Kinetics of induced absorption of polyatomic molecules upon two-quantum excitation

We study the kinetics of triplet‒triplet absorption of polyatomic molecules (on the example of such halogen-substituted fluoresceins as eosin and erythrosin) upon stepwise population of their high triplet electronically exited states by radiation from two single-pulse lasers with a controlled delay between pulses. It is shown that the efficiency of competing channels for the relaxation of highly excited triplet states of molecules can be estimated from the kinetics of induced absorption. In the absence of energy transfer to the medium and photochemical reactions, the main channel for the relaxation of high triplet states of molecules containing heavy halogen atoms is the reverse triplet‒singlet intercombination conversion. When interpreting the results, it is necessary to take into account the thermal effects caused by the heating of the medium as a result of nonradiative relaxation of high electronically excited states of molecules.