Microwave reflectance methods have been used to study the kinetics of photogeneration of hydrogen on p-Si
in acidic fluoride solutions. To estimate the rate constant for interfacial electron transfer, the magnitude of
the observed steady-state light-induced microwave reflectivity change has been compared quantitatively with
theoretical predictions based on numerical calculation of the electron and hole profiles in the silicon sample.
In addition, the transient and periodic microwave reflectivity responses to stepped and sinusoidally modulated
illumination, respectively, have been analyzed to obtain information about the electrode kinetics. The time
constants for relaxation of the light-induced conductivity derived from this analysis confirm that electron
transfer during hydrogen evolution is slow. The apparent inconsistency in the values of the phenomenological
electron-transfer rate constants derived from the steady-state and transient or periodic responses can be resolved
if it is assumed that hydrogen evolution proceeds via electron capture by protons followed by a slow bimolecular
step leading to molecular hydrogen. Surface charging as the result of slow kinetics is expected to lead to
band edge unpinning even at low light intensities, and the existence of this effect has been confirmed by
transient photocapacitance measurements.