Effective-Hamiltonian parameters for \emph{ab initio} energy-level calculations of SrCl2_{2}:Yb2+^{2+} and CsCaBr3_{3}:Yb2+^{2+}

Calculated energy levels from recent \emph{ab initio} studies of the electronic structure of SrCl$_{2}$:Yb$^{2+}$ and CsCaBr$_{3}$:Yb$^{2+}$ are fitted with a semi-empirical "crystal-field" Hamiltonian, which acts within the model space $4f^{14} + 4f^{13}5d + 4f^{13}6s$. Parameters are obtained for the minima of the potential-energy curves for each energy level and also for a range of anion-cation separations. The parameters are compared with published results parameters fitted to experimental data and to atomic calculations. The states with significant $4f^{13}6s$ character give a good approximation of the impurity-trapped exciton states that appear in the \emph{ab initio} calculations.Comment: Minor revisio

Trasplante pulmonar

A lung transplant is usually the final therapeutic option for patients with respiratory insufficiency. In spite of the many advances in immunology and the management of complications, mortality and morbidity associated with this transplant are far higher than with others. Acute rejection is an almost universal problem in the first year, while obliterative bronchitis reduces long term survival. Respiratory infections also play a significant role in the complications associated with lung transplants due to the constant exposure of the graft to the outside. However, the success of this therapeutic option, which basically depends on a suitable selection of donor and recipient, are evident, above all with respect to quality of life

Metal-to-metal charge transfer between dopant and host ions: Photoconductivity of Yb-doped CaF2 and SrF2 crystals

Dopant-to-host electron transfer is calculated using ab initio wavefunction-based embedded cluster methods for Yb/Ca pairs in CaF2 and Yb/Sr pairs in SrF2 crystals to investigate the mechanism of photoconductivity. The results show that, in these crystals, dopant-to-host electron transfer is a two-photon process mediated by the 4fN-15d excited states of Y b2+: these are reached by the first photon excitation; then, they absorb the second photon, which provokes the Y b2+ + Ca2+ (Sr2+) → Y b3+ + Ca+ (Sr+) electron phototransfer. This mechanism applies to all the observed Y b2+ 4f-5d absorption bands with the exception of the first one: Electron transfer cannot occur at the first band wavelengths in CaF2:Y b2+ because the Y b3+-Ca+ states are not reached by the two-photon absorption. In contrast, Yb-to-host electron transfer is possible in SrF2:Y b2+ at the wavelengths of the first 4f-5d absorption band, but the mechanism is different from that described above: first, the two-photon excitation process occurs within the Y b2+ active center, then, non-radiative Yb-to-Sr electron transfer can occur. All of these features allow to interpret consistently available photoconductivity experiments in these materials, including the modulation of the photoconductivity by the absorption spectrum, the differences in photoconductivity thresholds observed in both hosts, and the peculiar photosensitivity observed in the SrF2 host, associated with the lowest 4f-5d bandThis work was partly supported by a grant from Ministerio de Economía y Competitividad, Spain (Dirección General de Investigación y Gestión del Plan Nacional de I+D+i, Grant Nos. MAT2011-24586 and MAT2014-54395-P

Crops on the edge of a cliff: Storage at Castro S. João das Arribas (Northwest Iberia) in the Late Antiquity

The site of Castro S. João das Arribas is placed on the edge of a cliff over the Douro river (Miranda do Douro, Northeast of Portugal). Archaeological interventions in its highest area uncovered a main occupation from Late Antiquity. On its western part a functional space was recorded, which included two small above-ground structures and abundant charred carpological remains. These were found inside ceramic vessels and spread throughout the area, suggesting its destruction occurred after a fire event. A radiocarbon date places such episode in some moment between the late 6th and the first half of the 7th century CE. Carpological results revealed an assemblage dominated by cereal grains, mostly rye (Secale cereale). Naked wheat (Triticum aestivum/durum), barley (Hordeum vulgare), common millet (Panicum miliaceum) and foxtail millet (Setaria italica) were also found but in smaller amounts. The large amount of carpological remains in the above-mentioned contexts, suggests the space was used for storage, at least between the 6th and 7th centuries CE. Although some uncertainties remain regarding how crops were stored, evidence points that they were kept in ceramic vessels, outside and inside the small storage facilities, but also in other types of containers, eventually made of perishable materials. At Castro S. João das Arribas, past communities chose a diversity of crops, however, most of them show undemanding features in terms of soil and climatic conditions. The agricultural choices could have been motivated by several factors, but cereals like rye were certainly well-suited to the environmental conditions around the settlement.LS was financially supported by a PhD grant (Norte-08-5369-FSE000057) from the University of Porto (Faculty of Sciences) and the European Social Fund, through the North Portugal Regional Operational Program “Norte 2020”, under the announcement “Aviso Norte-69-2015-15-Formaç˜ao Avançada (Programas Doutorais)”. MMS was funded by the Beatriz Galindo program as Junior Distinguished Researcher (BG20/00076). JPT was supported by national funds through the Foundationfor Science and Technology (FCT)

Yb2+-doped SrCl2: electronic structure of impurity states and impurity-trapped excitons

First-principles electronic structure calculations of the excited states of Yb2+-doped Sr Cl2 crystals up to 65 000 cm-1 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 4f 135d  manifold, 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 4f 136s 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 4f 135d  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 4f 136s configuration among the 4f 135d manifolds, 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