(382b) Role of Adsorbate-Adsorbate Interactions in Dynamics of Surface Bond Breaking

Dynamics of surface reactions, though closely related to kinetics, is primarily dealing with the motion of individual atoms and molecules. Unraveling dynamics of elementary surface steps has been a long-term goal in surface science because of its significance in many technological applications, such as semiconductor processing, molecular sensing, and heterogeneous catalysis. Developing a fundamental understanding about important physical interactions that govern dynamics of surface bond breaking and forming is crucial for design of chemical processes with unprecedented energy efficiency and selectivity.

Recently, a transiently populated, weakly adsorbed CO precursor state on Ru(0001) has been identified with real-time observation of surface dynamics induced by a optical laser. The selective pump-probe experiments with intense femtosecond x-ray pulses and electronic structure calculations that include van der Waals interactions allow us to unambiguously probe the electronic signature of this species and distinguish it from other modes of excited molecules. In this talk, we will focus on discussing effects of various adsorbates (H, O) on CO desorption dynamics from Ru(0001) as a first step toward understanding reaction mechanisms of CO oxidation and hydrogenation reactions. We show that the long-ranged adsorbate-adsorbate interactions on metal surfaces have dramatic effects on the CO desorption mechanism, i.e., precursor-mediated vs. direct pathways, predicted by ab-initio molecular dynamics simulations. The observation is supported with ultrafast pump-probe experiments and elucidated with free energy surfaces of CO desorption obtained from the potential of mean force and metadynamics simulations. We emphasize that probing dynamics of surface bond breaking contributes to understanding the nature of molecule-surface interactions and developing detailed kinetics.

1. Beye, M. et al. Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001). Phys. Rev. Lett. 110, 186101 (2013).

2. Dell’Angela, M. et al. Real-Time Observation of Surface Bond Breaking with an X-ray Laser. Science 339, 1302–1305 (2013).