Photodetachment study of He^- quartet resonances below the He(n=3) thresholds

The photodetachment cross section of He^- has been measured in the photon energy range 2.9 eV to 3.3 eV in order to investigate doubly excited states. Measurements were made channel specific by selectively detecting the residual He atoms left in a particular excited state following detachment. Three Feshbach resonances were found in the He(1s2p ^3P)+e^-(epsilon p) partial cross section: a ^4S resonance below the He(1s3s ^3S) threshold and two ^4P resonances below the He(1s3p ^3P) threshold. The measured energies of these doubly excited states are 2.959260(6) eV, 3.072(7) eV and 3.26487(4) eV. The corresponding widths are found to be 0.20(2) meV, 50(5) meV and 0.61(5) meV. The measured energies agree well with recent theoretical predictions for the 1s3s4s ^4S, 1s3p^2 ^4P and 1s3p4p ^4P states, respectively, but the widths deviate noticeably from calculations for 1s3p^2 ^4P and 1s3p4p ^4P states.Comment: 10 pages, 3 figures, LaTeX2e scrartcl, amsmath. Accepted by Journal of Physics B; minor changes after referee repor

Photodetachment study of the 1s3s4s ^4S resonance in He^-

A Feshbach resonance associated with the 1s3s4s ^{4}S state of He^{-} has been observed in the He(1s2s ^{3}S) + e^- (\epsilon s) partial photodetachment cross section. The residual He(1s2s ^{3}S) atoms were resonantly ionized and the resulting He^+ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy E_r = 2.959 255(7) eV and a width \Gamma = 0.19(3) meV, in agreement with a recent calculation.Comment: LaTeX article, 4 pages, 3 figures, 21 reference

Ionization Structure and Spectra of Iron in Gaseous Nebulae}

The emission spectra and the ionization structure of the low ionization stages of iron, Fe I--IV, in gaseous nebulae are studied. This work includes: (i) new atomic data: photoionization cross sections, total e-ion recombination rates, excitation collision strengths, and transition probabilities; (ii) detailed study of excitation mechanisms for the [Fe II], [Fe III], and [Fe IV] emission, and spectroscopic analysis of the observed IR, optical, and UV spectra; (iii) study of the physical structure and kinematics of the nebulae and their ionization fronts. Spectral analysis of the well observed Orion nebula is carried out as a test case, using extensive collisional-radiative and photoionization models. It is shown that the [Fe II] emission from the Orion nebula is predominantly excited via electron collisions in high density partially ionized zones; radiative fluorescence is relatively less effective. Further evidence for high density zones is derived from the [O I] and [Ni II] spectral lines, as well as from the kinematic measurements of ionic species in the nebula. The ionization structure of iron in Orion is modeled using the newly calculated atomic data, showing some significant differences from previous models. The new model suggests a fully ionized H II region at densities on the order of $10^3$ cm$^{-3}$, and a dynamic partially ionized H II/H I region at densities of $10^5-10^7$ \cm3. Photoionization models also indicate that the optical [O I] and [Fe II] emission originates in high density partially ionized regions within ionization fronts. The gas phase iron abundance in Orion is estimated from observed spectra.Comment: AAS LaTex, 60 pages 18 figures. Astrophysical Journal. in pres

HYPERSPHERICAL ANALYSIS OF ELECTRONIC CORRELATIONS IN MULTIPLY-EXCITED STATES

Lorsque plusieurs électrons sont excités, les effets de corrélation deviennent notables. Nous présentons des progrès récents sur la compréhension des corrélations électroniques dans les états doublement et triplement excités d'atomes simples. La minimisation locale des énergies cinétique et potentielle détermine des modes propres de corrélation qui évoluent graduellement avec la taille du système. Cette image intuitive est mise en oeuvre naturellement par le formalisme hypersphérique qui sélectionne la taille globale du système multiélectronique comme unique variable R. Les modes propres de corrélation émergent comme états propres du hamiltonien à R fixe, et les valeurs propres correspondantes définissent des potentiels effectifs régissant l'expansion du système. Cette analyse hypersphérique des corrélations est illustrée sur des systèmes simples H-, He, He-.When a few electrons are excited, correlation effects become conspicuous. We present recent progresses on the understanding of electronic correlations for doubly - and triply-excited states of simple atoms. The local minimization of kinetic and potential energies determine correlation eigenmodes which evolve smoothly with the size of the system. This intuitive picture is naturally implemented by the hyperspherical formalism which selects the global size of the many-electron system as the unique radial variable R. Correlation eigenmodes emerge as the eigenstates of the fixed-R hamiltonian and the corresponding eigenvalues define effective potentials governing the system's expansion The hyperspherical analysis of correlation is illustrated on simple systems H-, He, and He-