  The mechanism behind fast intersystem crossing in transition-metal complexes
is shown to be a result of the dephasing of the photoexcited state to the
phonon continuum of a different state with a significantly different transition
metal-ligand distance. The coupling is a result of the spin-orbit interaction
causing a change in the local moment. Recurrence to the initial state is
prevented by the damping of the phonon oscillation. The decay time is faster
than the oscillation frequency of the transition metal-ligand stretch mode, in
agreement with experiment. For energies above the region where the strongest
coupling occurs, a slower "leakage-type" decay is observed. If the photoexcited
state is lower in energy than the state it couples to, then there is no decay.
