Filming the Birth of Molecules and Accompanying Solvent Rearrangement

Molecules are often born with high energy and large-amplitude vibrations. In solution, a newly formed molecule cools down by transferring energy to the surrounding solvent molecules. The progression of the molecular and solute–solvent cage structure during this fundamental process has been elusive, and spectroscopic data generally do not provide such structural information. Here, we use picosecond X-ray liquidography (solution scattering) to visualize time-dependent structural changes associated with the vibrational relaxation of I₂ molecules in two different solvents, CCl₄ and cyclohexane. The birth and vibrational relaxation of I₂ molecules and the associated rearrangement of solvent molecules are mapped out in the form of a temporally varying interatomic distance distribution. The I–I distance increases up to ∼4 Å and returns to the equilibrium distance (2.67 Å) in the ground state, and the first solvation cage expands by ∼1.5 Å along the I–I axis and then shrinks back accompanying the structural change of the I₂ molecule.