We report on first results of real-time studies on the photodissociation dynamics of electronically excited Cr(CO)6 and Cr(CO)6·(CH3OH) n heteroclusters generated in a molecular beam. The red part of the 1T1u metal to ligand charge-transfer state at 280 nm is excited, and the resulting photofragments are detected by multiphoton ionization. The decarbonylation dynamics is found to be much faster than the cross correlation width (230 fs) of our pump and probe pulses. The observed fragmentation pattern and its dynamics can tentatively be attributed to two competing dissociation channels, both starting from the primary photofragment Cr(CO)5, a simultaneous and a sequential channel. For the clusters we find that both these channels appear to be operative depending on pump laser intensity and that decarbonylation precedes evaporative loss of solvent molecules, which takes place on the picosecond time scale. Solvated cluster species as well as coordinatively unsaturated carbonyls are stabilized by evaporation.

We report on one-color femtosecond−pump/probe experiments applied to homogeneous chromium hexacarbonyl clusters which are prepared in a molecular beam. The pump pulse initiates bimolecular intra-cluster reactions by ultrafast photodissociation of a CO ligand from Cr(CO)6 in a 1T1u metal-to-ligand charge-transfer state. The resulting reaction products are detected mass-selectively after multiphoton ionization by the probe pulse. In contrast to nanosecond multiphoton ionization (Peifer, W. R.; Garvey, J. F. Int. J. Mass Spectrom. Ion Processes 1990 102, 1), which exclusively leads to appearance of the Cr+ mass peak, a number of novel species which may serve as interesting model systems for catalysis and surface chemistry are detected. In addition to mononuclear fragments, metal cluster ions, and coordinatively unsaturated polynuclear metal carbonyls, metal oxides and carbides are observed in the femtosecond mass spectra. The appearance of these species can be rationalized by formation of asymmetric π-bridging CO ligands in the neutral manifold. The dynamics extracted from the transients of different reaction products is interpreted using concepts of surface and inorganic cluster chemistry. Ultrafast metal−metal bond formation on a time scale much faster than our laser pulse width (ca. 150 fs) can be observed. Site exchange of CO from terminal to bridging positions on the picosecond time scale is proposed to occur in coordinatively unsaturated polynuclear metal carbonyls. Experiments at different center wavelengths (280 and 262.5 nm) reveal energy-dependent reaction channels which are closed when exciting the clusters at 280 nm. When exciting at 262.5 nm, impulsive formation of the corresponding products and decay on the picosecond time scale is observed.

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