Nanocatalysis and Femtochemistry - Research
Small, atomically defined metal clusters exhibit fascinating physical and chemical properties. In particular their ability to act as nanoscale catalytic reaction centers with size dependent activity for important chemical processes qualifies them as model systems for kinetic and spectroscopic investigations. Our experiments aim to reveal details of the hitherto unknown reaction mechanisms of these small metal nanoclusters with adsorbed molecules. The focus of our work is on the real-time 'Femtochemistry' observation of reaction dynamics in the gas phase and on surfaces with ultrafast lasers, as well as on the investigation of catalytic reaction mechanisms via gas phase kinetic measurements.
Chemical Reactions of Size-Selected Metal Clusters
Experimental setup gas-phase experiments
A radio frequency octopole ion trap is employed to investigate gas phase metal cluster reactions. Reaction kinetics by monitoring ion intensities as a function of stroage time in the trap.
One methane molecule will not be dehydrogenated by free gold dimer cations, but the cooperative interaction of two methane ligands initiates a catalytic ethylene formation cycle. The cycle is closed by the subsequent adsorption of a third methane molecule that cooperatively activates the release of ethylene. This detailed molecular picture of a new, highly selective cooperative C-C bond formation mechanism has been obtained by joint ab-initio calculations of Uzi Landman's group in Atlanta and our gas phase reaction kinetics measurements.
In addition, the femtosecond time resolved reaction dynamics of metal cluster complexes with molecular ligands can be investigated in the ion trap with a pump-probe laser experiment.
Ion trap reactivity studies reveal the formation and charge transfer induced photodissociation dynamics of noble metal complexes with benzene. In this case the first laser pulse excites one electron from the benzene 'substrate' to the metal cluster cation with five silver atoms which is subsequently neutralized. The dynamics of this cluster is probed with femtosecond time resolution by ionization with a second laser pulse.
Surfaces and Supported Clusters
Experimental setup surface experiments
Femtosecond-laser photoemission employing a time-of-flight spectrometer (TOF-PES) is applied to study ultra-thin metal oxide films on Mo(100) and small metal clusters on such oxide films.
Ultrathin metal oxide films (less than 10 atomic monolayers in height) and metal clusters supported on such oxide films exhibit fascinating new properties that can be controlled by adjusting the oxide layer thickness and the metal clusters size with atomic precision. Femtosecond laser photoemission spectroscopy reveals information about the electronic structure of such systems.
We employ pump-probe laser mass-spectrometry in a new approach (TOF-MS) to investigate the real-time photoreaction dynamics of molecular adsorbates on oxide films and supported metal clusters.
As an example, methyl iodide molecules adsorbed at submonolayer coverage on
an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption.