Time-resolved organometallic photochemistry -: Femtosecond fragmentation and adaptive control of CpFe(CO)2X (X = Cl, Br, I)

We report a systematic experimental study of the influence of metal-ligand bonding properties on the ultrafast photofragmentation dynamics and on quantum control of organometallic molecules in the gas phase. In the first part, the femtosecond pump-probe technique is combined with reflectron time-of-flight mass spectrometry to record transient mass spectra of the photodissociation products of CpFe(CO)(2)X (CP = C5H5; X = Cl, Br, I). Sequential dissociation steps are observed for the X = Cl species while concerted and more complex mechanisms are seen in the molecules involving the larger halogens. The overall dissociation rate increases from the lighter toward the heavier halogen ligands in this series which can be explained qualitatively by the different metal-halogen bonding properties and their stabilizing effects on unsaturated intermediate species. In the second part, adaptive quantum control experiments are performed on the X = Cl and X = Br molecules. A femtosecond laser pulse shaper provides spectrally phase-modulated electric fields which are then iteratively optimized by a learning evolutionary algorithm. Direct experimental feedback from the photoproduct mass spectra is used in this search for specific optimization goals. A comparison between the two molecular species shows that laser fields optimized for one molecule also lead to an improvement in the species with the substituted halogen ligand, but not to the same degree as when the substituted molecule is directly optimized. Despite their chemical similarities, adaptive pulse shaping is sensitive enough to detect differences in the investigated metal complexes which are due to the electronic metal-halogen bonding properties. These results suggest an application in chemical analysis where specific signatures are desired for the individual compounds in mixtures of chemically similar molecules. (C) 2002 Elsevier Science B.V. All rights reserved.