Ricerc@Sapienza

Ultrafast light-induced processes in molecular systems rely on the efficient conversion of photon energy into atomic motions and are tightly controlled by vibrational couplings between different normal modes. Upon light absorption the optical energy converted into vibrational motions is initially stored in an excited Franck-Condon manifold and is then funneled to different vibrational degrees of freedom. Vibrational Mode Coupling (VMC) is the bio-physical mechanism determining efficient and selective energy relaxation processes, characterizing the molecular multidimensional energy surface that describes how the potential energy surface (PES) of the system changes with modification of the geometrical configuration. Critically, establishing a spectroscopic protocol to access the VMCs that rule the ultrafast evolution of photoexcited molecules is typically hampered by the need of multidimensional spectroscopic probes detecting different energy scales with high temporal and frequency resolution and remains an open challenge. This project aims to precisely tackle this issue introducing a 2-Dimensional Femtosecond Coherent Raman Spectroscopy (2D-FCRS) scheme: the basic idea is to exploit a 8 fs pump pulse to selectively excite vibrational wave-packets in a desired electronic excited state, whose evolution is tracked by the joint action of a femtosecond probe pulse and a narrowband Raman pump. Since the evolution of these wave-packets is determined by the mode couplings in the vibrationally structured PES of the system, the proposed scheme will be the key to map the multidimensional energy surfaces involved in the process. 2D-FCRS will be applied to study the paradigmatic case of the Green Fluorescent Protein ultrafast dynamics, addressing the highly debated case of the excited state proton transfer in this essential bioimaging dye. We anticipate that the introduced approach will bring about impactful insights into the reaction dynamics of photo-active molecular compounds