Luminescence band scheme

In this case emission is not connected with electron transitions inside Ag ion and may be interpreted in energy band scheme (Gaft et al. 1989). Under UV lamp excitation at 300 K those minerals do not exhibit luminescence but at lower temperature of 77 K very strong emission appear with blue-green color for chlorargyrite, yellow-green for embolite and yellow-orange for embolite. 

The sol-gel co-immobilization of a non-fluorescent blue indicator bromothymol blue (BTB) with an europium (Ill)-complex intense antenna mediated lanthanide dye represents a new scheme for the fluorescence analysis38. Luminescence spectra of europium (Ill)-complex shown in Figure 12 were found to be independent of pH changes in the range 1-10. Therefore, BTB, a non-fluorescent pH indicator with alkaline absorption maximum close to main europium emission band was added to the sol-gel mixture to shield reversibly the emission of the europium (Ill)-complex at different pH s without quenching of the antenna function. 

Finally, it is important to mention that besides the possibility of energy transfer, a high concentration of centers can lead to new kinds of centers, such as clusters formed by aggregation or coagulation of individual centers. Thus, these new centers can have a different level scheme to that of the isolated centers, giving rise to new absorption and emission bands. This is, of course, another indirect mechanism of concentration quenching for the luminescence of the isolated centers, as happens in the next example. 

In somewhat related work of Venturi et al., the dinuclear complex [(bpy)2Ru(MACl)Os(bpy)2]4+ was examined (Scheme 2). Luminescence spectral data show that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os (II) unit. Electronic energy transfer from the Ru(II) to the Os(II) 3MLCT state takes place by two distinct processes (ken = 2.0 x 108 and 2.2 x 107s-1 at 298 K) [74],... 

In order to assign the bands which have been observed in the optical spectra of the octahedral uranate group, relativistic molecular orbital calculations on this uranium complex are in fact indispensable. However, no reliable theoretical studies have been devoted to the energy level scheme of the UO group up till now, so there is no direct information available about the transitions involved in the luminescence processes. 

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