Using in situ X-ray photoelectron spectroscopy measurements after reaction, we show that hydrogen-terminated Si(100) perturbs the bonding of physisorbed NH3 enabling a photochemical decomposition pathway at wavelengths different from those characteristic of either the molecule in the gas phase or the semiconductor bandgap. UV illumination only of gas phase NH3 at partial pressures from 0.1 to 100 Torr produced a maximum at 10 Torr in the N surface coverage. This is in good agreement with a model of the radical production rate showing that at this pressure the gas density balances the flux of photons at the surface with energies sufficient to dissociate NH3. UV illumination of both the gas phase and the surface produced a monotonic increase in the N coverage with pressure as well as coverages that were 3-10 times higher than when only the gas phase was illuminated. The amine saturation coverage scaled with the UV fluence at 10 Torr and 75 degrees C, reaching 6.9 X 10(14) atoms/cm(2) (similar to 1 N atom per Si surface atom) at 19 mW/cm(2) and 12 X 10(14) atoms/cm(2) (similar to 1.8 N per Si) at 35 mW/cm(2). Monochromatic illumination showed that the wavelengths driving deposition were not correlated with the Si bandgap, but instead were roughly the same as gas phase photodissociation (lambda < 220 nm). The primary driving force to replace the hydrogen termination with amine groups was direct photodissociation of NH3 molecules whose electronic structure was perturbed by interaction with the surface. Amine groups enhanced the surface reaction of water present as a contaminant in the source gas. These results show that molecules in weakly bound surface states can have a dramatic impact on the photochemistry.