Journal of Luminescence 22 (1981) 273—284 273 North-Holland Publishing Company RADIATIVE ENERGY TRANSFER I. GENERAL EQUATIONS J.C. CONTE and J.M.G. MARTINHO Centro de Quirnica Fisica Molecular, Complexo I, Inst ituto Superior Técnico, 1000 Lisboa, Portugal Received 11 April 1980 Revised manuscript received 29 September 1980 A set of equations is derived which makes possible to study the radiative energy transfer process whereby the photons emitted by the energy donor are absorbed by the energy acceptor and so increase the efficiency of the overall energy transfer. It is shown that the coefficients describing the radiative transfer which appear in the expressions for the intensities of the energy donor and the energy acceptor are not the same, due to the fact that past of the fluorescence absorbed by the acceptor comes from radiation which is not detected as donor emission when there is no acceptor present. The general equations derived are applied to two particular cases commonly considered: measurements in reflec- tion, where the fluorescence emission is observed from the same face of the absorption and measurements in transmission where the fluorescence emission is observed from the opposite face of the cell. 1. Introduction It is well known [1,2] that in a binary liquid system containing two fluorescent molecules Y and Z it is possible to observe fluorescent emission from both species when the solution is excited with uv radiation of adequate energy, even when the optical density for absorption by one of the species is so low that it is valid to assume that all the excitation is absorbed by the other. This effect shows that an energy transfer process between an energy donor (which absorbs almost all the energy of excitation) and an energy acceptor is taking place. This energy transfer process may be either radiative if photons are emitted by the energy donor and subsequently absorbed by the acceptor or nonradia- tive if it takes place without any emission followed by absorption. That both processes may take place is widely accepted. The fact that radiative transfer is mainly important for solutions where the concentration of the energy acceptor is low and the quantum yield of the energy donor is high [3] has been used by several authors to choose the systems under study such as 0022-2313/81 /0000—0000/$02.50 © North-Holland