Luminescence spectroscopy transitions

Despite the current lack of clarity regarding the relationship between glass transition and chemical reaction kinetics, it is still quite feasible that chemical and biochemical reaction rates may be governed by mobility, i.e., the mobility that is most rate limiting to a particular reaction scheme (e.g., water mobility, reactant mobility, molecular-level matrix mobility, local or microregion mobility), but perhaps not simply by an average amorphous solid mobility as reflected by the Tg. Ludescher et al. (2001) recommend the use of luminescence spectroscopy to investigate how rates of specific chemical and physical processes important in amorphous solid foods are influenced by specific modes of molecular mobility, as well as by molecular structure. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

Luminescence spectroscopy, whenever applicable, has the advantage that the low-temperature spectrum consists of only one electronic transition, whereas in absorption the transitions to the four spin-orbit components of g are superimposed. As a result the absorption spectrum is not as well resolved. 

An important example historically of the application of luminescence spectroscopy (1969) is [Fu(terpy)3] ( 104)3, whose spectrum (Figure 5.8) supported the assigned geometry before crystallographic details were available (crystallographic structural determination was a much slower business at that time). The Dq Fq transition is absent, whilst the Do transition consists of a singlet and a very slightly split (0.4 nm) doublet. The... 

CPL investigations on Eu(III) and Tb(III) nitrate complexes with the chiral D2-symmet-ric ligand 4/, 9/, 19/ ,24/ -3,10,18,25,3l,32-hexaazapentacyclo[25.3.1.1.0.0]-dotriaconta-l-(31),2,10,12,14,16(32),17,25,27,29-decaene (7 -pydach) (Scheme 12) and its S S -pydach enantiomer have shown that the strong measured CPL, g um = —0.19 at 596 nm ( Dq Fi transition), was only due to the twisted conformation (Tsubomura et al., 1992). In the case of the complexes, [Ln(7 -pydach)] + and [Ln( S S -pydach)] +, there was no evidence of a contribution from coordinated nitrate anions on the complex chirality as previously observed. The complex structures which have been characterized by NMR and luminescence spectroscopy have suggested that the nitrate anions were not coordinated to the lanthanide (III) ion in these species and, also, that approximately three water molecules were bound to the metal center. 

This volume of the Handbook on the Physics and Chemistry of Rare Earths adds five new chapters to the science of rare earths, compiled by researchers renowned in their respective fields. Volume 34 opens with an overview of ternary intermetallic systems containing rare earths, transition metals and indium (Chapter 218) followed by an assessment of up-to-date understanding of the interplay between order, magnetism and superconductivity of intermetallic compounds formed by rare earth and actinide metals (Chapter 219). Switching from metals to complex compounds of rare earths, Chapter 220 is dedicated to molecular stmctural studies using circularly polarized luminescence spectroscopy of lanthanide systems, while Chapter 221 examines rare-earth metal-organic frameworks, also known as coordination polymers, which are expected to have many practical applications in the future. A review discussing remarkable catalytic activity of rare earths in site-selective hydrolysis of deoxyribonucleic acid (DNA) and ribonucleic acid, or RNA (Chapter 222) completes this book. 

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. 

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