Singlet excitation, processes occurring after

Figure 18-13 Physical processes that can occur after a molecule absorbs an ultraviolet or visible photon. S0 is the ground electronic state. S, and T, are the lowest excited singlet and triplet electronic states. Straight arrows represent processes involving photons, and wavy arrows are radiationless transitions. R denotes vibrational relaxation. Absorption could terminate in any of the vibrational levels of S,. not just the one shown. Fluorescence and phosphorescence can terminate in any of the vibrational levels of Sq.

The various light-emission process are best described with reference to the Jablonski diagram. The diagram for a simple carbonyl compound is given in Fig. 5. The absorption process leads to the formation of excited singlet states (Sj, 2, etc.). A very rapid depopulation of the upper excited states occurs by internal conversion through vibrational relaxation processes. After deactivation to the first excited singlet state (Si), several procrases are possible for the molecule to reduce... 

Figure 4.4 Schematic illustration for competing radiative, transfer and migration processes which can occur after singlet excitation. Energy quantum absorbed by the chromophore can be dissipated by single chromophore, monomer emission, hVf, or by dimer chromophore, excimer emission, hv, or it can migrate along the polymer chain, and/or the excitation energy can be transferred to an acceptor molecule.

A schematic summary of the most important photophysical processes occurring in molecules is given in Fig. 4.2 After the absorption (A) of one photon the molecule is promoted from its electronic ground state to an electronic excited state. Because for most molecules the ground state is a singlet state, we denoted... 

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