Generalized oscillator strengths (GOS) for the dipole-forbidden 31.4-eV band in ${\mathrm{N}}_2$ have been determined both experimentally and theoretically. The experimental values for the GOS were obtained using a crossed-beam electron spectrometer at 1-keV impact energy. The theoretical results were determined using the first Born approximation with ab initio configuration-interaction target wave functions.

The cross sections for the double ionization, ionization plus excitation, and double excitation of ${\mathrm{H}}_2$ by electrons and protons in the range of 350 to 3500 keV/amu have been measured. In all cases, the cross section for electron bombardment was greater than that for equivelocity proton bombardment. The results are discussed in terms of first and second Born processes and interferences between the two.

A momentum-space analysis of the scattering-state wave function based on a formal integral equation for the wave function in electron–hydrogen-atom ionizing collisions leads to two simple results. These solutions are correct in the asymptotic region and are exact, respectively, to first and second orders in the interaction potentials. The result may be further improved and generalized and could be used in atomic ionization studies.

We present several theoretical estimates of D+D fusion rates for heavy-water molecular clusters impacting on TiD targets. Our considerations range from the simple thick-target yield to single- and multiple-deuteron knock-on to thermonuclear and thermal-spike models; detailed molecular-dynamics simulations are also presented. Each of our models fails, by many orders of magnitude, to reproduce the yields observed in recent experiments. We consider the extent to which experimental artifacts might contribute to the results claimed.

Directional effects on the motion of swift ions in anisotropic media are studied. The stopping power is a function of the direction of the velocity relative to the principal axes of the medium, and there is a nonzero lateral force on the ion tending to bend its trajectory. These effects arise from the anisotropy of the dielectric response, and are distinct from channeling. Simple expressions are derived for the stopping power and lateral force in the nonrelativistic, high-velocity limit, and results are presented for crystalline graphite.

Absolute absorption, ionization, and dissociation cross sections of HCl have been measured in the 980–720-Å spectral range. Dipole moments have been obtained for several transitions. A quantum-defect-theory model with quantum-defect functions is used to describe the observed structures. The possible predissociation mechanisms are discussed. The dissociation excitation spectrum of DCl is assigned in the same way.

The combined use of an eigenchannel R-matrix calculation in jj coupling, and of a generalized quantum-defect treatment of electron escape in a polarization potential, accounts quantitatively for high-resolution photodetachement spectra of ${\mathrm{Cs}}^{\mathrm{-}}$ and ${\mathrm{Rb}}^{\mathrm{-}}$. The calculation further predicts the existence of three odd-parity 6s6p ${}_{}{}^3{}_{}{}^{}$P° bound states in ${\mathrm{Cs}}^{\mathrm{-}}$ (with J=0,1,2) in addition to the well-known 6${\mathit{s}}^2$ ground state, confirming earlier evidence derived from studies by Fabrikant [Opt. Spektrosk. 53, 223 (1982); Eng. trans., Opt. Spectrosc. (USSR) 53, 131 (1982)], and by Fischer and Chen [J. Mol. Struc. 199, 61 (1989)] performed in LS coupling. The radiative lifetime of the J=1 excited bound state is predicted to be 0.013 sec, whereas the J=0,2 fine-structure components appear to be unusually long-lived. Photodetachment spectra predicted for ${\mathrm{Fr}}^{\mathrm{-}}$ exhibit resonances near the first excited ${}_{}{}^2{}_{}{}^{}$P thresholds of Fr, analogous to those familiar in ${\mathrm{Rb}}^{\mathrm{-}}$ and ${\mathrm{Cs}}^{\mathrm{-}}$.

Isotope shifts of 16 transitions in 4${\mathit{f}}^7$5d6${\mathit{s}}^2$${(}^9$D)-4${\mathit{f}}^7$5d6s6p${(}^9$D${,}^9$F) of Gd i for the stable isotopes with A=152–160 were measured by atomic-beam laser spectroscopy. The hyperfine constants A and B of ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ in two ${}_{}{}^9{}_{}{}^{}$D terms and the ${}_{}{}^9{}_{}{}^{}$F term were determined. The single-electron hyperfine parameters ${\mathit{a}}_{4\mathit{f}}^{10}$ for the 4f electron and ${\mathit{a}}_{5\mathit{d}\mathit{s}}^{\mathit{k}}$${\mathit{k}}_{\mathit{l}}$,${\mathit{b}}_{5\mathit{d}\mathit{s}}^{\mathit{k}}$${\mathit{k}}_{\mathit{l}}$ for the 5d electron were deduced. The electric quadrupole moments Q of ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ were obtained to be 1.27(5)b and 1.36(6)b, respectively. The J dependences of the isotope shifts in two ${}_{}{}^9{}_{}{}^{}$D terms and the ${}_{}{}^9{}_{}{}^{}$F term were observed. The parameter ${\mathit{z}}_{5\mathit{d}}$ of the crossed-second-order effect for the ${}_{}{}^9{}_{}{}^{}$D term of 4${\mathit{f}}^7$5d6${\mathit{s}}^2$ was derived, and the value of ${\mathit{z}}_{5\mathit{d}}$/λ was obtained to be 134(4) MHz/${\mathrm{fm}}^2$. We confirmed that the field shift is dominant in the crossed-second-order effect and the specific mass shift is negligible.

The spectrum of redistributed light that results from excitation by monochromatic near-resonant incident light has been observed in calcium perturbed by argon. The two components were found to be well described by the astrophysical redistribution functions ${\mathit{R}}_{\mathrm{II}}$ and ${\mathit{R}}_{\mathrm{III}}$ for the Rayleigh and fluorescent peaks, respectively. The Doppler width of the Rayleigh component was observed to be √2 larger than that of the fluorescent component, for our observation at right angle to the incident beam, clearly demonstrating the coherence of this part of the scattering. The ratio of fluorescent to Rayleigh intensity was independent of detuning within our range, which was well within the impact region, and increased linearly with argon pressure in a manner that is consistent with an identification of the cross section for redistributing collisions with that for line broadening.

The analytical exactness of estimating eigenquantum defects from Lu-Fano plots is examined. The results show that it is analytically untenable to estimate the eigenquantum defects from any two-dimensional Lu-Fano plot if more than two ionization thresholds are involved in the multichannel quantum-defect theory models, but that Lu and Fano’s original two-limit recipe [Phys. Rev. A 2, 81 (1970)] is rigorously valid.

We show that when three-photon excitation of a dipole-allowed transition is mediated by two laser beams at angular frequencies ${\mathrm{\omega }}_{\mathit{L}1}$ and ${\mathrm{\omega }}_{\mathit{L}2}$ that are crossed at an angle θ, large cooperative pressure-dependent shifts occur in the line shape for resonant excitation. When excitation of the state is by a 2${\mathrm{\omega }}_{\mathit{L}1}$±${\mathrm{\omega }}_{\mathit{L}2}$ process and either θ≪1 or ${\mathrm{\omega }}_{\mathit{L}1}$≫${\mathrm{\omega }}_{\mathit{L}2}$, the cooperative shift is large compared with the pressure-induced width of the line. For the 2${\mathrm{\omega }}_{\mathit{L}1}$+${\mathrm{\omega }}_{\mathit{L}2}$ case the shift is toward higher excitation energy, while for 2${\mathrm{\omega }}_{\mathit{L}1}$-${\mathrm{\omega }}_{\mathit{L}2}$ the shift is to lower excitation energies. In cases where the resonance can be reached by more than one pathway, differing only in the directions of propagation of the photons involved in the excitation, multiple peaks are predicted in the excitation line shape.

We present a relativistic self-consistent-field calculation of two-photon decay rates in atomic inner shells. By comparing our relativistic two-photon decay rates for Mo, Ag, and Xe with results for a series of hydrogenlike ions and existing nonrelativistic results, we identify and clarify the nature of the relativistic effects and atomic screening effects: (1) the relativistic effects reduce the two-photon decay rates compared with the nonrelativistic results; (2) screening reduces the atomic two-photon decay rates compared with the corresponding H-like ions. We also compare our results with recent experiments and with other theoretical calculations. Our results are in reasonable agreement with the experimental measurements, except for the 2s-1s transitions, and with other theoretical calculations, except for the nd-1s results of Mu and Crasemann [Phys. Rev. Lett. 57, 3039 (1986); Phys. Rev. A 38, 4585 (1988)].

The ion broadening parameters of several argon and carbon spectral lines are determined by fitting asymmetric theoretical profiles to spectral line profiles obtained in a wall-stabilized arc experiment. Good agreement with previous experimental results is obtained for most of the ‘‘blue’’ argon lines and carbon lines examined. Results are presented for several argon ‘‘red’’ and ‘‘blue’’ lines that have not been examined previously. In most cases, our ion broadening parameter values are larger than published theoretical values.

Radiative lifetimes for 17 levels of the ${\mathit{z}}^4$P°, ${}_{}{}^6{}_{}{}^{}\mathrm{°}$, ${\mathit{z}}^6$F°, and ${\mathit{z}}^6$P° terms in Cr i have been measured by the technique of time-resolved laser-induced fluorescence from sputtered metal vapor, extracted from a pulsed hollow cathode discharge. A XeCl-excimer laser-pumped distributed-feedback dye-laser with a pulse duration of 200 ps, a spectral bandwidth of 0.006 nm, and a repetition rate of up to 200 Hz was employed for the selective excitation of the levels investigated.

We have applied the collinear laser spectroscopy technique to investigate the Stark structure of argon Rydberg states in the vicinity of the n=18 hydrogenic manifold below the first ionization limit (3${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ ionic core). Stark spectra have been recorded for electric fields from 0 up to 120 V/cm. Although the studied energy range is only 4 ${\mathrm{cm}}^{\mathrm{-}1}$ wide, it is sufficient to observe how the 18f level progressively merges into the hydrogenic linear Stark structure arising from the n=18 (l≥4) manifold. The high resolution of the method allowed us to observe several fine-structure components for each Stark level. The complexity of these spectra mainly originates in the electrostatic interaction of the Rydberg electron with an anisotropic ionic core. Using jK coupling, a calculation is developed that quantitatively fits the observed spectra. The energy positions of the lines, as well as their relative intensities, are accurately reproduced by this calculation. The best fit between theory and experiment is obtained when we take into account the presence of the odd-parity perturber state 18d[3/2${]}_1$, which is known to lie within the studied energy range.

In this paper we discuss in detail the dynamics of a two-state system ruled by a non-Hermitian Hamiltonian. The relevance of the results to multiphoton ionization is also discussed and a comparison to earlier works is presented.

An investigation of the foundations of strong-field theories of the Keldysh type is undertaken. It is found first that the Volkov solution, upon which the Keldysh approximation depends, exists unambiguously only in the relativistic case. Accordingly, a fully relativistic theory of atomic photoionization is formulated from first principles. A strong-field approximation (SFA) is expressed, based only on the proposition that the photoionizing field dominates the atomic potential in the final state. The nonrelativistic limit of the SFA gives exactly one of the known Keldysh-type theories, but the original Keldysh theory itself is found to neglect some fraction of the strong-field effects. It is shown on a variety of grounds that ${\mathbf{A}}^2$ terms must be fully retained in Keldysh-type theories. Important in this matter are the proper application of asymptotic conditions, the interchangeability of A⋅p and ${\mathbf{A}}^2$ terms, and the consistent reduction to the nonrelativistic limit.

We investigate different kinds of polarization-correlated-emission schemes, including a maser and a laser concept. In these systems, coherently prepared atoms drive the electromagnetic field. We demonstrate that the atomic coherence can lead to a reduction of noise fluctuations within a linear theory. In the homogeneously broadened case the amplitude noise is strongly affected by the atomic coherence, leading to noise quenching in the laser and even squeezing in the maser. On the other hand, the phase fluctuations are unaffected by the coherence between the lasing levels. We then study the effect of inhomogeneous broadening of the atomic transition frequency. We find the interesting result that the noise reduction due to the atomic coherence is not destroyed but redistributed between the phase and amplitude of the electromagnetic field.

We discuss the influence of atomic memory effects on the field fluctuations of a laser in a general context. We derive a Fokker-Planck equation for the field that takes into account the long lifetime of the lasing atoms. Using this equation, we discuss two consequences of atomic memory. First, we find that even in the presence of saturation, memory effects can lead to a reduction of spontaneous-emission noise for short measurement times. Second, we quite generally show that atomic memory effects lead to time-dependent diffusion coefficients.

We calculate the spectrum of second-harmonic generation (SHG) for multimode input fields. We show that SHG cannot be described as the degenerate limit of sum-frequency generation (SFG) for multimode fields, because the dynamical equations describing SFG do not properly account for this degeneracy. We consider SHG for amplitude-modulated as well as frequency-modulated fundamental fields. The bandwidth of the second harmonic generated from an amplitude-modulated fundamental field depends on the fundamental input intensity and the conversion strength of the nonlinear medium. Additional frequencies that are not contained in the set of frequencies ${\mathrm{\omega }}_{1\mathit{i}}$+${\mathrm{\omega }}_{1\mathit{j}}$ will be created at intermediate conversion strengths. However, the growth of the spectral bandwidth is much less pronounced than in SFG. For very high conversion, the spectrum of the second harmonic narrows with increased conversion strength. For frequency-modulated input fields, the second-harmonic spectrum is independent of both the fundamental input intensity and the conversion strength.