Heterogeneous electron transfer involves the coupling of a dense manifold of highly delocalized electronic levels of the solid state to a discrete molecular state as well as an abrupt change in phase in the reaction coordinate. These features make this problem unique relative to homogeneous solution phase or gas phase reaction mechanisms which involve coupling between discrete states within a uniform medium. Recent advances in time domain optical methods are discussed in the context of studying interfacial charge transfer processes at single crystal semiconductor 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 and charge emission from a semiconductor to an acceptor. The reaction dynamics are investigated using a combination of nonlinear spectroscopies with an emphasis on mapping the electron transport and transfer and investigating the role of nuclear vs electronic relaxation mechanisms in the bamer crossing dynamics. A fundamental understanding at this level seeks to determine the criteria for fully optimizing charge separation at surfaces.

iv) This kind of heterogeneous system can be useful for some applications. Its use as a chemical sensor for the detection of gaseous oxidant has been d e v e l o~d ?~ Besides. in nonlinear oetics.Acknowledgment. This work has been partly supported by the Ministery of Research under the program on Molecular Engineering (MRES-19881.-. ;mall metallic clusters of A eiectrons can give rise to a ihird harmonic generation and a nonlinear refractive index.30Registry No. [EDTTTF(SC,,),], 117701-65-2; 12, 7553-56-2; behenic acid, 112-85-6.(29) Henrion, L.; Derost, G.; Barraud, A.; Ruaudel-Teixier, The carrier dynamics at dye-sensitized SnSz surfaces were studied using a variety of picosecond techniques. Fluorescence quenching studies of the rate of electron injection from adsorbed oxazine into the conduction band have determined the rate to be 3 X lo" s-I, corresponding to an electron-transfer time of 40 f 20 fs. The corresponding localization of the free carrier in the conduction band of the semiconductor is observed to occur in 1-10 ps. Picosecond pumpprobe studies of the ground-state recovery of the oxazine determine the back-electron-transfer process to occur on the 10-ps time scale. Theoretical justification for the initial electron injection time scale is offered using the complete molecular Liouville equation within the Mori-Zwanzig projection operator formalism.

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