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

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 S-1 state which can again relax in an ultrashort time through a JT induced conical intersection to S-0; the JT active coordinate (relaxation pathway) corresponds to pseudorotation. The total time to reach S-0 takes 110, 165 and 195 fs for the three carbonyls (calculated from the measured first four time constants). After arrival at S-0, M(CO)(5) oscillates coherently along a pseudorotation coordinate. In S-0, 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. (C) 2000 Elsevier Science B.V. All rights reserved.