Time-resolved two-laser probe of Cr(CO)₆ photodissociation dynamics

We have examined the photodissociation of jet-cooled Cr(CO)₆ using a time-resolved, two-laser multiphoton dissociation (MPD) technique with fluorescence detection of the atomic photofragments. We have observed that the rate of appearance of Cr(CO)₄ via 248-nm photolysis of Cr(CO)₆ is slower if the Cr(CO)₆ has first been cooled in a supersonic expansion. We suggest the difference in the observed rates is due to slower internal conversion in the transient Cr(CO)₅ following impulsive loss of the first CO ligand from the jet-cooled hexacarbonyl. This internal conversion is thought to be facilitated by low-energy OC-Cr-CO bending modes, which are probably not accessible by one-photon absorption from the ground state of Cr(CO)₆. We discuss the utility of our time-resolved MPD/atomic fluorescence technique as a general dynamical probe for metal carbonyl photodissociation.