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TWO-PHOTON PHOTOEMISSION SPECTROSCOPY OF WET ELECTRON STATE AND FEMTOSECOND ELECTRON DYNAMICS ON TiO2 SURFACES

by Li, Bin

Abstract (Summary)
Combination of the femtosecond laser time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science preparation techniques provides the possibility to study the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on single-crystalline surfaces, such as TiO2. The nearly perfect, stoichiometric TiO2 surface is prepared by a standard surface-preparation protocol, while various UHV surface preparation methods are available to modify the stoichiometric surfaces by introducing defects and/or adsorbed molecules. Two-photon photoemission (2PP) spectroscopy with near ultraviolet (400 nm) femtosecond laser pulses are used to investigate systematically the work function, and the occupied and unoccupied electronic structure of TiO2 surfaces due to the presence of defects and adsorbates. Adsorbates e.g. O2, H2O, CH3OH are introduced onto TiO2 surfaces to investigate their interaction with the TiO2 surface, as well as the ultrafast interfacial charge transfer dynamics. O2 molecules act as electron acceptors and titrate (heal) the surface O atom vacancy defects. H2O acts as an electron donor and forms a monolayer structure with an effective electric-dipole of 0.5 D pointing outwards. More remarkably, at ~1 monolayer coverage of water with minority -OH species present on TiO2 surfaces, an unoccupied state of 2.45 eV above the Fermi level is observed. Density functional theory shows this to be a ¡°wet-electron¡± state, representing the lowest energy nonadiabatic electron transfer pathway through the interface. The decay of the wet-electron state through the reverse charge transfer occurs within 15 femtoseconds, faster than the dielectric response time scale of the H2O overlayer. Similarly, the chemisorption of CH3OH molecules on TiO2 surfaces induces a related resonance at 2.3 0.2 eV above the Fermi level. Following the injection of electrons into the CH3OH overlayer we can follow by pump-probe measurements the ultrafast dielectric response of the interface leading to the solvation of injected electrons. Surprisingly, the solvation dynamics exhibit a strong deuterium-isotope effect. The excess charge is stabilized by the structural reorganization of the interface involving the inertial motion of substrate ions (polaron formation), followed by slower diffusive solvation by the molecular overlayer. According to the pronounced isotope effect on the electron lifetime, this motion of heavy atoms transform the reverse charge transfer from a purely electronic process (nonadiabatic) to a proton-coupled electron transfer (PCET) regime on ~30 fs time scale.
Bibliographical Information:

Advisor:Jeremy Levy; Daniel Boyanovsky; Hong Koo Kim; Hrvoje Petek; David Snoke

School:University of Pittsburgh

School Location:USA - Pennsylvania

Source Type:Master's Thesis

Keywords:physics

ISBN:

Date of Publication:09/29/2006

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