Structure dynamics of the proton in liquid water probed with terahertz time-domain spectroscopy

We study the hydration of protons in liquid water using terahertz time-domain spectroscopy and polarization-resolved femtosecond midinfrared pump-probe spectroscopy. We observe that the addition of protons leads to a very strong decrease of the dielectric response of liquid water that corresponds to 19±2 water molecules per dissolved proton. This depolarization results from water molecules (∼4) that are irrotationally bound to the proton and from the motion of water (corresponding to the response of ∼15 water molecules) involved in the transfer of the proton charge.