Measuring the Change in the Intermolecular Raman Spectrum during Dipolar Solvation

We demonstrate a method to directly measure the change in the spectrum of intermolecular solvent fluctuations
as a function of time after electronic excitation of a solute, and this method is applied to the dye Coumarin
102 (C102) in acetonitrile. The complete intermolecular response is captured following resonant excitation
with time domain third-order Raman spectroscopy. In a previous report, we introduced this method and used
it to probe one point in the intermolecular response as a function of time after solute excitation (Underwood,
D. F., Blank, D. A. J. Phys. Chem. A 2003, 107 (7), 956). Here we extend this approach to recover the
change in the entire intermolecular response as a function of time. To our knowledge the results provide the
first direct measurement of the difference in the equilibrated intermolecular response after excitation of a
solute and its evolution during a dipolar solvation event. Excitation of C102 results in a significant increase
in the solvent−solute interaction due to a large increase in the dipole moment. The observed change in the
intermolecular response is consistent with a rapid change in local solvent density, with intermolecular kinetic
energy transfer changing the response on longer time scales. Evolution of the response exhibits a strong
frequency dependence and suggests changes over longer distances at longer delay times. The measured change
in the spectrum of solvent fluctuations represents a direct experimental confirmation of the breakdown of
linear response and confirms predictions from molecular dynamics simulations.