Exciton Emission from Alkali Halides

This section. starts with the alkali halides, because the intrinsic luminescent center in these compounds shows a complicated relaxation in the excited state which has 

Consider as a specific example KCI, a very simple compound indeed. In Chapter 2, its lowest optical absorption band was mentioned to be due to the 3p - 3p 4s transition on the Cl ion. The excited state can be considered as a hole on the Cl ion (in the 3p shell) and an electron in the direct neighbourhood of the Cl ion, since the outer 4s orbital spreads over the K inns. Now we consider what happens after the absorption process. The hole prefers to bind two Cl ions forming a Vk centre this centre consists of a C pseudomolecule on the site of two O ions in the lattice. The electron circles around the Vk centre. In this way a self-trapped excitnn is formed. An exciton is a state consisting of an electron and a hole bound together. By the relaxation process (Cl Vk.c) the exciton has lowered its energy and is now trapped in the lattice. 

This example shows clearly that the emission process is very different from the (simple) absorption process. For all details the reader is referred to the literature [SJ. Finally we draw attention to the fact that the life time of the relaxed self-trapped exciton in the alkali halides is longer 10 s) than expected for an allowed transition (I0 - I0 s). This is ascribed to the fact that the emitting state contains an amount of spin triplet character. Such a triplet state arises when the spins of the electron and the hole are oriented parallel. The emission transition becomes (partly) forbidden by the spin selection rule (see Chapter 2). 

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