Monomolecular phosphorescence

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

For low excitation conditions, i. e. where the monomolecular decay prevails 7tta[ ]2<< / o[T ]> time dependence of the phosphorescence intensity derived from Eq. (16) is ... 

From Eq. (3.19) it is apparent that the DF intensity varies quadratically with the triplet concentration and hence with excitation light intensity as long as the singlet state is deactivated by monomolecular decay. The DF intensity decays exponentially with a lifetime half of the phosphorescence lifetime rphos. 

Phosphorescence. This term is commonly used for the thermal release of electrons (holes) from traps followed by radiative recombination at luminescence centers. From eq. (1) we see that the phosphorescence should rise exponentially with T and decrease exponentially with the trap depth E. Depending on the probability for retrapping of the released electrons, the phosphorescence is described as monomolecular or bimolecular. 

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