Conical intersections, pseudorotation and coherent oscillations in ultrafast photodissociation of group-6 metal hexacarbonyls Original Research Article

By time-resolved nonresonant (800 nm) multiphoton ionization we found five consecutive processes and pronounced coherent oscillations after excitation of M(CO)6, M=Cr, Mo and W at 267 nm in the gas phase. We suggest that the first two steps correspond to relaxation along a Jahn–Teller (JT) active coordinate and internal conversion between metal-to-ligand charge-transfer states via a JT induced conical intersection, whereas in the third step the molecules change over to a repulsive ligand-field surface and dissociate. The primary product is M(CO)5 in its S1 state which can again relax in an ultrashort time through a JT induced conical intersection to S0; the JT active coordinate (relaxation pathway) corresponds to pseudorotation. The total time to reach S0 takes 110, 165 and 195 fs for the three carbonyls (calculated from the measured first four time constants). After arrival at S0, M(CO)5 oscillates coherently along a pseudorotation coordinate. In S0, M(CO)5 eliminates a second CO in about 1 ps owing to its vibrational excess energy, a step which is suppressed in solution by cooling. All processes take place in the singlet manifold of states.