  When redox active molecules are covalently tethered to the surface of the
n-doped silicon, light induces their oxidation by the semiconductor. To
visualize this charge separation we have followed the formation and decay of
the surface photovoltage by an auxiliary transparent electrode, which serves as
a capacitive probe that picks up the transient surface photovoltage. This
method has been used in electronics and biophysics but is novel for the surface
redox chemistry. The probe provides both the direction of the charge transfer
and the resolution of the dynamics. Its application is independent of the
optical properties of the semiconductor. Laser initiation of the reaction
permits quantitative measurements of the reactions with the rate constants
~108-103 s-1. The electron injection from covalently tethered NTCI(A) molecules
into silicon was fast (k >109 s-1). The back electron transfer is poly-disperse
with the components ranging from ~10 microseconds and up.
