R. E. Irving

Lifetime Measurements in Sn II

Lifetime measurements are reported for levels arising from the 5s25d and 5s24f configurations in Sn II. Measured decay curves were jointly analyzed using the Arbitrarily Normalized Decay Curve (ANDC) method to remove the effects of cascade repopulation from the determination of the lifetimes of the 5s25d 2D3/2 level. The branching ratio of the decay of this level to the ground term fine-structure levels 2P1/2 and 2P3/2 was carefully measured, and we have obtained an accurate value for the absorption oscillator strength of the resonance transition to this level at 1400.52 A. The results are discussed in the context of interpreting vacuum ultraviolet absorption spectra observed with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope.

Lifetimes and Oscillator Strengths for the 6p7s3P0,1 Levels in Bi II

Lifetime measurements and empirical branching ratio estimates for the 6p7s 3P0 and 3P1 levels in Bi II are reported, together with supporting theoretical calculations. These results are combined to yield absorption oscillator strengths between these levels and those of the 6p2 ground configuration. The results are discussed in the context of spectroscopic data obtained from orbiting telescopes.

Determination of polarizabilities and lifetimes for the Mg, Zn, Cd and Hg isoelectronic sequences

The dipole polarizability and adiabatic correlations of atomic ions are related to the energy levels and oscillator strengths of the levels through bracketing inequalities. For systems with a ground state ns 2 1 S0, the total oscillator strength is dominated by the unbranched intrashell ns 2 1 S0‐nsnp 1 P o 1 transition, and the remaining oscillator strength can be narrowly bracketed using the f -sum rule. Thus measurements of the lifetime of the lowest resonance transition can be used to specify the polarizabilities and, alternatively, measurements of the polarizabilities can be used to deduce lifetimes. Moreover, isoelectronic regularities in line strengths can be used to obtain a comprehensive database from a small number of precision lifetime determinations. These methods are applied homologously to produce values for polarizabilities and lifetimes for the Mg, Zn, Cd and Hg isoelectronic sequences. To confirm the reliability of these methods, fast ion beam lifetime measurements in P IV were performed using the arbitrarily normalized decay curve (ANDC) method, and a test of the f -sum was made for Ba II, for which precision measurements of both the polarizability and lifetimes exist.

Oscillator strengths for ultraviolet transitions in Cl II and Cl III

Oscillator strengths for transitions in Cl II and III are derived from lifetimes and branching fractions measured with beam-foil techniques. The focus is on the multiplets at 1071 A in Cl II and 1011 A in Cl III whose lines are seen in spectra acquired with the Far Ultraviolet Spectroscopic Explorer. These data represent the first complete set of experimental f-values for the lines in the multiplets. Our results for Cl II λ1071 agree very well with the most recent theoretical effort and with Mortons newest recommendations. For Cl III, however, our f-values are significantly larger than those given by Morton; instead, they are more consistent with recent large-scale theoretical calculations. Extensive tests provide confirmation that LS coupling rules apply to the transitions for these multiplets.

OSCILLATOR STRENGTHS FOR ULTRAVIOLET TRANSITIONS IN P ii

We report lifetimes, branching fractions, and the resulting oscillator strengths for transitions within the P II multiplet (3s23p2 3P-3s23p4s 3Po) at 1154 A. These beam-foil measurements represent the most comprehensive and precise set currently available experimentally. Comparison with earlier experimental and theoretical results is very good. Since Mortons most recent compilation is based on the earlier body of results, phosphorus abundances for interstellar material in our Galaxy and beyond derived from λ1154 do not require any revision.

Lifetimes of the 5d 9 6p levels in Au ii

We report experimental lifetimes for 11 of the 12 5d96p levels of Au ii, determined by means of the beam-foil excitation method. The data are compared with theoretical calculations based on the Cowan group of programs. These first studies of Au ii lifetimes and transition probabilities were motivated largely by the need for such data in the analysis of high-resolution stellar spectroscopy done with the Goddard High-Resolution Spectrograph on the Hubble Space Telescope.

The need for branching fraction measurements in multiply-charged ions

Much progress has been made in the measurement of oscillator strengths in neutral and singly-ionized atoms, providing a database for use in important applications. However, for multiply-charged ions, measured data for oscillator strengths are almost exclusively limited to low-lying unbranched transitions. Although extensive measurements of ionic lifetimes exist, the lack of branching fraction measurements in multiply-charged ions prevents these data from being converted to oscillator strengths. A significant factor leading to this deficiency involves the lack of adequate line intensity calibration standards in the vacuum ultraviolet spectral region 2000‐400 A. Here we review the interrelationships connecting these rate parameters, indicate some of the important applications for which they are needed, describe the experimental limitations that currently exist, and suggest possible methods for extending these measurements below 1000 A.

LIFETIMES OF THE 4D95P LEVELS IN CD III

Lifetimes are reported for the twelve 4d95p fine structure levels of Cd III, measured using beam-foil excitation. Isoelectronic trends are studied through comparisons with earlier measurements for Ag II (Irving R E et al 1995 Physica Scripta 51 351).

Oscillator strengths for ultraviolet transitions in P II and Cu II

Analysis of ultraviolet absorption from the dominant ions in interstellar clouds provides information on the mineralogy of the solid material in space and on the synthesis of elements in our Galaxy and beyond. For the most part the spectra are acquired with the spectrometers on the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. In order to convert the amount of absorption into an accurate abundance, determinations of oscillator strengths of sufficiently high quality are needed. We present our most recent beam-foil measurements in this area. Lifetimes, branching fractions, and oscillator strengths for all transitions within the P II multiplet at 1154 A are compared with available results. Close agreement between our laboratory measurements and theoretical and semi-empirical calculations suggests a means for calibrating fast-beam instruments in the far ultraviolet. As for Cu II, our results for the line at 1358 A provide further evidence for a short lifetime for the upper state of interest.

Lifetime measurements in N III using the beam-foil technique and cascade corrections

Lifetime measurements are reported for several levels in the 2s2p3p, 2s2p3d, 2s2p2, 2p3 configurations of boronlike N III. Beam-foil excitation methods were used with cascade repopulation analyzed by the correlated decay curve ANDC technique. A small but significant J-dependence in the lifetimes of the 3d 4F levels was observed. The experimental lifetimes for the levels in the 2s2p2 and 2p3 were found to be in good agreement with recent theoretical results.