Ultrafast photoreactions are governed by multidimensional excited-state Potential Energy Surfaces (PESs), which describe how the molecular potential varies with the nuclear coordinates. Nature has tailored electronically excited PESs, in which the molecular geometry is specifically modified from the ground state equilibrium configuration to efficiently convert the absorbed light energy into specific nuclear rearrangements, driving the system photochemistry. This can be rationalized by the displacements between two different PESs, crucial quantities that are encoded in the Franck-Condon overlap integrals. Conventional spectroscopic approaches probe transition amplitudes, only accessing the absolute value of nuclear displacements; herein we introduce an experimental technique, based on broadband impulsive Raman response to directly measure the magnitude and the sign of excited state displacements, revealing the first steps of photoreaction processes.
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