The light intensity and concentration dependence of the photoproduct yield are investigated in a monophotonic process, The relationship of the photoproduct yield with the laser intensity and the complex concentration for a monophotonic process is derived under laser flash photolysis. The relationship is confirmed experimentally in a monophotonic process, i.e., triplet-triplet transition for a Cu(I) complex Cu-6(DMNSN＇)(6) (DMNSN＇ = 4,6-dimethylpyrimidine-2-thiolate). At low light intensity, the relationship can be approximated by a linear inverse square root dependence on the light intensity. Based on this equation, a method is proposed to determine the intrinsic back electron transfer rate constant kb,in photoinduced intermolecular electron transfer reactions, precluding the effect from the diffusional encounter pairs. The Marcus "inverted region" is observed by using the method in photoinduced back electron transfer reactions of [Au-2(dppm)(2)](ClO4)(2) (dppm = bis(diphenylphiosphino)methane) with a series of substituted pyridinium acceptors.