Surface Femtochemistry

Charge transfer excitation of adsorbed atoms molecules on metal or metal oxide surfaces can turn on chemical forces that lead to nuclear wave packet motion and possibly to chemical reaction.  We follow the photoinduced charge transfer dynamics by interferometric time-resolved two-photon photoemission (ITR-2PP) spectroscopy.  Alkali atoms represent some of the simplest adsorbates on metal surfaces with well-known electronic structure.  Photoinduced charge transfer excitation creates a neutral excited state atom at the position of the ionic ground state.  The repulsion experienced by alkali atoms leads to frustrated desorption on the repulsive excited state electronic surface.

In other experiments we study photocatalytic processes on TiO₂ surface.  We have identified the "wet electron" states as the lowest energy electron acceptor states on H₂O/TiO₂ surfaces.  Such wet electron states on CH₃OH/TiO₂ surfaces provide evidence for the decay of photoexcited electron population by proton coupled electron transfer (PCET).  The coupling of electron and proton motions at a protic solvent/metal oxide interface is the key to understanding photocatalysis.

We have recently discovered the t2g-eg transitions in TiO2 crystal orientation and other environmental effects, such as molecular adsorption.