The Energy Level Diagrams of Individual Ions

On the utmost left-hand side (crystal field A = 0) we find the energy levels of the free ion. Many of these levels split into two or more levels for A 7 0, as for example in a solid. The lowest level, i.e. the ground state, coincides with the x-axis. For the free ion the levels are marked where S presents the total spin quantum 

however, that this transition is a forbidden one, since it orxurs between levels of the d shell. Therefore the parity does not change. Actually the color of transition metal ions is never intense, as far as this type of transitions, also called crystal-field transitions, are involved. ITie parity selection rule is relaxed by coupling of the electronic transition with vibrations of suitable symmetry [13], In tetrahedral symmetry, however, the centre of symmetry is lacking, and the parity selection rule is also relaxed in another way, viz., by mixing small amounts of opposite-parity 

How Docs a Luminescent Material Absorblis Excitation Energy  

The more-electron cases ate considerably more complicated (Figs 2.9 and 2.10). However, remembering the selection rules, we can derive the absorption spectra to be expected. For an ion with three electrons, for example CP (3d the ground level is A2. Optical absorption will, in good approximation, occur only to the spin-quartet (2.S -1-1 4) levels in view of the spin selection rule. There are only three of these transitions possible, viz. A2 - T2, T 1 ( F) and T1 ( P). Indeed the absorption spectrum of the 01 ion consists of three absorption bands with low intensity (parity selection rule). This is shown in Fig. 2.12. The spin-forbidden transitions can usually be observed only in very accurate measurements. 

The last example is the configuration, of which Mn, used in many luminescent materials, is a well-known representative. The Tanabe-Sugano diagram is given in Fig. 2.10. In octahedral coordination the ground level is A. All optical absoiption transitions are parity and spin forbidden. In fact the Mn ion is practically colorless. 

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