Singlet-Triplet Mixing Under Spin-Orbit Coupling

5 Singlet-Triplet Mixing Under Spin-Orbit Coupling 

In Section 8.5, we discussed the singlet and triplet excited states, I s and 4 t, respectively, resulting from a singlet ground state 1 g. In the absence of SOC, the excited states (I s and 4 t do not interact. The spin parts of the singlet and triplet 

Under SOC, these two states interact because the interaction term (T Hso S) is nonzero. Namely, according to equation 24.52, 

This expains why the transition from the triplet excited state I t to the singlet ground state I g (i.e., phosphorescence) takes place, though very weakly. The forbidenness of this transition is true only when the effect of SOC is neglected. 

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