Ultrafast dynamics in noble metal clusters: The role of internal vibrational redistribution

We present a theoretical study of the ultrafast dynamics in noble metal clusters interacting with molecular oxygen which is of fundamental importance for the understanding and design of cluster-based heterogenous nanocatalysts. We demonstrate that intrinsic dynamical properties can significantly promote the reactivity of small noble metal clusters towards O2. This concept is illustrated by performing collision simulations between Ag6- and Au6- clusters and O2 in the framework of the ab initio molecular dynamics (MD) using density functional theory (DFT). We show that different nature and efficiency of the internal vibrational energy redistribution (IVR) during the collisions with O2 are responsible for considerably different sticking probabilities of O2 to silver and gold clusters, respectively. In the case of Au6-+O2, resonant IVR between the cluster and the O2 subunit activates the O-O bond and promotes the subsequent oxidation reaction. In contrast, in the case of Ag6-+O2 fast dissipative IVR on the time scale of ∼1 ps leads to a higher sticking probability for O2 but the O-O bond is very rapidly deactivated and cannot participate in further oxidation processes. These findings allow us to introduce the nature of IVR as a criterion for promoting the reactivity of noble metal clusters. Such different behaviour of silver and gold clusters colliding with O2 originates from difference in relativistic effects which are considerably more pronounced in the case of gold clusters causing more directional rigid bonding in contrast to silver clusters with more s-metallic floppy character. Moreover, we demonstrate that breaking of O-O bond can be induced in Au6O2- by a selective excitation of the O-O bond with an ultrashort pulse in the infrared spectral range. This opens the perspective to control the action of nanocatalysts by employing shaped laser pulses and thus bridges the fields of femtochemistry and cluster nanocatalysis.