The Luminescent Center in Inorganic Phosphors

Although we have previously shown in Chapter 5 that the excitation transition for Sn2+ is iS = Pi, the actual mechanism is, in reality, 

The excitation transition is shown as So = Pi. It is this state which relaxes to the 3 Pi state from which emission occurs. However, if we measure the excitation band for several different Sn2+ activated phosphors, we usually will obtain two closely connected bands. There are two possibilities. One is that the excitation transition also involves Sq 3Pi, a transition which is not allowed since AS = 2. The other is that they are crystal field states. 

If we compare the excitation bands of various Sn2+- activated phosphors, i.e. - those having various host crystals, we find that activated phosphors, the excitation bands vary significantly, but the high energy band, I.e.- the So iPi transition, seems to predominate. 

Because the two states are so close in peak energy, one finds it difficult to delineate the actual excitation transition. 

Now consider a different type of activator cation such as Cr3+. Suppose we use AI2O3 Cr3+ as the example. We do this to contrast  

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