ULTRAFAST NONLINEAR-OPTICAL STUDIES OF SURFACE-REACTION DYNAMICS - MAPPING THE ELECTRON TRAJECTORY

Heterogeneous electron transfer involves the coupling of a dense manif old of highly delocalized electronic levels of the solid state to a di screte molecular state as well as an abrupt change in phase in the rea ction coordinate. These features make this problem unique relative to homogeneous solution phase or gas phase reaction mechanisms which invo lve coupling between discrete states within a uniform medium. Recent a dvances in time domain optical methods are discussed in the context of studying interfacial charge transfer processes at single crystal semi conductor surfaces as a means to probe the primary processes governing heterogeneous electron transfer. Two distinct boundary conditions are discussed: charge injection into a semiconductor from an adsorbate an d charge emission from a semiconductor to an acceptor. The reaction dy namics are investigated using a combination of nonlinear spectroscopie s with an emphasis on mapping the electron transport and transfer and investigating the role of nuclear vs electronic relaxation mechanisms in the barrier crossing dynamics. A fundamental understanding at this level seeks to determine the criteria for fully optimizing charge sepa ration at surfaces.