Direct excitation of higher excited state and kinetics of photoreactions.

Excited-state reactions (ESR) play an essential role in chemical, physical, and biological processes. The mathematical models are usually used to study ESR in kinetics and steady-state regimes. In these models, the excitation pulse populates the first excited state (the first singlet level) of the primary molecular form. Recently, researchers' paid growing attention to the reactions excited via the higher energy levels. We modeled these reactions using the system of linear differential equations. Exact analytical expressions of the kinetics of N* and P* populations were derived for the general case when excitation performed via the higher S singlet state by the delta pulse. The graphical forms of these expressions were N and P time-dependent pulses. We detected the changes of the pulses' shapes, their maxima locations, the time behavior of the populations, and the total yield of the P* population. The changes occur due to the populating of the product excited state in the kinetic and thermodynamic reaction regimes. Numerical analysis performed for different ESR parameters revealed peculiarities of the N* and P* populations. Kinetics properties of these population characterize systems with varying rates of reversible ESR and various contributions of anti-Kasha (AK) reaction (from the S state) to P* population. Modeling data presented in graphical form, allowed to understand better (a) the impact of the AK reaction on the kinetic properties of the excited states of the molecular systems operating in various mode of ESR (kinetic, reversible and intermediate); (b) the photochemical processes' mechanisms. Also, this modeling allowed establishing the criteria for revealing the effect of the AK reaction for improving the efficiency of anti-Kasha processes.