ULTRAFAST STUDIES OF IMAGING PROCESSES

Interfacial electron transfer is the most fundamental process driving all imaging technologies. The advent of ultrafast lasers has led to th e development of novel experimental techniques to probe such dynamics. This article presents five time domain spectroscopies that allow dire ct measurement of different segments of the electron trajectory across a heterogeneous interface. Electro-optic sampling measures field-assi sted transport of carriers to the surface. Time-resolved two-photon ph otoemission enables measurement of electron relaxation at surfaces. Ti me-correlated single-photon counting, transient grating, and transient absorption techniques are implemented to determine electron transfer rates at interfaces. With these real-time approaches, the primary phot ophysical and photochemical processes at semiconductor/liquid interfac es and dye-sensitized semiconductors can be studied directly. The new information forthcoming from such studies is that electron transfer pr ocesses can be extremely fast at surfaces, in a range approaching adia batic coupling conditions between the delocalized bond states and disc rete molecular donor or acceptor states. This observation leads to a n ew conceptual framework for understanding photoinduced interfacial cha rge transfer processes.