Intersystem crossing rotational energy transfer

Intersystem crossing (ISC) is the intramolecular crossing from one state to another of different multiplicity without the emission of radiation. In Fig. 3.9 (ISC), shows the transfer from the first excited singlet state S, to the first excited triplet state T,. Since the process is horizontal, the total energy remains the same and the molecule initially is produced in upper vibrational and rotational levels of T, from which it is deactivated as shown by the vertical wavy line. Similarly, (ISC)2 shows the intersystem crossing from T, to upper vibrational and rotational states of the ground state S(l, from which vibrational deactivation to v" = 0 then occurs. 

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