Report：Excited electrons in interfacial chemistry

Professor: UCSB Prof. Alec Wodtke

Place:  A722 

Time：2009/6/22 , 10:30－11:30am

Excited electrons in interfacial chemistry

Alec M. Wodtke

Department of Chemistry and Biochemistry, UCSB

Developing a predictive understanding of surface reactivity incorporating the possible breakdown of the Born-Oppenheimer approximation represents one of the most important challenges to current research in chemical dynamics. Furthermore, to the extent that Born-Oppenheimer breaks down, we have no predictive theory of surface chemistry. This means we are working in an exciting environment where new phenomena might be discovered through experiments and inspire new theoretical developments. This lecture will present recent experimental results from our group that bears on this topic. For example, when molecules with low levels of vibrational excitation collide with metal surfaces, vibrational coupling to electron-hole pairs is not found to be strong unless incidence energies are high. However, there is accumulating evidence that coupling of large amplitude molecular vibration to metallic-electron degrees-of-freedom can be strong and becomes more important at reduced incidence translational energies. This can occur due to charge transfer between the surface and the molecule and the high kinetic energies associated with bond compression/formation. These conditions found in these experiments simulate reaching a chemical transition-state also and we are intrigued to pose the basic question: are electronically non-adiabatic couplings important at transition-states of reactions at metal surfaces? This implies theoretical approaches relying on the Born-Oppenheimer approximation may not accurately reflect the nature of transition-state traversal in reactions at metal surfaces. In related work, we have been looking at the energy transfer processes between molecules and surfaces that enable molecular trapping, the first elementary step in “Langmuir-Hinshelwood” reactions.  translational energies.We find remarkably large amounts of translational energy can be channeled to a metal surface and are able to look at this for different quantum-state to state scattering channels. While our results require additional theoretical comparisons, it appears that phonon coupling may not be sufficient to explain these experimental observations.