Extended analysis of intensity anomalies in the Al I isoelectronic sequence

Abstract

This paper reports an extended experimental investigation, using the beamfoil technique, of the intensity ratio of the fine-structure components in the 3s23p 2Pl,,, 3,2-3s3p2 2S,,, and 2Pl,, resonance transitions along the AI I isoelectronic sequence. These intensity ratios are strongly influenced by cancellation/enhancement effects arising from spin-orbit induced level mixings, and are thus sensitive probes of the detailed atomic structure. New data are given for Si 11, P 111, S IV, K VII, Sc IX, Fe XIV and Ni XVI.Together with previous experimental results this comprehensive isoelectronic study reveals significant and systematic deviations from theoretical predictions.

Critical Assessment of Theoretical Calculations of Atomic Structure and Transition Probabilities: An Experimenter’s View

“…The level mixing affects the relative line intensities of 2s-2p transitions. This effect has been systematically studied in beam-foil spectra [149,150]; the observations qualitatively confirm various computational predictions.…”

“…Al-like ions have been of interest to spectroscopy in many aspects, the line intensity anomalies resulting from level mixing are an example already mentioned above [150]. In the story of the identification of intercombination transitions in the EUV spectrum of the solar corona [195,196] from beam-foil spectra recorded in the accelerator basement, the 3s 2 3p 2 P o -3s3p 2 4 P lines have played a key role, because their wavelength and intensity pattern was so characteristic.…”

3s23p-3s3p2transitions in S iv

“…Engström et al (1989) undertook beam‐foil measurements, and reported that the experimental intensity ratio of these two lines differs significantly from the algebraic value of 2.0 which would be obtained by assuming LS coupling and by ignoring the difference between the wavelengths of these two transitions. In a more recent extended analysis, Engström et al (1995) gave a revised value of 1.12±0.1 for this ratio. Previous calculations, using the multiconfigurational Dirac–Fock (MCDF) method (Huang 1986) or using the Relativistic Parametric Potential method (RELAC) (Farrag, Luc‐Koenig & Sinzelle 1982) obtain a value of 1.38 for this ratio.…”

EUV Beam-Foil Spectra of Titanium, Iron, Nickel, and Copper

“…(Numerous data sets have been lost due to the mishaps of data storage media and computer system changes over several decades.) The recordings were made at the Bochum tandem accelerator laboratory about 30 years ago starting out with observations of electric dipole (E1) transitions, decay curves of regular and displaced terms, and various isoelectronic studies of specific configurations (for examples of such work on elements of present emphasis, see [45][46][47][48][49][50][51][52][53][54]). From this work, a quest evolved for finding spin-changing intercombination lines in iron group ions with an open n = 3 valence shell, and for measuring their transition rates (see, for example, [55][56][57][58][59][60]).…”

“…Given the limitations of atomic structure computations at the time, it is no surprise that much later computations of Al-like ions (see [73]) find a fair number of inconsistencies between the various computations and experimental line identifications, including some based on Bochum beam-foil work [74,75]. Nevertheless, beam-foil work on Al-like ions [49] verified assumptions on the variation of line intensity ratios in the n = 3 shell for a sequence of elements in the Al I isoelectronic sequence, and the unique properties of the beam-foil light source in combination with advanced computations of Si-like ions [37,38] helped to pin down inconsistencies in earlier term analysis, a problem that popped up with the availability of improved coronal spectra and will be explained in the section on Fe. An atomic structure problem closely related to the two-electron systems is the one-electron-one-vacancy system of singly excited Ne-like ions.…”