Impurity-Trapped Excitons

The combination of methods described above is flexible enough to allow for the appearance of unexpected excited states. This is what happened along a study of the electronic states of in the strong field hexafluoride environment of the CsaGeFe host. Whereas calculations had been set up to obtain the 5/, 5f6d, and 5f7s manifolds, the characteristics of the latter, namely, very short U-F bond lengths and unnormal, delocalized 7s shell, pointed at their impurity-trapped exciton nature as described in the Introduction by Ordejon et al. [29]. 

Impurity-trapped excitons are thought to be responsible for anomalous emissions in some Eu + and Yb + containing crystals [82]. They are also thought to participate in energy transfer and decay processes in scintillators. The widespread description of their electronic structure corresponds very well with the characteristics of the high-energy excitons we have found and described above, where the excited electron delocalizes symmetrically towards 

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Abstract In this chapter, we summarize recent accomplishments in the area of high-pressure luminescence spectroscopy of phosphor materials. The effect of pressure on the luminescence related to f-f, d-d, and d-f transitions is discussed. Several recent examples from the literamre are presented to illustrate the influence of pressure on luminescence energy, intensity, lineshape, luminescence kinetics, and luminescence efficiency. Especially, the unique ability of pressure to investigate the influence of impurity-trapped exciton states, which are created after ionization and charge-transfer transitions, on the luminescence of TM and RE ions in solids and energy-transfer processes are presented. 

Chemical-composition and high-pressure smdies share the goal to identify the physical and chemical factors that control the optical properties of the materials. The final objective is to progress toward the rational design of novel phosphors with required properties. High-pressure spectroscopic smdy also provides fundamentally unique information on quantum mechanical mixing of the localized states related to transition-metal and lanthanide ions with the band-delocalized states of the host lattice, which are responsible for the creation of intermediate states of the impurity-trapped exciton. 

Grinberg M (2013) Impurity trapped excitons under high hydrostatic pressure. Opt Mater 35 2006... 

Grinberg M, Mahhk S (2008) Impurity trapped excitons experimental evidences and theoretical concept J Noncryst Solids 354 4163... 

Srivastava AM, Beers WW (1999) On the impurity trapped exciton luminescence in La2Zr207 Bi J Lumin 81 293... 

Altogether, impurity states and impurity-trapped excitons define the realm of lanthanide activated solid-state materials. This is a realm where experiment and theory should meet but where the research work conducted is overwhelmingly experimental. Their structure and optical properties are complex and rich. They are a genuine challenge for quantum chemists. What is needed is not massive production of theoretical results, which follow experiments (which, in any case, would probably be very difficult to attain, given the pace of experimental work and sophistication of the theoretical methods apphcable). What is needed is to answer basic questions that cannot be answered by experimental techniques alone so that their electronic structures are mastered beyond simple model and beyond empirical model descriptions, to the point where the intensive and constant search for new materials could count on the ability to predict, which is characteristic of ab initio quantum chemical methods when it is found how to stretch them to the limits of their capabilities. 

Ordejon B, Seijo L, Barandiaran Z. Geometry and electronic structure of impurity-trapped excitons in Cs2GeF6 U " cyrstals. The 5f 7s manifold. J Chem Phys. 2007 126 194712. Sanchez-Sanz G, Seijo L, Barandiaran Z. Yb +-doped SrCb Electronic structure of impurity states and impurity-trapped excitons. J Chem Phys. 2010 133 114509. 

MP Reid, PS Senanayake, J-P R Wells, G Berden, A Meijerink, AJ SaUceld, C-K Duan, RJ Reeves. Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in Cap2 Yb +. Phys Rev B. 2011 84 113110. 

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