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Abstract.We investigate the formation of Nd - lines in the atmospheres of A-type stars with a comprehensive atomic model including 1651 levels of Nd , 607 levels of Nd  and the ground state of Nd . NLTE leads to overionization of Nd  which weakens the Nd  lines relative to the corresponding LTE line strengths at mild neodymium overabundance ([Nd/H] < 2.5) and amplifies them at higher [Nd/H] values. NLTE abundance corrections grow with the effective temperature and reach 0.6 dex at T eff = 9500 K for [Nd/H] = 2.5. The Nd  lines are strengthened compared with LTE in all cases, and NLTE abundance corrections lie between −0.3 dex and −0.2 dex for T eff between 7500 K and 9500 K. NLTE effects are larger for an inhomogeneous vertical abundance distribution compared with a homogeneous one resulting in positive NLTE abundance correction up to 1.3 dex for the Nd  lines and in negative ones down to −0.5 dex for the Nd  lines. The neodymium distribution in the atmospheres of roAp stars γ Equ and HD 24712 is deduced from NLTE analysis of the Nd  and Nd  lines and a strong evidence is found for the existence of enhanced Nd abundance layers above log τ 5000 = −3.

The Sn IX-Sn XII spectra excited in a vacuum spark have been analysed in the 130-160 Å wavelength region. The analysis was based on the energy parameter extrapolation in the isonuclear Sn VI-VIII and Sn XIII-XIV sequence. 266 spectral lines belonging to the 4d m -(4d m−1 4f + 4p 5 4d m+1 ) (m = 6-3) transition arrays were classified in the Sn IX-Sn XII spectra for the first time. All 18 level energies of the 4d 3 configuration and 39 level energies of the strongly interacting 4d 2 4f and 4p 5 4d 4 configurations were established in the Sn XII spectrum. The energy differences between the majority of the 4d m levels and about 40 levels of the 4d m−1 4f + 4p 5 4d m+1 configurations were determined in each of the Sn IX, Sn X and Sn XI spectra (m = 6-4). As a result, all intense lines were classified in the 130-140 Å region relevant to the extreme ultraviolet (EUV) lithography. It was shown that the most of the intense lines in the 2% bandwidth at 135 Å belong to the transitions in the Sn XI-Sn XIII spectra.

Extreme UV spectra of the gadolinium and terbium ions excited in the laser-produced plasma and vacuum spark sources were recorded in the 40-120 Å region and investigated on the basis of the Hartree-Fock calculations using Cowan code. The intense peaks in the 65-75 Å region of the vacuum spark spectra were interpreted as a manifold of the 4d 10 4f m -4d 9 4f m+1 transitions in the ions with a partially filled 4f shell. The drastic narrowing of these peaks was observed in the spectra of the laser-produced plasma. It was explained by a change of the 4d 10 4f m -4d 9 4f m+1 (m > 2) transition arrays mostly contributing to the intensity of the peaks in the vacuum spark spectra for the 4-4 transitions in the simplest spectra of the 4p 6 4d k (k = 8-10) and 4d 10 4f m (m = 1-2) ground configuration ions predominantly excited in hotter laser-produced plasma. The most intense lines of the 4d 10 4f 2 -4d 10 4f5d transitions in the Gd XVII and Tb XVIII spectra were classified for the first time.

Context. The chemically peculiar stars of the upper main sequence represent a natural laboratory for the study of rare-earth elements (REE). Aims. We want to check the reliability of the energy levels and atomic line parameters for the second REE ions currently available in the literature, and obtained by means of experiments and theoretical calculations. Methods. We have obtained a UVES spectrum of a slowly rotating strongly magnetic Ap star, HD 144897, that exhibits very large overabundances of rare-earth elements. Here we present a detailed spectral analysis of this object, taking effects of non-uniform vertical distribution (stratification) of chemical elements into account. Results. We determined the photospheric abundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe), we obtained a stratification model that allows us to produce a satisfactory fit to the observed profiles of spectral lines of various strengths. All the stratified elements but Cr show a steep decrease in concentration toward the upper atmospheric layers; for Cr the transition from high to low concentration regions appears smoother than for the other elements. The REEs abundances, which have been determined for the first time from the lines of the first and second ions, have been found typically four dex higher than solar abundances. Our analysis of REE spectral lines provides strong support for the laboratory line classification and determination of the atomic parameters. The only remarkable exception is Nd iii, for which spectral synthesis was found to be inconsistent with the observations. We therefore performed a revision of the Nd iii classification. We confirmed the energies for 11 out of 24 odd energy levels that were classified previously, and derived the energies for additional 24 levels of Nd iii, thereby substantially increasing the number of classified Nd iii lines with corrected wavelengths and atomic parameters.