Separation of rotational diffusion and level kinetics in transient absorption spectroscopy

Transient absorption spectroscopy on electronic levels of molecules in the liquid phase is governed by population kinetics as well as rotational diffusion. The goal of transient absorption experiments has been the true level kinetics free of rotation. Moreover, to extract the rotational time from transient photodichroism experiments the knowledge of true population kinetics is instrumental. Three methods for separating rotational and level kinetics are described theoretically, and one of them is performed experimentally using a repetitive picosecond spectrometer for the measurement of rotational behaviour of fluorescein 27 in solvents of different viscosity.