Ultrafast dynamics of transition metal carbonyls.: II.: Picosecond evaporation after photodissociation of Cr(CO)6•(CH3OH)n heteroclusters at 280 nm

Results of femtosecond pump-probe real-time studies on the evaporation dynamics following photodissociation of electronically excited Cr(CO)(6).(CH3OH)(n) heteroclusters generated in a molecular beam are presented. The red part of the T-1(1u) metal-to-ligand charge transfer state at 280 nm is excited and the resulting photofragments are detected by multiphoton ionization. The time scale of the decarbonylation dynamics of Cr(CO)(6) within the clusters can be explained by a sequential mechanism when applying moderate pump intensities. Decarbonylation is found to be pulse-width limited and solvent evaporation from the solvated neutral photoproducts occurs on the picosecond time scale. Evidence for two different evaporation rates is provided, which by similarity to condensed-phase results from the literature can be interpreted in terms of vibrational energy transfer from low-frequency molecular modes to the solvent (similar to 13 ps) and relaxation of high-frequency C-O stretch modes to anharmonically coupled low-frequency molecular modes (similar to 100 ps). Solvated cluster species as well as coordinatively unsaturated carbonyls are stabilized by evaporation. (C) 1998 Elsevier Science B.V. All rights reserved.