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The delayed luminescence consists in the prolonged in time and ultra-weak emission of optical photons by a system after the light source is turned off. The overall intensity of the DL is 103÷105 times less than that of the fluorescence and it is characterized by very long decay times up to hundreds of ms or even seconds.
The DL is so prone to noise contamination. Enough refined technics and expedients are therefore necessary to reveal it. The DL physically significant parameters are the total number of photons emitted and the dynamic decays of the spectral and integral (i.e. in whole visible range) emissions. The decay temporal trend accords often with a hyperbolic curve or with a linear combination of some decays hyperbolic even if, this trend exhibits a bi- or multi-modal behavior (sum of terms which are power laws) when  the interval of observation is prolonged in time. Obviously the hyperbolic fit is a practical way to express the experimental data and was used in the literature to express the trend of the DL, but there is no definitive theory in this regard. The assumptions made in the literature attribute this trend or to  bi-equimolecular recombination processes (for which dn/dt∝n2), or to the decay of a continuous set of  many equilibrium states whose decay constants are distributed according to appropriate probability density functions  (I (t)∝ ∫p (γ) exp (-γt) dγ), or even to the decay of a coherent field, existing within the cell, which would play an important role in the promotion and control of biological processes.

Figure 1 - Example of trends of DL decay for different biological systems (SPIE 3602 (1999))

The phenomenon of the DL, according to Jablonski diagram, is related to the existence of metastable or triplet states with energies lower than excited state of singlet. Phenomena of back reaction by the metastable state and / or intersystem crossing from the triplet state bidirectional lead to repopulation of the first excited state singlet, whose decay gives rise to the DL.
The research carried out by our group showed that the DL is associated to the structural characteristics of the target system. In fact, the measures carried out on the sulphide Cadmium (a polycrystalline material), and on the seaweed Acetabularia acetabulum (a unicellular organism of large dimensions and characterized by almost one-dimensional ordered structures constituting the cytoskeleton) have shown how to disintegrate the structures (in the case of cadmium) or to their damage (in the case of the alga) there is a decrease in the emission of DL, no fluorescence. (see "physical models" section for more details).


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