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Fundamental electrical, optical, and photchemical properties of solid-state surfaces are determined by the dynamical response of carriers to internal and external fields. The near-equilibrium properties of carriers in most materials are well understood from classical studies of transport and optical conductivity. Due to strong interactions of carriers among themselves and with the lattice, studies of nonequlibrium dynamics on femtosecond time scales (10 - 15 s) are just emerging. In particular, we are interested in using nonequilibrium excitations of electrons and holes in metal and semiconductor surfaces to drive surface femtochemistry. In our group, a particularly versatile and powerful technique, time-resolved two-photon photoemission (TR-2PP) spectroscopy, has been developed for studying the carrier excitation and relaxation processes in solid-state materials. From the dependence of the energy and momentum distributions of photoemitted electrons were are able to study the optical coherence in photoexcitation at solid surfaces, the unoccupied electronic structure of pristine and molecule modified surfaces, and changes in the surface electronic structure in response to surface photochemistry. In particular, we are interested in photocatalytic processes related to splitting water into H2, or a reduction of CO2 to hydrocarbon fuels. We are studying the unoccupied electronic states and interaction of holes of protic solvent covered TiO2 surfaces, where we have found evidence for ultrafast proton coupled electron transfer.
Understanding of the carrier dynamics under quantum confinement is a key to advancing nanoscale science and technology. Although with the existing scanning probe techniques we can potentially study dynamics of individual nanostructures, there is also a clear need for ultrafast imaging microscopic techniques in studies of dynamics in complex systems of nanostructures that could comprise ultrafast electronic or optical device. Photoemission electron microscopy (PEEM) is a well-developed surface science technique for imaging nanostructures on metal and semiconductor surfaces. By combining femtosecond pump-probe excitation with PEEM, capable of imaging with <10 fs time and 50 nm spatial resolution. The time-resolved PEEM method allows us to record movies of electromagnetic modes of a metal/vacuum interface, i.e., surface plasmon polaritons, propagating at near the speed of light in vacuum. We are studying the fundamental properties of surface plasmon polaritons and their interactions with nanofabricated metal structures. Using the lithographic techniques available at the Petersen Institute for NanoScience and Engineering we fabricate plasmonic optical elements and test their function by imaging their interactions with optical fields. In 2012, we plan to upgrade our PEEM microscope to an instrument, which will be capable of Low -energy Electron Microscopy (LEEM) and PEEM imaging with <10 nm resolution.
Imaging single molecule dynamics
With the support of the W. M. Keck Foundation, we are developing ultrafast single-molecule imaging techniques based on a combination of ultrafast laser excitation and scanning tunneling microscopy (STM) single molecule imaging. The goal is to use femtosecond pump-probe pulses with a variable delay to control current through a metal-molecule-metal tip junction. We expect that scanning the delay between the pump and probe pulses should allow us to follow the nuclear dynamics associated with the electronic excitation of the target molecule. So far we have used the STM imaging to discover atom-like (superatom) orbitals of C60 molecules and single molecule wide "molecular wires" of self-organized CO molecules. We have also demonstrated a single-molecule switch base on STM tunneling current induced motion of an internal cluster within an endohedral fullerene molecule. We are also pursuing theoretical studies of single molecule switches. For instance, we have described theoretically a single molecule electro-opto-mechanical switch based on internal femtochemistry within an endohedral fullerene, which is actuated by electron tunneling or photoinduced charge transfer.