Journal of Quantitative Spectroscopy & Radiative Transfer 109 (2008) 1968–1985 Electric dipole transitions for La I (Z ¼ 57) Betu¨l Karac - oban, Leyla O ¨ zdemir à Physics Department, Sakarya University, 54187 Sakarya, Turkey Received 15 June 2007; received in revised form 26 January 2008; accepted 29 January 2008 Abstract Wavelengths, weighted oscillator strengths and transition probabilities for electric dipole (E1) transitions for some excited levels in neutral lanthanum (La I, Z ¼ 57) have been calculated using the multiconfiguration Hartree–Fock (MCHF) method within the framework of Breit–Pauli relativistic corrections. Results obtained have been compared with other calculations and experiments. A discussion of these calculations for La I in this study has also been given in view of the MCHF method. r 2008 Elsevier Ltd. All rights reserved. Keywords: MCHF method; Relativistic corrections; Wavelengths; Oscillator strengths; Transition probabilities 1. Introduction The rare earths consist of two series of elements, the lanthanides and the actinides, which roughly speaking involve filling of the 4f and 5f subshells, respectively. The lanthanides are the group of elements with atomic number Z ¼ 57–71. The lanthanides are sometimes considered to consist of the elements Z ¼ 58–71 (Ce to Lu). In the solid state, these atoms are generally triply ionized and hence show valence subshells running from 4f 1 to 4f 14 [1]. The unique properties of lanthanide elements, and also of lanthanum in particular, are a direct result of the small radii of the 4f orbital, which is smaller than that of the 5s electron. The collapse of the 4f orbital from a small quantum-defect, large-radius hydrogenic orbital to a large quantum-defect, small-radius non-hydrogenic orbital occurs at lanthanum (Z ¼ 57). This collapse makes the accurate calculations of wavefunctions in this atom extremely complex [2]. This collapse has been rather frequently considered in the literature. The fact that a huge number of levels arise from the 4f N configurations makes the analysis of the lanthanide spectra extremely complex and time consuming [3]. Although all lanthanide spectra are exceptionally complex, those in the right half (Gd, Tb, Dy, Ho, Er, Tm, Yb) of the series tend to be somewhat simpler than those in the left half (La, Ce, Pr, Nd, Pm, Sm, Eu), in that the former spectra tend to show contrasting strong and weak lines whereas lines of the latter tend to more uniform intensity. The reasons for this are associated with the gradual contraction of the 4f orbital with increasing Z, so that the 4f N subshell becomes more and more deeply buried within the atom, and therefore interacts less and less strongly with the outer valence electrons [1]. ARTICLE IN PRESS www.elsevier.com/locate/jqsrt 0022-4073/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2008.01.020 à Corresponding author. Tel.: +90 264 2956079; fax: +90 264 2955950. E-mail address: lozdemir@sakarya.edu.tr (L. O ¨ zdemir).