Filming the Birth of Molecules and Accompanying Solvent Rearrangement

Abstract

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 I-2 molecules in two different solvents, CCl4 and cyclohexane. The birth and vibrational relaxation of I-2 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 similar to 4 angstrom and returns to the equilibrium distance (2.67 angstrom) in the ground state, and the first solvation cage expands by similar to 1.5 angstrom along the I-I axis and then shrinks back accompanying the structural change of the I-2 molecule.