Radiative lifetime measurements in Er II by time-resolved laser spectroscopy

Abstract

Radiative lifetime measurements were performed with time-resolved laser-induced fluorescence techniques for 24 levels of Nd II in the energy range 20 500-32 500 cm −1 . For 17 levels, no previous experimental data exist. These results have allowed the testing of new theoretical calculations with the relativistic Hartree-Fock method taking configuration interactions and core-polarization effects into account, and a satisfying agreement has been found for this complex ion. A new set of calculated oscillator strengths, accurate within a few per cent for the strongest transitions, is presented for 107 lines of astrophysical interest appearing in the wavelength range 358.0-1100.0 nm. These results will be useful to evaluate abundance values of neodymium in chemically peculiar stars in relation with cosmochronology.Key words: atomic data -atomic processes -stars: chemically peculiar. I N T RO D U C T I O NThe consideration of the r-process abundance patterns in some stars allows an age determination of the earliest galactic halo. In particular, the comparison of the abundances of the heavy elements (i.e. thorium and uranium) synthesized soon after the formation of our Galaxy with those of lighter stable neutron capture elements allows the estimation of the age of this halo. However, some singly ionized rare-earths, interesting for cosmochronometry (in relation with the third n-capture peak), have only poorly determined transition probabilities (Sneden, Lawler & Cowan 2002). As a consequence, narrowing the uncertainties in age estimates relies clearly on further improvements in r-process theory but also on high-resolution spectroscopy of very metal-poor stars itself depending upon accurate atomic data.The age of some very metal-poor stars (see e.g. Cayrel et al. 2001) can be estimated, by comparison of abundance ratios of radioactive and stable nuclear species, using different pairs of elements such as U/Th, U/Os and U/Ir. Nd II is one of the elements which can be used as a stable nuclear species in cosmochronology for estimating the age of the Galaxy (Lawler, Whaling & Grevesse 1990). The present study is mainly aimed at obtaining a new set of atomic data for this ion having in mind contribution to the cosmochronometry based on the pair U/Nd. E-mail: E.Biemont@ulg.ac.be In a more general context, accurate atomic data for lanthanide ions are required for refining the stellar elemental abundances and for more firmly establishing the odd-even effect observed in many stars: in fact the elements with an even atomic number appear generally more abundant than the odd elements. In some stars, however, this odd-even effect does not appear clearly because some of the lines (which are frequently only a few of them) used for deducing abundance values are affected by blending with the abundant elements (iron-group elements) and by possible unknown hyperfine structure effects.Singly ionized neodymium is frequently observed in astrophysical spectra and the analysis of Nd II lines leads to strong overabundance estimates of th...

Improved Laboratory Transition Probabilities for Eriiand Application to the Erbium Abundances of the Sun and Fiver‐Process‐rich, Metal‐poor Stars

“…Although the comparison to Musiol & Labuz (1983) is not as favorable as one might hope, it is better than the comparisons to theoretical results in Figures 5 and 6. Figures 5 and 6 are, respectively, comparisons of our results against relativistic Hartree Fock calculations (Xu et al 2003) and semiempirical results.…”

“…There have been some LIF lifetime measurements (e.g., Bentzen et al 1982;Xu et al 2003Xu et al , 2004, but a large set of experimental transition probabilities based on the best modern methods was not available before this work. Recent and extensive TR-LIF lifetime measurements by Stockett et al (2007) provide a foundation for determining a large set of atomic transition probabilities from FTS data.…”

Branching fractions andAvalues in singly ionized tantalum (Ta II)

“…Details of the experimental setup used in the present experiment were described elsewhere (see e.g. Bergström et al 1986;Xu et al 2003Xu et al , 2004Fivet et al 2006Fivet et al , 2008 and only a brief description will be provided here.…”

Transition probabilities of astrophysical interest in the niobium ions Nb$\mathsf{^+}$ and Nb$\mathsf{^{2+}}$

“…The TR-LIF technique previously provided accurate lifetimes in many different systems (see Fivet et al 2006Fivet et al , 2008. A detailed description of the experimental setup is reported in Bergstöm et al (1988) and Xu et al (2003Xu et al ( , 2004 and only a brief description is presented in this paper. Niobium ions in different excited states were generated in a laser-produced plasma obtained by focusing a 5320 Å Nd:YAG laser pulse (Continuum Surelite) onto a rotating niobium target.…”