Ultrafast vibrational and structural dynamics of the proton in liquid water

The dynamical behavior of excess protons in liquid water is investigated using femtosecond vibrational pump-probe spectroscopy. By resonantly exciting the O-H⁺ -stretching mode of the H₉O₄⁺ (Eigen) hydration structure of the proton and probing the subsequent absorption change over a broad frequency range, the dynamics of the proton is observed in real time. The lifetime of the protonic stretching mode is found to be approximately 120 fs, shorter than for any other vibration in liquid water. We also observe the interconversion between theH₉O₄⁺ (Eigen) and H₅O₂⁺ (Zundel) hydration structures of the proton. This interconversion, which constitutes an essential step of proton transport in water, is found to occur on an extremely fast (<100 fs) time scale.