We have measured the radical pair quantum yield of reaction centers of photosystem II of green plants and of the purple bacterium Rhodobacter sphaeroides as a function of excitation density using sub-picosecond pulses at 77 K. The saturation behavior of both RCs can be described consistently by taking into account stimulated emission and the geometry of the polarized (magic angle) laser experiment. Our results show that the quantum yield of charge separation for the PS II RC is very high, about 1, and that no annihilation occurs under these conditions.