Ultrafast Dipole Solvation Measured in the Far Infrared

Ultrafast visible and near-UV pump, far-IR (terahertz) probe spectroscopy on the solvation dynamics of two dyes in polar solution is reported. In both cases, the pump pulse produces an excited state in the dye with a permanent dipole moment that is significantly different from that in the ground state. This dipole moment change induces changes in the solvent librational spectrum. Coherent beats in the data indicate that the solvent response is partially diffusive and partially impulsive demonstrating the inhomogeneity of the response.