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Center for Nonlinear Studies |
An ability to model photoinduced energy transport between anharmonicaly coupled vibrational modes in polyatomic molecular systems in condensed phase is crucial for understanding and controlling a verity of photo-induced chemical reactions as well as photo-physical processes, e.g., light harvesting in photosynthetic antenna complexes, T-jump protein folding/unfolding, and optically induced detonation of energetic materials. Such phenomena occur on sub-picosecond to nanosecond time scale, and can be efficiently initiated, manipulated, and probed using ultra-fast time resolved optical and infrared laser techniques. To model the energy transport pathways, we have developed and implemented as a computational tool, the first principles (DFT) based approach for the anharmonic potential energy surfaces reconstruction in polyatomic materials with periodic translational symmetries. As a result the anharmonic couplings between the phonons and/or vibrons can be extracted and the energy transport pathways can be identified. The knowledge of the anharmonic couplings should further allow us to simulate the infrared and Raman frequency-resolved vibrational spectra as well as the time-resolved nonlinear, e.g., photon-echo, responses. The comparison of these spectra with available experimental results should provide method validation, and should help one to gain an insight into the hidden microscopic aspects of the experimentally probed dynamics. The applications to pentaerythritol tetranitrate (PETN) molecular crystal, an important energetic material, will be discussed to illustrate the approach.