[en] Electron-electron correlation influences the spectroscopy of atoms, when both electrons are simultaneously excited into states of high principal quantum numbers. As the number of available photoexcitation channels increases with increasing photon energy, search for a simple and intuitive understanding of the correlation process becomes tenaciously difficult. In this work, the systematics of the photoabsorption processes are sorted out and the main photoabsorption channels are identified. Using the method of hyperspherical coordinates, selection rules for the excitation of doubly-excited states are presented. Simple two-electron formulas give accurately the positions of the doubly-excited states to any high quantum number

[en] The scattering of unpolarized electrons from hydrogen atoms is treated using the hyperspherical coordinate and the multichannel quantum defect approach with particular emphasis on the formation of quasi-stable resonant e^- -H complexes. Resonant positions and widths, formed below the H(n = 2), H (n = 3), and H(n = 4) thresholds, are calculated for symmetries ^1,3S^e, ³P^o, and ¹D^e. Resonant cross sections are also given for the singlet and triplet L = 0 partial waves. Good agreement is found with other more computationally intensive calculations. (author)

[en] Ever since the classic work of Wannier in 1953, the process of treating two threshold electrons in the continuum of a positively charged ion has been an active field of study. The authors have developed a treatment motivated by the physics below the double ionization threshold. By modeling the double ionization as a series of Landau-Zener transitions, they obtain an analytical formulation of the absolute threshold probability which has a leading power law behavior, akin to Wannier's law. Some of the noteworthy aspects of this derivation are that the derivation can be conveniently continued below threshold giving rise to a open-quotes cuspclose quotes at threshold, and that on both sides of the threshold, absolute values of the cross sections are obtained

[en] A simple and accurate procedure for calculating the rate of double ionization of open-quotes two-electronclose quotes systems by X-ray photons is presented. Arguments are given to support the validity of the method used. In particular, the authors show that the many-body perturbation theory diagrams depend asymptotically on the choice of the gauge for the electric dipole operator. The ratio of double-to single-ionization is calculated to be 1.68% in agreement with the recent synchrotron measurements. For H^- and Li⁺, they predict ratios of 1.51% and 0.89%, respectively

[en] The method of hyperspherical coordinates is used in obtaining very highly excited two-electron adiabatic potential curves up to the n=12 hydrogenic threshold. The present numerical effort exploits the analytic nature of the solutions to the Schroedinger equation at small and large distances to diagonalize the hyperspherical Hamiltonian in the combined basis set. Numerical deficiencies resulting from a linear dependency of the total nonorthogonal basis set are overcome in an automatic fashion. A high degree of diabaticity is observed which limits the channel interaction to within a select set of hyperspherical channels. The validity of this quasiconstant of motion as reported by H. R. Sadeghpour and C. H. Greene (SG) [Phys. Rev. Lett. 65, 313 (1990)] for H^- is investigated for the photoabsorption of He and higher-z members of the He isoelectronic sequence. A generalized two-electron Rydberg formula capable of predicting the positions of the dominant He (ridge and valley) resonances is given. A new feature of the very-highly-excited resonance structure for H^- and He [observed only recently by Domke et al., Phys. Rev. Lett. 66, 1306 (1991)] is discussed. Semiempirical multichannel-quantum-defect fits are also made to the photodetachment spectra of H^-, supporting the conclusions reached in SG

[en] The knowledge of the long-range interaction between atoms and molecules is of fundamental importance for low-energy and low-temperature collisions. The electrostatic interaction between the charge distributions of two ground-state alkali-metal atoms can be expanded in powers of 1/R, the internuclear distance. The coefficients corresponding to the expansion powers of R^-6, R^-8, and R^-10 are calculated exactly using a numerical technique originally developed for determining resonance profile near an absorption line. These coefficients are evaluated by integrating the dynamic electric multipole polarizabilities at imaginary frequencies which are in turn obtained by solving two coupled inhomogeneous differential equations. Highly accurate one-electron model potentials are developed to represent the motion of the valence electron in the field of the closed alkali positive ion core. The numerical results for the constants C₆, C₈, and C₁₀ for the homo- and hetero-nuclear alkai diatoms are compared with other calculations

[en] An inhomogeneous differential equation method is used to calculate the Rayleigh and Raman scattering cross sections for atomic hydrogen and atomic caesium as functions of wavelength below and above the ionization limits. The profile of the scattering cross section near the 6s-6p resonance of atomic caesium is determined by including an energy-independent radiative width in the scattering formula. The effect of replacing the electric dipole operator by a modified form that takes account of the polarization of the core by the valence electron is explored. (author)

[en] We present results of an eigenchannel R-matrix calculation of the photoabsorption spectrum of He below the He⁺ (n = 3) threshold. The observed _n{υ}_m^A =₃ {1}₄⁺ resonance is found to decay with an 84.4% branching ratio to the ₂{1}⁰ ('2pεd') channel, in violation of a radial correlation propensity rule, ΔA = 0. The Δυ 0 propensity rule is satisfied, however, indicating that angular correlation is preserved in the decay of this resonance. (author)

[en] The existence of a fundamental limit to the efficiency of spin-exchange optical pumping of ³He nuclei by collisions with spin-polarized alkali-metal atoms is established. Using accurate ab initio calculations of molecular interactions and scattering properties, requiring no adjustable parameters, it is demonstrated that attainable polarization of ³He nuclei by spin-exchange collisions with K atoms is limited by the anisotropic hyperfine interaction. The theory is specifically applied to the spin-exchange between potassium and ³He. In a complementary calculation, it is suggested that it may be possible to overcome this limit by using atomic silver (Ag) as a collision partner in spin-exchange optical pumping experiments.

[en] We consider the transfer of a two-species Bose-Einstein condensate (BEC) into an optical lattice with a density such that a Mott-insulator state with one atom per species per lattice site is obtained in a deep lattice regime. Depending on collisional and transition parameters, the result could be either a 'mixed' or a 'separated' Mott-insulator phase. Such a mixed two-species insulator would be well suited for the formation of dipolar molecules via photoassociation. The resulting array of dipolar molecules could then be used for the formation of a dipolar molecular condensate or for computation. For the case of a ⁸⁷Rb-⁴¹K two-species BEC, however, the large interspecies scattering length makes it difficult to obtain the desired mixed Mott-insulator phase. To overcome this difficulty, we investigate the effect of varying the lattice frequency on the mean-field interaction and propose a favorable parameter regime, under which a lattice of dipolar molecules could be generated