Real-Time Measurement of the Orientational Dynamics of Aqueous Solvation Shells in Bulk Liquid Water

We present a study of the orientational dynamics of aqueous solvation shells of halogenic anions in bulk water solution with femtosecond two-color midinfrared spectroscopy. These orientational dynamics have time constants between 2.5 and 12 ps, depending on the type of anion and the temperature. We find that the solvation shell of the larger ion $I^{-}$ shows faster dynamics than that of the smaller ions ${\mathrm{Cl}}^{-}$ and ${\mathrm{Br}}^{-}$.