Ultrafast vibrational Stark-effect spectroscopy: exploring charge-transfer reactions by directly monitoring the solvation shell response.

We present the first implementation of transient vibrational Stark-effect spectroscopy as an ultrafast probe of solvation dynamics. The method is applied to the phototriggered intramolecular charge-transfer reaction of Betaine-30, where the vibrational Stark shifts of the nearby solvent molecules--arising from the change in the electrostatic environment--are measured using a three-pulse photon echo probe. This new experiment provides a direct subpicosecond measure of the chromophore's excited-state dynamics and back electron transfer as viewed from the solvent's perspective. We develop a simple ab initio model that offers semiquantitative prediction of the experimental Stark shifts.