Activators optical energy

Phosphors usually contain activator ions in addition to the host material. These ions are dehberately added in the proper proportion during the synthesis. The activators and their surrounding ions form the active optical centers. Table 1 Hsts some commonly used activator ions. Some soflds, made up of complexes such as calcium tungstate [7790-75-2] CaWO, are self-activated. Also in many photolurninescence phosphors, the primary activator does not efficiently absorb the exciting radiation and a second impurity ion is introduced known as the sensitizer. The sensitizer, which is an activator ion itself, absorbs the exciting radiation and transfers this energy to the primary activator. 

In principle, all of the elements of the periodic table can be used to iucorporate foreign ions in crystals. Actually, only a number of elements have been used for optically active centres in crystals in other words, only a number of elements can be incorporated in ionic form and give rise to energy levels within the gap separated by optical energies. The most relevant centers for technological applications (although not the unique ones) are based on ions formed from the transition metal and rare earth series of the periodic table, so we will focus our attention on these centers. 

It has in fact been anticipated for many years that the CT free energy surfaces may deviate from parabolas. A part of this interest is provoked by experimental evidence from kinetics and spectroscopy. Eirst, the dependence of the activation free energy, Ff , for the forward (/ = 1 ) and backward i = 2) reactions on the equilibrium free energy gap AFq (ET energy gap law) is rarely a symmetric parabola as is suggested by the Marcus equation,Eq. [9]. Second, optical spectra are asymmetric in most cases and in some cases do not show the mirror symmetry between absorption and emission.In both types of experiments, however, the observed effect is an ill-defined mixture of the intramolecular vibrational excitations of the solute and thermal fluctuations of the solvent. The band shape analysis of optical lines does not currently allow an unambiguous separation of these two effects, and there is insufficient information about the solvent-induced free energy profiles of ET. 

Moreover, from the data in Table 2 it is possible to estimate the fi ee energy of activation of the reaction studied. In order to do this we must assume, above all, that the optical electron transfer, to which the band correspond, is the same as the themud electron transfer, to which the k values correspond. However, the data in Table 3 cannot be used directly for estimating the activation free energy of the thermal electron transfer reaction. The first obvious reason comes from the fact that the band corresponds to the (Co(NH3)4(pzC02)] -[Ru(CN) ion-pair instead of to the [Co(NH3)4(pzC02)] -[Fe(CN) J "... 

Table 3.- Activation free energies estimated for the optical electron transfer within the [Co(NH3)4(pzC02)] -[Fe(CN)J ion-pair (see the text). T=298.2 K.

Optical Activity and Energy Discrimination (H.G. Smith Memorial Lecture 1972). 

In most covalent NCS it is found that AE, the thermal activation energy of the conductivity is about half the magnitude of the optical energy gap. This means that Ep is not far from the center of the mobility gap. Does this mean that these materials are intrinsic In the case of crystalline semiconductors the word intrinsic is used to mean that the conduction properties are not affected by the presence of localized impurity states. The position of Ep is then determined by the equality... 

Optical Energy Provides Activation Energy for Exoergic Reactions]... 

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