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The time dependence of the delayed light in the green alga Chlorella pyrenoidosa has been examined quantitatively in the 1 to 12 msec range after excitation with light pulses (A = 6328 A) of 100 psec and 4.5 msec duration. We have confirmed the data of Tollin, Ruby, and Bertsch et a / . , on the time course of the delayed light in the msec range. New experiments, with 100 psec flash excitation, on the time dependence of the delayed light emitted by ChloreNu treated with 3-(3,4-dichlorophenyl)-I , I -dimethylurea (DMU), hydroxylamine, methyl violgen, and various combinations of these chemicals are presented. Also, data on the dependence of the delayed light intensity on the intensity of the excitation light in the 1.5 and 5.0 msec range are reported. The square law dependence, reported by Jones, in the 140 and 250 msec range is confirmed in the 1.5 and 5.0 msec range at very low light levels.The experimental data on delayed light has been analyzed in terms of a model which incorporates triplet exciton fusion. The following major points result from this analysis: (1) A triplet exciton kinetic model can explain both the time dependence and the excitation intensity dependence of the delayed light emitted by Chlorella.(2) The density of triplet excitons predicted by the model from the observed delayed light intensity is much less than that which can be detected by flash photolysis measurements. Therefore, the failure of such measurements to detect triplet states in vivo does not disprove the model. (3) The possibility of changes in the rate of electron transfer reactions of photosynthesis is included in the kinetic model. The predictions from the model are compared with the effects of chemical additives on the time dependence of the delayed light decay. (4) The proposed triplet exciton model predicts that the delayed light intensity may, under certain specific conditions, be affected by a magnetic field. The negative result of an attempt to observe this effect is reported and discussed. ( 5 ) It is concluded that the proposed triplet 'fusion' model is a valid alternative to the electron-hole recombination model. A. I N T R O D U C T I O N THE CHLOROPHYLL molecules in green plants emit delayed light [ 11. Since its discovery by Strehler and Arnold in 195 1 [2], many workers have attributed this emission to an electron-hole recombination process in the chloroplast [3-81. Others have suggested that triplet states of chlorophyll may be involved[9-121. In a process analogous to electron-hole recombination two triplet excitons might annihilate (triplet exciton fusion) to produce an excited singlet state which could decay with the emission of a photon. This mechanism has been well established for a number of organic solids and solutions [ 131. The resulting light would have the same spectral characteristics as the normal prompt fluorescence but a slower decay time due either to the longer lifetime of the triplets relative to that of the singlets, or to a slow rate of triplet formation.

Blue-light-excited delayed fluorescence is used to study the temperature dependence of monomolecular intersystem crossing in crystalline anthracene. The associated rate constant k is found to obey an equation of the form k = k1 + k2exp(− E / kT), where E = 900 ± 100 cm−1 and k2 / k1 = 800 ± 200. A calculation of the relevant Franck–Condon factors using the method developed by Siebrand gives good agreement with experiment. At temperatures less than 90°K, the delayed fluorescence appears to be dominated by trapping effects similar to those reported in red-light-excited measurements.