The theory of transient spectra obtained in pump-probe experiments for molecular systems with ultrafast chemical transitions

In this paper we develop a theory of transient spectra of molecular systems in which superfast chemical reactions proceed. The dynamics of chemical reactions is calculated in the framework of multi-channel stochastic model which includes the reorganization of the medium and high-frequency intramolecular vibrational modes. This model is well adapted for the calculation of the spectral dynamics as it initially operates with the particle distribution functions. To validate the theory we consider donor-acceptor pairs the excitation of which in the charge-transfer band is accompanied by ultrafast charge recombination in the ground state of the pair. In this case the photochemical transformation is reduced to the charge recombination that is the non-radiative transition of the pair into the electronic ground state. These transitions lead to the appearance of a new absorption band due to the absorption by the particles in the ground state, which returned there from the excited state. The band gives a positive contribution to the pump-and-probe signal and is located between two strong negative bands corresponding to bleach and excited state absorption. The conditions for the appearance of a positive signal in the total transient spectrum due to the ground state absorption are clarified. The results of simulations are compared with the transient spectra of betaine-30, previously studied experimentally.