  We investigate the cascade decay mechanism for ultrafast intersystem crossing
mediated by the spin-orbit coupling in transition-metal complexes. A
quantum-mechanical description of the cascading process that occurs after
photoexcitation is presented. The conditions for ultrafast cascading are given,
which relate the energy difference between the levels in the cascading process
to the electron-phonon self energy. These limitations aid in the determination
of the cascade path. For $Fe^{2+}$ spin-crossover complexes, this leads to the
conclusion that the ultrafast decay primarily occurs in the manifold of
antibonding metal-to-ligand charge-transfer states. We also give an
interpretation why some intermediate states are bypassed.
