Ultrafast photochemistry of metal carbonyls

We investigated M(CO)(6) (M = Cr, Mo, W), Fe(CO)(5), Ni(CO)(4) and M-2(CO)(10) (M = Mn. Re) by femtosecond spectroscopy. The molecules were pumped by one photon at 267 nm and then probed by multiphoton ionization at 800 nm and mass selective detection of the resulting parent and fragment ions. Whereas it was previously believed that such metal carbonyls are excited to a repulsive potential, leading to elimination of one or several CO, we find that only one CO is photochemically split off in times typically below 100 fs and that this is already a multistep process involving relaxation between excited-state surfaces. The second elimination takes place in the So of the unsaturated carbonyl and requires much longer time (> 1 ps with our pump wavelength of 267 nm). The unsaturated carbonyl is initially generated in its first excited singlet state S-1. If this molecule has four- or fivefold coordination, it can relax from this S-1 state to S-0 within about 50 fs. the pathway leading through a symmetry-induced conical intersection involving pseudorotation of the ligands. Coherent oscillations along such coordinates were observed in several molecules. In the case of threefold coordination (Ni(CO)(3)), however, there is no such relaxation pathway. Therefore, this photofragment shows a beautiful luminescence with > 10 mus lifetime. All processes only involve the singlet manifold. Intersystem crossing takes at least 500 ps.