ИСТИНА

Time-resolved spectroscopy is a powerful tool for studying excited-state dynamics in various objects. Amultitude of different experimental configurations allows to evaluate various dynamical characteristics ofa system and produces a number of experimental methods including pump-probe spectroscopy, four-wave mixing techniques, and others. Interpretation of results of non-linear spectroscopic experiments isnot straightforward and has to be performed via theoretical modeling. Most approaches to calculation ofnon-linear signal intensities use an interaction picture ansatz combined with the perturbation approachwith respect to electric field intensity and a number of subsequent simplifications which rely on certainassumptions regarding the system parameters.In this work an alternative approach to calculation of the nonlinear spectroscopic signal is presented. Fora system with a constant relaxation tensor, an ansatz based on the tensor eigenvalues allows to derive aclosed-form expression for the signal intensity in a system with any relation between the pulse duration,optical period and characteristic times of the system. This allows to avoid using the commonly usedapproximations such as the snapshot, slow envelope or rotating wave approximations. For two frequentlyused spectroscopic methods, namely pump-probe and three-pulse photon echo spectroscopy, theresulting closed-form expressions for the signal are provided explicitly with rotational averaging. Samplecalculations of non-linear spectroscopic response in a natural light-harvesting complex are provided asan illustration.