ULTRAFAST DYNAMICS OF Cr(CO)_6 IN CLUSTER ENVIRONMENT

摘要

Femtosecond pump/probe experiments aimed at elucidating the dynamics of transition metal carbonyls in cluster environment are reported on. When exciting Cr(CO)_6·(CH_3OH)_n heteroclusters with weak pump pulses to a ~1T_(lu) state, picosecond dynamics of solvated Cr(CO)_4 is observed. Comparing with solution data, the short-time dynamics is explained by coupling of low-energy molecular modes to cluster modes and the long-time dynamics by relaxation of excited CO stretch vibrations to low-energy molecular modes (IVR). At higher pump intensities, while maintaining pump wavelength, different solvation time scales are found. Absorption of an additional pump photon by nascent Cr(CO)_5 leading to different solvated fragments is proposed. Exciting homogeneous clusters [Cr(CO)_6]_n leads to ultrafast bimolecular intra-cluster reactions producing a large number of novel products. The dynamics of these products is dominated by metal-metal bond formation on the femtosecond time scale. Applying concepts of surface science, the picosecond dynamics of polynuclear coordinatively unsaturated carbonyls is explained by site exchange from terminally bound to bridging ligands. Transients corresponding to a precursor to CO dissociation which most likely contains a π-bonded CO bridge show similar time scales. In addition, energy-dependent reaction channels are observed.