T. Andersen

Lifetimes of some excited states in the rare earths: La ii, Ce ii, Pr ii, Nd ii, Sm ii, Yb i, Yb ii, and Lu ii

Radiative lifetimes for excited states in La ii, Ce ii, Pr ii, Nd ii, Sm ii, Yb i, Yb ii, and Lu ii have been determined by means of the beam-foil technique or the zero-field level-crossing method. The lifetimes for La, Ce, Pr, Nd, Sm, and Yb are shorter than those computed by summing the transition probabilities of Corliss and Bozman by a factor of up to ~5. The large discrepancies between the abundance of La, Ce, Pr, Nd, and Sm in the solar photosphere and in meteorites are eliminated or greatly reduced if the abundance determinations of the solar photosphere are based upon the gf values of Corliss and Bozman, corrected for by the present lifetimes.

Beam-Foil Spectroscopy at Low Initial Ion Energies. Radiative lifetimes measurements for Na, Ne, Ar, Kr and Xe

The possibility of determining atomic mean-lives of excited levels in heavier atoms by the beam-foil technique using low initial ion energies was investigated with emphasis on correct energy-loss determinations. Cascade corrections are facilitated at low energies, especially for excited states with long lifetimes. The mean-life of the 3p2P01 level in Na I was determined at 50 keV to be 16.2?0.5 ns. A number of mean-lives of excited levels in neutral and ionized neon, argon, krypton, and xenon were measured at ion energies ranging from 50 to 300 keV. The present xenon mean-lives are considerably shorter than reported in previous beam-foil studies.

Measurement of the Absolute Photoionization Cross Section of C+ near Threshold

The absolute photoionization cross section of the ground-state C+ ion has been measured from the ionization threshold at 24 eV to 31 eV using a merged ion-photon beam setup with synchrotron radiation from an undulator. The experimental results have been compared with nonrelativistic theoretical data from the Iron Project and the agreement is generally good in the near-threshold region, but differences in the magnitude of the continuum cross section of 15%-35% at higher energies and several deviations in the resonance structure are observed. The present measurements are a part of an experimental program to test the extensive theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.

Absolute photoionization cross section of K+ ions from the 3p to the 3s threshold

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb ±10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p6np 2P (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing 2P resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.

Absolute, State-selective Measurements of the Photoionization Cross Sections of N+ and O+ Ions

The absolute single-photoionization cross sections of singly charged nitrogen and oxygen ions have been measured from 29-80 eV and 30-150 eV, respectively, by merging a synchrotron-radiation beam from an undulator with a 2 keV ion beam. In the case of O+, separate data sets were obtained for the 4S ground state and the 2D/2P metastable states by attenuating the target-ion beam with N2 gas, exploiting the fact that cross sections for charge exchange between O+ ions, in the 4S or in the 2D/2P states, and N2 are different. The spectral structures were very different for the ground and the metastable states, whereas the cross sections for photoionization into the continua were nearly identical. The agreement between the experimental data and data calculated using the model function by Verner et al. or obtained from R-matrix calculations performed within the Opacity and the Iron projects is rather good. Finally, double photoionization (direct) was observed for O+ ions, with an approximately linear onset at 90 eV and a maximum cross section of 0.04 Mb.

Radiative lifetimes in Zn(II), Cd(II) and Hg(II) measured by fast-beam zero-field level-crossing technique

Abstract The zero-field level-crossing technique has been used to determine radiative lifetimes of excited states in singly ionized zinc, cadmium, and mercury. The excited levels in Zn(II), Cd(II) and Hg(II) were populated by collisions between fast ions and helium gas. Particular attention has been paid to the systematic errors occurring in radiative lifetimes by this technique. The results are compared with the lifetimes obtained by beam-foil, phase-shift, delayed-coincidence techniques and theoretical calculations.

Absolute cross sections for the photoionization of 4d electrons in Xe+ and Xe2+ ions

The partial and total absolute cross sections for the photoionization of Xe+ and Xe2+ ions, respectively, have been measured in the 4d excitation region (50-130 eV). The experimental cross sections are compared with calculated values obtained by the random phase approximation method taking exchange and rearrangement into account. The maximum total cross section values are 27(3) Mb for both ions, a value also reported for the photoionization of the Xe atom. The reliability of the experimental cross sections has been tested by measuring the absolute photoionization cross section for the He+ ion.

Doubly Excited States in Be III

The triplet spectrum of doubly excited Be III has been studied in the wavelength region of 75-5000 A in order to test the validity of the theoretical term values reported by Lipsky et al. The beam-foil excitation technique was applied to effectively populate the doubly excited states. The identified lower-lying, doubly excited states 2p2 3P, 2pnp 3P, or 3D, and 2pnd 3P, or 3D (n = 3, 4) show that the theoretical term values should be slightly modified.

Structure and dynamics of the negative alkaline-earth ions

Recent progress in experimental and computational methods for the study of negative atomic ions has greatly improved the knowledge about the structural and dynamic properties of the negative alkaline-earth ions. This review deals in particular with the stable ground states in , , and and with the metastable quartet states (, , and ) observed in , , , and . Future perspectives are discussed.

Lifetimes of Excited Levels in MgIII, CaIII, KIV–V, CaV, and Ge I

Radiative lifetimes of excited levels in Mg III, Ca III, K IV, K V, Ca V, and Ge I have been measured by means of the beam-foil technique in order to test the validity of recent theoretical methods to calculate lifetimes for heavier atoms and ions. Good agreement with theoretical lifetimes is obtained.