A theory for the nuclear-magnetic-resonance (NMR) absorption of chemically interacting species is derived which is valid when the correlation time ${\tau }_c$ for the molecular motion—which gives rise to NMR relaxation—is less than ${\tau }_e$, the chemical exchange time. The previous theory is valid only if ${\tau }_c$ were much less than ${\tau }_e$. This latter theory assumed that the relaxation was defined using the usual Fermi Golden Rule form where the time integration is from zero to infinity. When ${\tau }_c$ begins to approach ${\tau }_e$, it becomes necessary to replace the $0-\infty$ time integration by a $0-t$ time integration where $t$ is the time of the next exchange collision. This modification of the previous theory leads to a correction to the previous theory for which the lowest-order term goes as $\frac{{\tau }_c}{{\tau }_e}$.

A linear, steady-state analytical model of the optogalvanic effect in a positive column on the 587.6-nm He transition is presented. Absolute measurements of the optogalvanic effect in a positive column on the 587.6-nm He transition are reported. The model and the experiment are found to agree over a substantial range of direct currents and over a factor of 10 in column-radius-pressure product. This model relates the absolute magnitude of the optogalvanic effect on one transition to a known ionization rate. The structure of this model should be useful in determining ionization rates of electronically excited levels from optogalvanic measurements.

The ion contribution to the Stark broadening, as described by a suitably defined asymmetry function $A_L$, was investigated from the asymmetry behavior of the vacuum-uv spectral lines of N I at 119.955 nm (${}_{}{}^4{}_{}{}^{}P\to {}_{}{}^4{}_{}{}^{}S_{}^0{}_{}{}^{}$), 141.194 nm (${}_{}{}^2{}_{}{}^{}D\to {}_{}{}^2{}_{}{}^{}P_{}^0{}_{}{}^{}$), 149.467 nm (${}_{}{}^2{}_{}{}^{}P\to {}_{}{}^2{}_{}{}^{}D_{}^0{}_{}{}^{}$), and 174.273 nm (${}_{}{}^2{}_{}{}^{}P\to {}_{}{}^2{}_{}{}^{}P_{}^0{}_{}{}^{}$). The line radiation, which was emitted under controlled optical depths, was generated in a dense equilibrium plasma (wall-stabilized arc; electron number density $4.0\times {10}^{22} {\mathrm{m}}^{-3\mathrm{}}\le n_e\le 3.7\times {10}^{23} {\mathrm{m}}^{-3\mathrm{}}$; temperature $11000\le T\le 15000$ K). Deviations between experimental asymmetry functions of this study and those calculated from recently published data by Griem, have been found for the wings of N I 119.955 nm (normalized wavelength distance from the line center $\frac{\Delta \lambda }{\Delta {\lambda }_{\frac{1}{2}}}\ge 1$; $A_L^{\mathrm{Griem}}-A_L^{\mathrm{this}\mathrm{study}}\le 0.1$). For N I 141.194, N I 149.467, and N I 174.273 nm, the asymmetry functions based on line profiles calculated by Griem are in agreement with the asymmetry functions of this study within the given uncertainties. In addition, Stark-width and Stark-shift parameters were experimentally determined for the N I spectrum between 118 and 180 nm. The Stark-width parameters calculated by Griem and by Wilson and Nicolet agreed with the data of this study within ± 20%. Only for N I 131.054 and N I 132.663 nm did the Stark-width parameters calculated by Wilson and Nicolet deviate from the corresponding values of this study by approximately + 31%. The calculated Stark-shift parameters of Wilson and Nicolet, however, were considerably higher than the corresponding data of this study (maximum deviation + 78%), whereas the Stark-shift parameters calculated by Griem showed smaller deviations from the data of this study (maximum deviation + 49%).

The theoretical treatment of the combined effects of strong magnetic and motional electric fields on a simple atom, such as ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, is developed. It is shown that normal selection rules are relaxed by the external fields and that many previously forbidden transitions become allowed. The transition moment is shown to be velocity dependent, resulting in new and unique line shapes for these transitions. The theory is presented in a form directly applicable to the experiments reported in the accompanying paper.

Forbidden transitions have been observed from the $7{}_{}{}^1{}_{}{}^{}S$ state of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to the manifold of states, $n=9,L=1-8\mathrm{}$. These transitions are tuned into resonance with C${\mathrm{O}}_2$ laser lines by an applied magnetic field of 0-140 kG. Transitions to the odd-parity states are characterized by a velocity-dependent, motional Stark effect line shape which has been described previously. Transitions to the even-parity states have, in addition, a velocity-dependent transition moment creating a more complex line shape whose form is derived in an adjoining article. Analysis of the positions of over 20 observed resonances allows a determination of the zero-field separations of all the $n=9,L=1-8\mathrm{}$ states of He, and the quantum defects for He singlet states with $L>\mathrm{}0\mathrm{}$.

We report the effects of static electric fields on the doubly excited $6p_{\frac{1}{2}}\mathrm{nl}$ states of barium in the region $n^{*}=12\mathrm{}$. Using a three-step laser excitation scheme we have measured the electric-field dependence of the positions and autoionization widths of the $6p_{\frac{1}{2}}-17s,16p,15d,12l \mathrm{}(l=4-10\mathrm{}),12f, \mathrm{and} 16s$ states at field values of up to 4.92 kV/cm. The photoexcitation spectra of the autoionizing states exhibit interference effects which are well understood and provide a better understanding of the character of these states in electric fields. The systematic study of the spectra and positions allows us to explain the field dependence of the autoionization widths in terms of electric-field mixing of the discrete portions of the auoionizing states. The relationship of these effects to dielectronic recombination rates of ions and electrons in plasma microfields is discussed.

Using a high-resolution crystal spectrometer we have measured the relativistic angular-momentum splittings of the $5g-4f$ and $5f-4d$ transitions in pionic Ti and Fe atoms. The observed fine-structure splittings of 85.3±3.0 eV in ${\pi }^{-}$ Ti and 158.5±7.8 eV in ${\pi }^{-}$ Fe agree with the calculated splittings of 88.5 and 167.6 eV, respectively, arising from the Klein-Gordon equation and from small corrections due to vacuum polarization, strong interaction, and electron screening.

A general theory of the motion of a two-level atom in a resonant or near-resonant electromagnetic wave of arbitrary amplitude and phase, including effects of radiative relaxation due to interaction with the quantized vacuum field, is developed from first principles. Particular emphasis is placed on the effects of quantum-mechanical fluctuations of the radiation force and on the associated diffusion of atomic momentum due to spontaneous and induced absorption and emission processes. Analytic results and numerical examples are presented for (1) the lower bound on the temperature achievable by radiation cooling in a standing wave tuned below resonance, (2) the heating rate in a strong resonant standing wave, (3) the maximum confinement time for an atom in a Gaussian radiation trap, (4) the deflection and spreading of an atomic beam transversely illuminated by a strong resonant running wave, and (5) the transverse cooling of an atomic beam by a strong running wave tuned below resonance.

Relative cross sections for the reaction ${\mathrm{K}}^{-}+h\nu \to {\mathrm{K}}^{*}+e^{-}$ where ${\mathrm{K}}^{*}$ denotes the $4{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{},4\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}, \mathrm{and} 4{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ levels of neutral potassium have been measured. A crossed ion-laser beam technique was used to obtain measurements from threshold for the $P$ levels at photon energies from ∼17 030 ${\mathrm{cm}}^{-1\mathrm{}}$ to 20 486 ${\mathrm{cm}}^{-1\mathrm{}}$. A calibrated photometer detected the fluorescence due to the $4P\to 4S$ decay for each of the fine-structure levels. For photon energies well above threshold the branching ratios are flat with values of 0.50 for the $4{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ level, 0.17 for the $4{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ level, and 0.33 for the $4{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ level. The latter two are in accord with the 1:2 statistical weight ratio of these levels. Near the $P$ thresholds considerable structure is seen which suggests a final-state interaction.

We report initial results of a systematic study of the calculation of near-edge features in the x-ray absorption spectra of an atom in a molecular cluster as a function of different chemical environments and of varying prescriptions for the effective molecular potential of the excited electron. For a test-case comparison of the $K$ edges of Ge${\mathrm{Cl}}_4$ and Ge${\mathrm{H}}_4$, we find that the chemically induced changes in the molecular potential lead to large changes in the occurrence and strengths of bound-state and shape-resonance spectral features, in semiquantitative agreement with observation.

The beam-foil time-of-flight technique has been used to measure the wavelengths and lifetimes of the $\Delta n=0\mathrm{}$ transitions to the ground state $2p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2},\frac{1}{2}}^{}{}_{}{}^{}\to 2s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ in lithiumlike ${\mathrm{Kr}}^{33+}$ and the intercombination transition $2s2p{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\to 2s^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ in berylliumlike ${\mathrm{Kr}}^{32+}$. The following results are obtained for the transition, wavelength (Å), and lifetime (nsec), respectively: ${\mathrm{Kr}}^{33+}$: ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{},174.15\mathrm{}\pm 0.26,0.32\pm 0.03$; ${\mathrm{Kr}}^{33+}$: ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{},91.08\mathrm{}\pm 0.10,0.052\pm 0.003$; ${\mathrm{Kr}}^{32+}$: ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{},169.9\mathrm{}\pm 0.5,2.3\pm 0.3$. These results are compared with calculations.

High-resolution $L_{2,3}{M}_{2,3}{M}_{4,5}$ and $L_{2,3}{M}_{4,5}{M}_{4,5}$ Auger electron spectra from ${\mathrm{Br}}_2$ and Kr have been measured using electron-beam excitation. The spectra have been decomposed into their components and interpreted by comparison with calculated energies and intensities. The theoretical calculations have been carried out in the mixed coupling scheme applying $\mathrm{jj}$ coupling for the initial state and intermediate coupling for the final state of the transition. Good agreement between the calculated and experimental results has been found for the $L_{2,3}{M}_{4,5}{M}_{4,5}$ and $L_3{M}_{2,3}{M}_{4,5}$ spectra of both bromine and krypton. There is, however, noticeable disagreement between theory and experiment for the $L_2{M}_{2,3}{M}_{4,5}$ spectra. The structure for the high-energy group is unusual and its intensity is anomalously low; a similar pattern is also seen for gallium, germanium, and arsenic.

Abrupt radiative excitation into a one-electron atomic continuum of distinct zero-width energy levels is investigated in a simple energy-conservative atomic model. Induced off-diagonal coherence is shown to decay irreversibly. That is, it is shown that one cannot recover off-diagonal coherence by the application of subsequent radiative pulses, ruling out the occurrence of long-time or repetitive coherent transients such as photon echoes in this model of continuum transitions. A brief comparison with the case of transitions in a Doppler-broadened continuum, where coherent transients are commonly observed, is made.

The excitation spectrum arising from photon density-density correlations has been considered in a system of two interacting identical atoms. The two-level atoms are pumped near resonance by a strong electromagnetic field while they interact through their dipole-dipole interaction and radiate to each other as well. In such a system the photon density-density correlations arise from the interference spectrum due to the beating of the frequency of the pump field and the atomic transition frequency, as well as from the spectrum induced by the dipole-dipole and radiative interactions between the atoms. The spectral function of this system has been calculated in the limit of high photon densities. It consists of four terms describing four bands which are similar to those appearing in the two-atom resonance fluorescence spectrum for the symmetric modes when the central peak is missing. The collective behavior as well as the dynamics of the system are discussed in detail. By using the spectral function, an expression for the photon density-density correlation function at different times has been derived where thermal effects have been also included.

An individual barium ion, continuously observed by laser fluorescence, has been isolated in a Paul rf quadrupole trap at room temperature. By optical sideband cooling its microscopically measured image has been reduced in thickness to ∼ 2 μm in the object plane, the diffraction limit. Estimated ion temperatures reached are $T_i\simeq 10 \mathrm{to} <36\mathrm{}$ mK. With cooling, the ion could be held indefinitely, without cooling ∼ 30 s. In the future the technique seems capable of attaining kinetic temperatures ∼ ${10}^{-8\mathrm{}}$ K, much lower than realized so far by other means, with corresponding far-reaching implications.

Four-wave mixing processes with double and triple resonances are discussed in detail. Explicit expressions for the resonant nonlinear susceptibilities of various cases are derived. It is shown that in some cases, four-wave mixing can yield a spectrum with reduced inhomogeneous broadening. Possible applications of multiresonant four-wave mixing as a spectroscopic technique are considered. They include high-resolution spectroscopy, deduction of transition matrix elements, study of transitions between excited states, measurement of longitudinal relaxation times, distinction between resonant Raman scattering and resonant fluorescence, etc. Selective polarization arrangement allows us to suppress the nonresonant background, separately measure the real and imaginary parts of the resonant nonlinear susceptibility, minimize the effects of absorption and laser intensity fluctuations, and greatly enhance the signal-to-noise ratio.

The propagation of a vector electromagnetic beam in a linear homogeneous dielectric half-space is considered using the Whittaker potentials for the electromagnetic fields. A relation between the Whittaker potentials and the vector and scalar potentials of the electromagnetic theory is obtained. The polarization properties of the beam are discussed in the paraxial approximation and beyond.

The saturation of resonant third-harmonic generation in absorbing and phase-matched high-density systems is investigated theoretically as well as experimentally. It is shown that the saturation is determined by the two-photon absorption [imaginary part of ${\chi }_S^{\mathrm{}\left(3\right)\mathrm{}}\left(\omega \right)$]. It gives rise to a depletion of the fundamental wave and leads to a change of the level populations and of the effective refractive index destroying the phase matching. This has to be compared with nonresonant systems which are limited by the Kerr effect, the intensity-dependent changes of the refractive index [real part of ${\chi }_S^{\mathrm{}\left(3\right)\mathrm{}}\left(\omega \right)$]. Optimum conditions for resonant systems are reported.

The moments of the fluorescent radiation of a $J^2$-invariant, cooperative fluorescent group of atoms are calculated exactly. Both photon bunching and antibunching can occur in the fluorescent radiation. All finite-order moments have a critical-point behavior at ${\Omega }_c=J\gamma$ in the cooperative limit of $J\to \infty$. The maximum normalized intensity correlation is 6/5 which shows greatly reduced intensity fluctuations compared to noncooperative resonance fluorescence. Enhanced antibunching in all the fluorescent correlation functions occurs in the threshold region.

A new method of observing two-photon optical transients is proposed, namely, by intensity switching of incident cw laser beams. This method, which is an alternative to the standard dc Stark or frequency-switching techniques, employs the second-order optical (ac) Stark effect to throw the two-photon active levels in or out of resonance by appropriately switching the intensities of the two laser beams. Theoretical treatment for the three basic transients optical nutation, free induction decay, and two-pulse echo in thin samples using intensity switching, is given for both the cases: co- and counter-propagating beams. The feasibility of the method for the $5S_{\frac{1}{2}}-3S_{\frac{1}{2}}$ transition in Na vapor is demonstrated. An important feature is the apearance of an intensity-dependent beat frequency in the optical FID signal.