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One of the most widely used methods for registration of energy transfer is luminescence analysis based on measurement of various luminescent characteristics of donor and acceptor: fluorescence or phosphorescence intensity; decay kinetics, lifetime of excited state, polarization degree, excitation and emission spectra (both stationary and transient ones), etc. Nonluminescent methods are, however, also employed for energy transfer measurements. Primarily there are fast absorption methods permitting the kinetics of appearance and deactivation of electronically excited states to be registered in short time intervals (millisecond, microsecond, nanosecond, picosecond, and even femtosecond). For this purpose the laser technique is used, as a rule: first, the donor molecules are brought to the excited state by a powerful laser impulse, then a second laser produces a sounding impulse that registers the kinetics of disappearance of excited donors or enhancement of excited acceptors from optical density changes. At present it is possible not only to follow the kinetics of the process but also to register the transient absorption spectra of excited molecules.
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