K-shell x-ray production cross sections are reported for ${}_4{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ ions incident upon thin ${}_9{}^{}{}_{}{}^{}\mathrm{F}$, ${}_{11}{}^{}{}_{}{}^{}\mathrm{Na}$, ${}_{13}{}^{}{}_{}{}^{}\mathrm{Al}$, ${}_{14}{}^{}{}_{}{}^{}\mathrm{Si}$, ${}_{15}{}^{}{}_{}{}^{}\mathrm{P}$, ${}_{17}{}^{}{}_{}{}^{}\mathrm{Cl}$, and ${}_{19}{}^{}{}_{}{}^{}\mathrm{K}$ targets. Incident-beam energies range from 0.5 to 2.5 MeV. It is found that the first Born approximation (plane-wave Born approximation plus the Oppenheimer-Brinkman-Kramers treatment by Nikolaev) greatly overpredicts the data, while the predictions of the perturbed-stationary-state theory with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) are generally in good agreement with the data. There is a low-velocity discrepancy between the data and the ECPSSR predictions which may be due to multiple ionization effects on the fluorescence yields used to convert total ionization to x-ray production cross sections.

A general formalism to treat radiative emission of a colliding three-particle system is developed in the semiclassical approximation. The matrix element of the matter-radiation interaction is separated into three terms representing the center of mass, the intersystem bremsstrahlung, and the electron bremsstrahlung. At each step we impose the off-shell orthogonality property of the mechanical wave functions. Instead of internuclear bremsstrahlung, used so far, a more general concept of intersystem bremsstrahlung is introduced. The electron bremsstrahlung is calculated using eikonal distorted wave functions, and good agreement is found with experiment. Radiative electron excitation and deexcitation have also been calculated. The former is found to be very small, while the spectrum associated with the latter produces the continuous emission around the characteristic line, induced by the passing projectile.

We introduce in the context of a simple soluble model for laser-enhanced autoionization a new representation—the dressed-resonance representation (DRR). The model includes one autoionizing state, one flat electron continuum, one continuous driving laser of arbitrary strength, and two types of relaxations, i.e., spontaneous decay of the continuum and autoionizing state back to the initial state (spontaneous recycling) and phase-jitter noise in the driving laser. The DRR is a natural extension of the dressed-states representation, employed in strong-field discrete-discrete transitions, to discrete-continuum transitions. We focus on two observables, i.e., the temporal initial state population and the photoelectron spectrum. The emphasis is on strong-field effects. The DRR yields simple, transparent, analytical expressions for the above observables in the no-relaxations limit of the model. These results are complemented by a systematic analysis of the exact solution in all qualitatively distinct physical regimes.

Resonant photoelectron spectra of atomic Eu have been measured in the photon-energy ranges of the ‘‘4d→4f ’’ (110–170 eV) and 3d→4f (1120–1165 eV) excitations. For the ‘‘4d→4f ’’ giant resonance, the decay process results mostly in ${\mathrm{Eu}}^{+}$ main-line configurations, including some fractional enhancement directly into the 4d continuum. The 3d→4f excitations, however, are characterized by competition between the main-line decay channels and channels corresponding to highly excited (satellite) ionic states. We find that decay after 4d excitation leads predominantly to the 4f continuum, whereas several channels participate in the decay following 3d excitation. The observed decay characteristics are discussed with regard to orbital-collapse phenomena.

We describe the application of the collective vector method to the evaluation of the moments of atomic transition arrays. We use these moments with the Lanczos algorithm to obtain a Stieltjes δ-function representation of the array. As an example the procedure is applied to a test array containing over 5000 lines in the exact spectrum and is seen to give a good representation of the array with only moderate computational requirements. In comparison the more familiar Gram-Charlier expansion leads to uninterpretable negative excursions making it unusable except for completely unresolved transition arrays.

A theoretical study of the two-photon emission rate from the 1s2s ${\mathrm{}}^1$S state of two-electron ions is presented. High-precision values of the nonrelativistic emission rate for ions with nuclear charge Z up to 36 are obtained by use of correlated variational wave functions of the Hylleraas type. Summations over intermediate states are performed by finite-basis-set methods. The results are used to obtain an accurate extrapolation formula and to study the convergence characteristics of a 1/Z expansion for the decay rate. The leading hydrogenic term is calculated to an accuracy which substantially exceeds Klarsfeld’s, and the next term is obtained for what is believed to be the first time. Relativistic effects are taken into account by means of a screened hydrogenic approximation. The predicted decay rate for two-electron ${\mathrm{Kr}}^{34+}$ is (2.993±0.012)×${10}^{10}$ ${\mathrm{s}}^{\mathrm{-}1}$, where the error is the uncertainty in the relativistic correction. This lies significantly higher than the value (2.934±0.030)×${10}^{10}$ ${\mathrm{s}}^{\mathrm{-}1}$ recently measured by Marrus et al.

Molecular Rydberg states with l>2 exhibit essentially no core penetration, and so interact with the molecular ion core principally through its polarizability and multipole moments. If the core is sufficiently excited vibrationally or rotationally, these interactions can give rise to autoionization. The ionization rates are readily calculable using methods developed previously to calculate the energy-level structure of these states. Detailed calculations are described for the Rydberg nf states of ${\mathrm{H}}_2$. Since the structure of nonpenetrating Rydberg states varies only slightly from molecule to molecule, many of these results are readily generalized. Several significant conclusions can be drawn. The first is that autoionization usually dominates radiative decay when it is energetically possible, even for states with rather high l. Second, rotational autoionization is typically more rapid than vibrational autoionization even in cases where both can occur. However, the rates vary tremendously with the total angular momentum N. Finally, the effects of quantum interferences affect the rates significantly, so it is important to perform a detailed calculation using accurate wave functions. Preliminary results of an experiment on the autoionization of the nf states of the ${\mathrm{H}}_2$ molecule are presented and compared with ab initio calculations. The good agreement suggests that this simple model suffices to account for the principal decay processes of these states.

By high-resolution double-resonance spectroscopy experiments in the millimeter-wavelength domain, we have determined precise values of the quantum defects in the s and p Rydberg levels of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ (principal quantum number n ranging from 18 to 40). A detailed comparison of the ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ data has yielded the value of the specific isotopic shift in the np series, which is the first evidence to our knowledge of an exchange effect between a tightly bound core electron and a very weakly bound Rydberg electron in an alkali-metal atom.

We have measured coincidences between Ne K Auger electrons emitted after impact excitation by hydrogenlike ${\mathrm{F}}^{8+}$ and bare ${\mathrm{F}}^{9+}$ projectiles at 0.53 and 0.23 MeV/u collision energies and the recoiling Ne target ion in a well-defined charge state. The quasiresonant K-K charge-tgas-atom pairs to calculate adiabatic potentials for some low-lying states. Using the calculated potentials, broadening and shift rates of resonance lines of ${\mathrm{Sr}}^{+}$, ${\mathrm{Ca}}^{+}$, and ${\mathrm{Mg}}^{+}$ perturbed by Ar and He have been calculated and compared with experiments. The overall agreement is good.

The psuedopotential model of Baylis has been applied to alkaline-earth-metal (singly ionized)–rare-gas-atom pairs to calculate adiabatic potentials for some low-lying states. Using the calculated potentials, broadening and shift rates of resonance lines of ${\mathrm{Sr}}^{+}$, ${\mathrm{Ca}}^{+}$, and ${\mathrm{Mg}}^{+}$ perturbed by Ar and He have been calculated and compared with experiments. The overall agreement is good.

We have theoretically studied the electronic autoionization between 17 and 18.3 eV of the Rydberg series converging to the B ${\mathrm{}}^2$${\Sigma }^{+}$ state of ${\mathrm{CO}}^{+}$ ($R_B$ series). We have obtained the partial and vibrationally resolved differential and total photoionization cross sections by combining ab initio electronic quantities with a variant of the two-step multichannel quantum-defect theory (MQDT). The formalism of the Feshbach projection operators is used to separate the electronic quantities needed in the first and second steps of the MQDT. We have introduced the vibrational motion within the Condon approximation. Our results lead to new unambiguous assignments of the $R_B$ series. The vibrationally resolved cross sections, the associated branching ratios, and the background and resonant features are discussed in detail and compared with the available experimental data. There is an overall agreement between theory and experiment except for the angular distribution of photoelec- trons.

The induced dipole moment of hydrogen- (${\mathrm{H}}_2$) argon pairs is obtained with an estimated accuracy of about 2% by treating the collisional complex like a molecule in self-consistent-field (SCF) and coupled-electron-pair calculations. The basis-set superposition error is effectively avoided by the use of nonorthogonal local orbital sets. Based on the four-term, ab initio, orientation-dependent dipole function and a reliable isotropic potential model, the rototranslational absorption spectra of free and bound ${\mathrm{H}}_2$-Ar pairs are computed from an exact quantum formalism. Both the spectral profile and absolute intensity of the ab initio spectra are shown to be in agreement with Dore and Birnbaum’s recent measurement. The work demonstrates once more what was seen previously for the He-Ar and ${\mathrm{H}}_2$-He far-infrared absorption spectra, namely that the precision attainable in such ab initio computations matches or exceeds that of the best available measurements of the same spectra [Meyer and Frommhold, Phys. Rev. A 33, 3807 (1986), and Phys. Rev. A 34, 2771 (1986).

Transitions in the wavelength region 4.9–8.1 Å from Br , , , , and have been identified in spectra from laser-produced plasmas. The identifications were made using ab initio calculations of wavelengths and oscillator strengths. Spatially resolved spectra were recorded for laser irradiation intensities from 3×${10}^{13}$ to 4×${10}^{14}$ W/${\mathrm{cm}}^2$. The dependence of the spectral lines on the distance from the target and on the irradiation intensity was very useful in distinguishing the transitions from different ionization stages.

The avoided crossings in an electric field of K s and p states with the nearly hydrogenic Stark manifold states have been investigated using 300-K blackbody radiation. Anticrossings between states of the same ‖$m_j$‖ for which the ‖$m_l$‖ values are the same or differ by one are encountered. The anticrossings between states of the same ‖$m_l$‖ are quite wide as they are due to core penetration and polarization effects. The anticrossings for which ‖$m_l$‖ differs by one are produced by the spin-orbit interaction and are accordingly quite small. We present results for the ns states for n in the range 18–29 and for the np states for n in the range 19–23.

We present our study on the feasibility and limitations of a purely one-laser trap concept, a ‘‘corner cube trap,’’ for trapping neutral K atoms. The confinement of the atoms in the two dimensions perpendicular to the laser will be provided in the cavity of a high-power alexandrite laser operating in the cw ${\mathrm{TEM}}_{01}^{\mathrm{*}}$ mode (‘‘doughnut mode’’) tuned slightly to the blue side of the resonance line of the K atom. By reflecting the ${\mathrm{TEM}}_{01}^{\mathrm{*}}$ laser back on itself with two mirrors, one ‘‘caps’’ the ends of the cylindrical trap, resulting in a slightly weaker end plug. This trap concept employs not laser cooling, but rather counterstreaming ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms which are cooled to ≲1.5 K, to drastically cool the K atoms to thermal energies well below the trap depth (expected to be ∼10 K). We have also examined various loss mechanisms for the trapped atoms. In particular, K atoms can be lost to the trap if they are multiphoton ionized, if they are heated by absorption and emission of many photons (‘‘recoil’’ or ‘‘diffusional’’ heating), if they simply have much higher energy than the vast majority of other atoms at 1.5 K, or if they form KHe (or ${\mathrm{KHe}}_2$, etc.). Results from these investigations suggest crude lifetimes for trapped atoms of the order of 1 sec.

A broadband multimode dye laser, detuned from the λ=570.68 nm resonance line of Sm atoms, was used to provide three incident beams in the standard phase-conjugate four-wave-mixing geometry. The phase-conjugate output intensity was measured as a function of polarization and external magnetic field configurations, and of the pressure of several buffer gases. We have confirmed the theoretical prediction that correlated stochastic fluctuations in the incident light field make a contribution to the four-wave-mixing signal. Collisional cross sections for reorientation among the m sublevels in the J=1 ground state of Sm have been obtained for He, Ar, and Xe. Differences in polarization characteristics between the collision-induced Hanle resonances in Na and Sm are pointed out.

Two-photon ionization of hydrogenic atoms with principal quantum number n=8 has been studied experimentally over the wavelength range 10.45–10.59 μm. The ionization was resonantly enhanced by the n=12 intermediate state, with a large n=8 to n=12 coupling responsible for a fractional resonance width of about 0.1%. On resonance, ionization rates of order 10 MHz were achieved with nominal 2-mJ, 40-nsec laser pulses, with 40 mJ being required at wavelengths 1% away from resonance. The data are interpreted in terms of the extended two-level model for multiphoton ionization. The peak laser electric field strength reached values where the top of the instantaneous potential energy barrier was near the intermediate-state energy value.

Photoabsorption spectra of tetrachlorosilane, tetramethylsilane, and tetrafluorosilane have been measured using synchrotron radiation in the 1830–1900-eV photon-energy range, i.e., in the vicinity of the silicon 1s edge. Intense resonances are observed in the discrete and the continuum part of the spectrum, and compared with previously published photoabsorption spectra obtained near the silicon 2s and 2p edges. Resonances in the discrete part of the spectrum are qualitatively interpreted using structural and electronic properties of the core-equivalent P$X_4$ (X=F, ${\mathrm{CH}}_3$, or Cl) phosphoranyl radicals. The interpretation of features in the continuum are discussed in terms of shape resonances, within the tetrahedral geometry.

We present calculations of two- and three-photon ionization of Xe in the energy region between and slightly above the two ionization thresholds. The resonance structure between the thresholds is treated by multichannel quantum-defect theory and the summation over intermediate states by truncating the sum. Predictions for resonance profiles and photoelectron angular distributions are also presented.

A semiclassical calculation of density-broadening and line-shift coefficients in the vibrational-rotational spectrum of molecular hydrogen is presented. These parameters are obtained as functions of temperature and upper-state quantum numbers, and the calculated results are in good agreement with experimental values obtained from fundamental and overtone electric quadrupole spectra, except for the 4-0 overtone density-broadening coefficient. Results from Raman and electric-field-induced spectra in the fundamental band are also discussed. The calculation includes construction of an intermolecular potential for the ${\mathrm{H}}_2$-${\mathrm{H}}_2$ interaction. The vibrational phase shift resulting from collision is shown to account for the vibrational state dependence of the broadening and shift coefficients, and the observed temperature dependence of the line shift is reproduced by the calculation.