Photoionization of tungsten ions: experiment and theory for W4+^{4+}

Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion W$^{4+}$. Absolute cross sections have been measured employing the photon-ion merged-beams setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans were performed at 200~meV bandwidth in the 40 -- 105~eV range. Theoretical results have been obtained from a Dirac-Coulomb R-matrix approach employing basis sets of 730 levels for the photoionization of W$^{4+}$. Calculations were carried out for the $4f^{14}5s^2 5p^6 5d^2 \; {^3}{\rm F}_{J}$, $J$=2, ground level and the associated fine-structure levels with $J$=3 and 4 for the W$^{4+}$ ions. In addition, cross sections have been calculated for the metastable levels $4f^{14}5s^2 5p^6 5d^2 \; {^3}{\rm P}_{0,1,2},{^1}{\rm D}_{2},{^1}{\rm G}_{4},{^1}{\rm S}_{0}$. Very satisfying agreement of theory and experiment is found for the photoionization cross section of W$^{4+}$ which is remarkable given the complexity of the electronic structure of tungsten ions in low charge states.Comment: 15 pages, 3 figures, to appear in the Journal of Physics B: Atomic, Molecular and Optical Physic

Near--K-edge double and triple detachment of the F- negative ion: observation of direct two-electron ejection by a single photon

Double and triple detachment of the F-(1s2 2s2 2p6) negative ion by a single photon have been investigated in the photon energy range 660 to 1000 eV. The experimental data provide unambiguous evidence for the dominant role of direct photo-double-detachment with a subsequent single-Auger process in the reaction channel leading to F2+ product ions. Absolute cross sections were determined for the direct removal of a (1s+2p) pair of electrons from F- by the absorption of a single photon

Single-photon single ionization of W+^{+} ions: experiment and theory

Experimental and theoretical results are reported for photoionization of Ta-like (W$^{+}$) tungsten ions. Absolute cross sections were measured in the energy range 16 to 245 eV employing the photon-ion merged-beam setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans at 100 meV bandwidth were performed in the 16 to 108 eV range. In addition, the cross section was scanned at 50 meV resolution in regions where fine resonance structures could be observed. Theoretical results were obtained from a Dirac-Coulomb R-matrix approach. Photoionization cross section calculations were performed for singly ionized atomic tungsten ions in their $5s^2 5p^6 5d^4({^5}D)6s \; {^6}{\rm D}_{J}$, $J$=1/2, ground level and the associated excited metastable levels with $J$=3/2, 5/2, 7/2 and 9/2. Since the ion beams used in the experiments must be expected to contain long-lived excited states also from excited configurations, additional cross-section calculations were performed for the second-lowest term, 5d^5 \; ^6{\rm S}_{J}, $J$=5/2, and for the $^4$F term, 5d^3 6s^2 \; ^4{\rm F}_{J}, with $J$ = 3/2, 5/2, 7/2 and 9/2. Given the complexity of the electronic structure of W$^+$ the calculations reproduce the main features of the experimental cross section quite well.Comment: 23 pages, 7 figures, 1 table: Accepted for publication in J. Phys. B: At. Mol. & Opt. Phy

Photoionization of tungsten ions: Experiment and theory for W5+

Experimental and theoretical cross sections are reported for single-photon single ionization of W5+ ions. Absolute measurements were conducted employing the photon-ion merged-beams technique. Detailed photon-energy scans were performed at (67 ± 10) meV resolution in the 20-160 eV range. In contrast to photoionization of tungsten ions in lower charge states, the cross section is dominated by narrow, densely-spaced resonances. Theoretical results were obtained from a Dirac-Coulomb R-matrix approach employing a basis set of 457 levels providing cross sections for photoionization of W5+ ions in the ground level as well as the and metastable excited levels. Considering the complexity of the electronic structure of tungsten ions in low charge states, the agreement between theory and experiment is satisfactory

Vicinal Surfaces, Fractional Statistics and Universality

We propose that the phases of all vicinal surfaces can be characterized by four fixed lines, in the renormalization group sense, in a three-dimensional space of coupling constants. The observed configurations of several Si surfaces are consistent with this picture. One of these fixed lines also describes one-dimensional quantum particles with fractional exclusion statistics. The featureless steps of a vicinal surface can therefore be thought of as a realization of fractional-statistics particles, possibly with additional short-range interactions.Comment: 6 pages, revtex, 3 eps figures. To appear in Physical Review Letters. Reference list properly arranged. Caption of Fig. 1 slightly reworded. Fig 3 (in color) is not part of the paper. It complements Fig.