Femtosecond mid-infrared spectroscopy of liquid water and aqueous solutions
In this chapter we report on the results obtained on the dynamics of water and aqueous solutions with nonlinear femtosecond vibrational spectroscopic techniques like transient absorption spectroscopy, infrared photon-echo spectroscopy, and two-dimensional vibrational spectroscopy. These techniques allow the real-time study of the energy relaxation, hydrogen-bond and molecular reorientation dynamics of selected subensembles of water molecules.
For different isotopic variations of pure liquid water it is found that the excited molecular vibrations of the water molecule relax on a time scale ranging from 0.2 to 2 picoseconds, depending on the character of the vibration (stretch, bend). The hydrogen-bond dynamics and the molecular reorientation take place on two distinct time scales: on a time scale <100 femtoseconds associated with local variations in the length of the hydrogen bonds and librational motions, and a time scale of 1–3 picoseconds associated with the collective reorganizations of the hydrogen-bond network.
For aqueous solutions of ions and hydrophobic molecular groups it is observed that the water molecules hydrating these ions/molecular groups have 3–5 times slower reorientation and hydrogen-bond dynamics than the water molecules in bulk liquid water. For water molecules near interfaces like the surfaces of reverse micelles similar effects are observed. Finally, recent results on the role of water molecules in the transfer of protons and hydroxide ions through liquid water are described. It is found that water molecules actively participate in this transfer by conducting the charge of the proton/hydroxide ions over chains of hydrogen-bonded water molecules.