Delayed Fluorescence
The following text supposes that the reader is familiar with the basic mechanisms and properties of chlorophyll fluorescence and the fluorescence induction. If you are not, please go first to the Chlorophyll Fluorescence section of this website.
What is delayed fluorescence?
Delayed fluorescence, which is also called delayed luminescence, delayed light emission, or afterglow, is the extremely weak light emitted by preilluminated photosynthetic samples. This phenomenon was discovered by Strehler and Arnold (1951) and was related by them with the processes of photosynthesis. The authors postulated that it is in fact chemiluminescence of the chlorophyll, caused by reversal of the photosynthetic reactions. The close relationship between DF and the photosynthetic reactions was confirmed undoubtedly in many studies and sometimes DF was found even more sensitive than the prompt fluorescence (see Kramer and Crofts, 1996). These reviews are also recommended to the interested reader: Lavorel et al., 1986, Jursinic, 1986, Radenović et al. 1994.
Mechanism of delayed fluorescence
Similar to PF, DF is emitted by the chlorophyll a molecules in the PS2 antennae. For that reason, DF has virtually the same spectral properties as PF and it is not possible to separate PF and DF by optical filters. However, when the illumination of the sample is stopped, PF vanishes almost instantly (in a matter of nanoseconds) while DF can be detected seconds, minutes, or even hours later.
Fig. 6. Mechanism of delayed fluorescence 
According to the recombination theory, the primary excited state of the pigments is converted into a meta-state, which has lower energy and longer lifetime. If this process is reversed, the meta-state can re-excite the pigment and a quantum of delayed fluorescence can be emitted by the re-excited molecule. In Photosystem 2, the excited reaction centre P680 reduces pheophytin which then transfers the electron to QA. At this step, part of the energy is lost, and the charge separation is stable - this is the meta-state. There is certain probability that the electron transfer is reversed and the electron is returned on P680 — charge recombination. The energy released in the charge recombination can excite P680 and the excited pigment can emit a photon of delayed fluorescence. The same reaction chain, but this time including QB, is represented in Fig. 6. Steps 1—5 are the forward reactions and steps 6—9 are the reverse reactions leading to DF emission.
Kinetics of delayed fluorescence
The afterglow of a leaf put in darkness is not constant but decays in time with the depletion of the meta-states accumulated during the illumination. There can be many different meta-states depending on the distance that the charges have travelled from the reaction centre pigment before returning back. The longer is that distance, the lower is the energy of the state, and the longer is its lifetime. Because of that, the DF decay is a complex time function with characteristic times in nanoseconds, microseconds, milliseconds, and so on. The DF registered in different time windows reflects different steps of the photosynthetic process. The most commonly registered, and the one registered by us, is the millisecond DF, which is a product of the charge recombination between Z+ and QA–.