. It is shown that this frequency shift depends on the acceleration, the index of refraction, and the width of the slab and is independent of the velocity of the slab.

The Breit interaction is found to substantially affect predicted multiplet splittings of double-inner-shell-vacancy configurations in heavy atoms. Not only can the magnitude of the splitting be greatly changed in some cases but the level order can also be altered with respect to that predicted from the Coulomb interaction alone. Inclusion of the full Breit interaction in the splitting of [$\mathrm{KL}$] hole state reduces the $K{\alpha }_1$ x-ray hypersatellite shift by ∼10 eV and increases the $K{\alpha }_2^h$ shift by ∼16 eV at $Z$=80; the intensity ratio of $K{\alpha }_1^h$ to $K{\alpha }_2^h$ is reduced by ∼25% at $Z$=18 and increased by ∼7% at $Z$=60.

When a beam of spin-polarized metastable ${\mathrm{He}}^{+}2s_{\frac{1}{2}}$ ions is quenched by an electric field $\overrightarrow{\mathrm{E}}$, the emitted radiation intensity contains an asymmetry term which is proportional to $\hat{k}·\overrightarrow{\mathrm{E}}\times \overrightarrow{\mathrm{P}}$, where $\overrightarrow{\mathrm{P}}$ is the spin-polarization vector and $\hat{k}$ is the direction of observation. The effect is due to interference between spontaneous magnetic-dipole ($M1$) and induced electric-dipole ($E1$) decay modes to the ground state. At $\left|\overrightarrow{\mathrm{E}}\right|=38.14\mathrm{}$ V/cm, the measured asymmetry is (0.323±0.085) × ${10}^{-3\mathrm{}}$ in agreement with the theoretical value 0.3443 × ${10}^{-3\mathrm{}}$. The experiment provides the first measurement of the relativistic $M1$ matrix element for the $2s_{\frac{1}{2}}-1\mathrm{}{s}_{\frac{1}{2}}$ transition in a hydrogenic ion. The paper contains an extensive discussion of all possible asymmetry effects, including higher-order relativistic and electric-field perturbation corrections.

The photo-Auger ionization process is a higher-order contribution to the direct photoelectric effect in which photoexcitation of an inner-shell electron is followed by the emission of an Auger electron. The frequency-integrated photo-Auger ionization cross section for ions of the lithium isoelectronic sequence is calculated in the isolated resonance approximation. The effects of transitions into all accessible intermediate states are explicitly included. Results are compared with the frequency-integrated direct photoionization cross section. The relative contribution of the photo-Auger effect for three-electron ions is a maximum in the Ne(7 +) region, where it is of order 70% of the direct process.

The relative level populations in beam-foil-excited sodiumlike argon (Ar VIII) and copperlike krypton (Kr VIII) have been studied at 2-MeV projectile energy. In Ar VIII, the $3p$ and the $3d$ terms are strongly excited. The level population decreases rapidly with increasing value of the principal quantum number $n$, reaches a minimum for levels with $n=5\mathrm{} or 6$, but increases then and reaches a strong maximum for levels with $n\simeq 11$. As an example, the $11d$ level is populated a factor of 2 more than the $3d$ level. Also the Kr VIII data show a population maximum for levels with $n=11\mathrm{}$. The strong $3p$ and $3d$ level excitations in Ar VIII are explained as selective inner-shell processes which can be understood in the molecular-orbital electron-promotion picture. The preferential population of highlying Rydberg states is explained as resulting from a near-resonance electron transfer from the valence band of the carbon foil to the projectile. This process takes place when the projectile leaves the back of the foil, and the distribution of population upon different $n$ levels will vary with charge state. The level population for levels with fixed $n$ increases rapidly with increasing value of the orbital-angular-momentum quantum number for levels in Kr VIII, Kr X, Kr XI, and Kr XIII. The results are discussed, and their influence on decay curves as well as their relations to multiply charged ion-atom collisions are pointed out.

Harmonic generation in the free-electron laser offers a possible means to extend the wavelength range of the device towards high frequency. Numerical solutions to the basic equations describing this process are shown for cw operation using a linearly polarized wiggler. Higher harmonic emission becomes enhanced as the magnetic field is increased and as the energy spread in the electron beam is reduced.