  Light absorption of myoglobin triggers diatomic ligand photolysis and spin
crossover transition of iron(II) that initiate protein conformational change.
The photolysis and spin crossover reactions happen concurrently on a
femtosecond timescale. The microscopic origin of these reactions remains
controversial. Here, we apply quantum wavepacket dynamics to elucidate the
ultrafast photochemical mechanism for a heme--carbon monoxide (heme--CO)
complex. We observe coherent oscillations of the Fe-CO bond distance with a
period of 42 fs and an amplitude of $\sim$1 \AA{}. These nuclear motions induce
pronounced geometric reorganization, which makes the CO dissociation
irreversible. The reaction is initially dominated by symmetry breaking
vibrations inducing an electron transfer from porphyrin to iron. Subsequently,
the wavepacket relaxes to the triplet manifold in $\sim$75 fs and to the
quintet manifold in $\sim$430 fs. Our results highlight the central role of
nuclear vibrations at the origin of the ultrafast photodynamics of
organometallic complexes.
