Interfacial electron-transfer dynamics have been characterized for a heterogeneous semiconductor-liquid junction. Molecular beam epitaxy was used to grow As-capped GaAs (100) surface quantum wells. Removal of the As layer created a pristine, low defect, surface which was studied in situ in direct contact with an outer-sphere, ferrocene, redox couple using time-correlated single photon counting. The surface quantum well structure was used specifically to remove field-dependent transport effects from the photocarrier dynamics. Concentration-dependent decay profiles indicate that a significant fraction of electrons undergo electron transfer. A fit to the data gives an electron-transfer cross section of (2.4 +/- 0.8) x 10(-15) cm(2), which corresponds to charge transfer in the adiabatic coupling regime. This work illustrates that the electronic coupling between the solid state and molecular states at the surface can be sufficiently strong to produce adiabatic reaction conditions even for weakly physisorbed outer-sphere accepters.