  We present a novel method to measure transient photovoltage at nanointerfaces
using ultrafast electron diffraction. In particular, we report our results on
the photoinduced electronic excitations and their ensuing relaxations in a
hydroxyl-terminated silicon surface, a standard substrate for fabricating
molecular electronics interfaces. The transient surface voltage is determined
by observing Coulomb refraction changes induced by the modified space-charge
barrier within a selectively probed volume by femtosecond electron pulses. The
results are in agreement with ultrafast photoemission studies of surface state
charging, suggesting a charge relaxation mechanism closely coupled to the
carrier dynamics near the surface that can be described by a drift-diffusion
model. This study demonstrates a newly implemented ultrafast diffraction method
for investigating interfacial processes, with both charge and structure
resolution.
