C. RiveraRadiative transition probabilities and oscillator strengths for 164 lines arising from 5s 2 ns, 5s 2 np, 5s 2 nd, 5s 2 nf, 5s 2 ng and 5s 5p 2 configurations of Sn II have been calculated. These values were obtained in intermediate coupling (IC) and using ab initio relativistic Hartree-Fock (HFR) calculations. We use for the IC calculations the standard method of least square fitting of experimental energy levels by means of computer codes from Cowan. The results of calculations for radiative transition probabilities are presented and compared with the experimental result presents in the literature and with other theoretical values. These values, although in general agreement with the rare experimental data, do present sorne noticeable discrepancies.

The optical emission from a laser produced plasma (LPP) generated by a I O 640 A laser irradiation, with a flux of 2 x 10 10 W cm-2 , on Pb targets was recorded and analysed between 2000 and 7000 A: spectral lines emitted by the first two ionization species of Pb were studied in a wide range of plasma parameters. First experimental values ofthe atomic transition probabilities for ten spectral lines of Pb III have been determined by measurement of ernission line intensities from an optically thin LPP of Pb in an atmosphere of Ar. The studied plasma has a temperature of about 25 300 K and an electron density of 10 17 cm-3 . Local thermodynamic equilibrium conditions and plasma homogeneity has been checked.

The preparation and characterization of two polymorphic phases of AgNd(WO 4 ) 2 are described. The high-temperature phase of AgNd(WO 4 ) 2 is prepared as a polycrystalline powder and as a single crystal. X-ray diffraction analysis indicates that the crystal has at 300 K the tetragonal symmetry of the space group (SG) I4 h (No. 82), with two independent crystal sites, 2b and 2d, for Nd 3+ cations and structural disorder around them. The 5 K ground state optical absorption of this tetragonal crystal clearly differs from that corresponding to the monoclinic SG C2/m (No. 12) ordered phase found in polycrystalline samples prepared below 800 °C. Four times larger bandwidths and a weaker crystal field (CF), that is, lesser CF splitting for all Nd 3+ 2S+1 L J manifolds, are observed for the tetragonal phase. Well-defined S 4 polarization rules have been determined in the tetragonal phase, and then the observed 99 Nd 3+ energy levels were labeled with the appropriate Γ 7,8 or Γ 5,6 irreducible representations. A detailed Hamiltonian of 26 free ion and CF parameters have been used in the simulation of the phenomenological energy levels and associated wave functions of the 4f 3 configuration of Nd 3+ in the tetragonal AgNd(WO 4 ) 2 single crystal, with final σ ) 12.6 cm -1 . The validity of the above set of CF parameters and wave functions has been established through the good reproduction of the thermal variation of the measured anisotropic paramagnetic susceptibility χ. As a result of this simulation it is shown that the larger bandwidths of the tetragonal phase contain nonresolved contributions from the two Nd 3+ sites. A method to control overheating events is proposed on the basis of the nonreversibility of the tetragonal phase into the monoclinic one.

A molecular theory of biaxial nematic ordering in the system of bent-core molecules has been developed in the two-particle cluster approximation which enables one to take into account short-range polar correlations determined by both electrostatic dipole-dipole interaction and polar molecular shape. All orientational order parameters and short-range correlation functions are calculated numerically as functions of temperature in the uniaxial and in the biaxial nematic phases, and the results are compared with the ones obtained in the mean-field approximation and in the cluster approximation but without taking into consideration the dipole-dipole interaction. It is shown that short-range polar correlations and, in particular, the dipole-dipole correlations dramatically increase the temperature of the transition into the biaxial nematic phase and enhancing its stability range. The results are also very sensitive to the value of the opening angle of a model bent-core molecule.

Transition probabilities and oscillator strengths of 382 lines with astrophysical interest arising from 5d96s26p, 5d106snl, 5d106s2, 5d106p2, 5d106p7s and 5d106p6d configurations and some levels radiative lifetimes of Pb iii have been calculated. These values were obtained in intermediate coupling (IC) and using relativistic Hartree–Fock calculations including core‐polarization effects. We use for the IC calculations the standard method of least‐square fitting from experimental energy levels by means of Cowan computer code. The inclusion in these calculations of the 5d106p7s and 5d106p6d configurations has facilitated us a complete assignment of the levels of energy in the Pb iii. Transition probabilities, oscillator strengths and radiative lifetimes obtained are generally in good agreement with the experimental data.