Radiationless processes intramolecular vibrational relaxation

Early studies of the photophysical radiationless processes of molecular systems were carried out on molecules in condensed media, liquids, rigid matrices, and high-pressure gases. This experimental situation introduces the complication associated with the presence of the possible occurrence of a number of different competing photophysical relaxation processes in the same molecular system in a fashion that mimics the complexity of a full photochemical reaction scheme. In order to study the primary photophysical radiationless transitions, it is optimal to consider experiments in which only the elementary individual processes of interest appear. Such investigations often involve the experimental determination of radiationless transition rates in isolated collision-free molecules. " For instance, collision-free experiments enable the consideration of the important phenomena of electronic relaxation and intramolecular vibrational redistribution. Studies on isolated molecules have greatly contributed to our... 

For the selective enhancement of the wanted reaction channel by laser excitation of the reactants, the time span At between photon absorption and completion of the reaction is of fundamental importance. The excitation energy n hco (n = 1,2,...) pumped by photon absorption into a selected excited molecular level may be redistributed into other levels by unwanted relaxation processes before the system ends in the wanted reaction channel. It can, for instance, be radiated by spontaneous emission, or it may be redistributed by intramolecular radiationless transitions due to vibrational or spin-orbit couplings onto many other nearly degenerate molecular levels. However, these levels may not lead to the wanted reaction channel. At higher pressures collision-induced intra- or intermolecular energy transfer may also play an important role in enhancing or suppressing a specific reaction channel. 

Following electronic excitation of the fluorophore and the subsequent relaxation processes that lead to the lowest vibrational level of the Si excited state (see Section 4.2), systems in the Si D-B-A state can rapidly lower their energy by undergoing a radiationless intramolecular ET, see Scheme 26. ... 

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