J. Chem. Phys., 133, 114509, 2010


G. Sánchez-Sanz, L. Seijo, and Z. Barandiarán.

Yb2+ -doped SrCl2 Electronic structure of impurity states and impurity-trapped excitons.

First-principles electronic structure calculations of the excited states of Yb2+-doped SrCl2 crystals up to 65000cm1 reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f135manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short Yb–Cl distances) and 4f136s impurity states (more stable at long Yb–Cl distances); the former are found to be preionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl8 moiety. Spin-orbit coupling between the double-well states and the lower lying 4f135d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread. To some extent, the dual electronic structure (impurity-trapped exciton–impurity state) of some excited states expresses and gives support to hypotheses of interaction between Yb2+ and Yb3+ pairs proposed to understand the complex spectroscopy of the material and conciliates these hypotheses with interpretations in terms of the existence of only one type of Yb2+ defect. The results presented confirm the presence of impurity states of the 4f136s configuration among the 4f135manifolds, as proposed in literature, but their energies are very different from those assumed. The Yb-trapped excitons found in this chloride host can be seen as precursors of the luminescent Yb-trapped excitons characterized experimentally in the isomorphous SrF2 crystals.

J. Chem. Phys., 133, 114506, 2010


G. Sánchez-Sanz, L. Seijo, and Z. Barandiarán.

Electronic spectra of Yb2+ -doped SrCl2.

The absorption and emission spectra of Yb2+-doped SrCl2 have been calculated on the basis of ab initio quantum chemical calculations which consider recently found, unexpected excited states with double-well energy curves and complex electronic structure, resulting from avoided crossings between Yb-trapped excitons and Yb impurity states, which influence prominent spectral features. The root mean square deviation and largest absolute error of the calculated energy levels are 394 and 826cm1, respectively. The YbCl8 moiety breathing mode vibrational frequencies and bond lengths of the lowest states are consistent with observed vibrational progressions and energy shifts induced by uniaxial compression. Photoionization is predicted above 49000cm1 as a consequence of the spin-orbit induced spreading of the Yb-trapped exciton character in the upper part of the spectrum and three new emission bands are predicted with origins at about 33 800, 36 400, and 43600cm1. The electron correlation methods used overestimate the relative stabilization of the 4f14ground state and this leads to a constant error of the whole absorptionspectrum of about 3500cm1 (23%–7%). Although this energy shift is customarily considered an adjustable parameter, it is a nonparametric, direct product in an ab initio route which shows the limitations on the proper representation of differential correlation between the 4fN.

Chem. Phys. Lett., 498, 226, 2010


G. Sánchez-Sanz, Z. Barandiarán, L. Seijo,

Energy level shifts in two-step spin–orbit coupling ab initio calculations

We point out a problem with two-step spin–orbit ab initio calculations in which the energy levels of spin–orbit free Hamiltonians are shifted as a means to including dynamic correlations at low cost in small spin–orbit configuration interaction calculations. The usual shifts driven by the energy order of the states can lead to anomalous results when avoided crossings exist with significant change of wave function character, which take place at different nuclear positions in the configurational spaces of the first and the second steps. In these cases, the shifts of the spin–orbit free energy levels must be assigned according to the characters of the wave functions.

Spectrosc. Lett., 43, 393, 2010


G. Sánchez-Sanz, L. Seijo, and Z. Barandiarán.

Energy shift of the 4f 13 6s1 excited states of Yb2+ from gas phase to CsCaBr3 solid.

Ab initio calculations on Yb2+-doped CsCaBr3 show a large increase (20000 cm−1) of the 4f 14 → 4f 136s 1transitions from gas phase to CsCaBr3, due to large embedding effects originated beyond the first bromide coordination shell. The 4f 136s 1 states of (YbBr6)4- in vacuo undergo a sudden energy and electron density change at short Br-Yb distances resulting in the formation of an Yb-trapped exciton, which evolves to full ionization, making Yb(II) unstable in hexabromide coordination in gas phase. Embedding in CsCaBr3localizes the 6s electron inside the Br6 cage, which increases the 6s-Br6 repulsion and increases the energy of the 4f 136s 1 states.

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