Structure and dynamics of aqueous solvation shells
We discovered that the dynamics of water molecules in aqueous solvation shells of ions can be studied with unprecedented selectivity using nonlinear femtosecond mid-IR spectroscopy. With this technique we observed that the hydrogen bonds between water molecules and halogenic anions fluctuate with a characteristic time constant of 10-25 ps (depending on the ion), which is 20-50 times slower than the hydrogenbond fluctuations in bulk liquid water. We also found that the water molecules in the solvation shells reorient on a time-scale that is ~3 times slower than in bulk. This reorientation is not due to rotation of the water molecules within the shell, but to orientational diffusion of the complete solvation structure. These results show that the first solvation shell of ions forms a surprisingly rigid and stable structure. In contrast, we have observed that the water dynamics outside the first solvation shell of the ion are only negligibly affected by the presence of ions. This result shows that ions do not act as structure breakers or makers of the hydrogen-bond structure of liquid water.

Anomalous temperature dependence of the vibrational relaxation of water
We found that the OH stretch vibrations of water show a very rapid relaxation with a time constant of 260 femtoseconds at 300 K. With increasing temperature, this relaxation becomes slower, a phenomenon which is now being acknowledged as one of the 41 anomalous properties of liquid water.

Energy dynamics of single embedded water molecules
We observed that a single water molecule embedded by acetone forms two fluctuating hydrogen bonds with the C=O groups of two neighbouring acetone molecules. We found that these hydrogen-bond fluctuations can tune the O-H vibrations of the water molecule into resonance, thereby enabling energy transfer between the two OH groups.

Dynamics of water in nanodroplets
We studied the dynamics of water in nanodroplets with a diameter of 1-10 nanometers (10-10.000 molecules). For a droplet with a diameter of ~1 nm, the OH-vibrational lifetime increases by a factor of 4 to ~1 picosecond compared to the bulk. We also found that the water layer at the surface of the nanodroplet forms a very rigid structure, whereas the water in the core of even a small nanodroplet shows the same orientational mobility as bulk liquid water. This result shows that the centre of a nanodroplet of water does not show an ice-like structure, as was commonly believed.

Proton-transfer reactions in liquid water
We studied the mechanism of acid-base reactions in water with femtosecond visible-pump mid-IR probe spectroscopy on an aqueous system of a photoacid and an accepting base. The conventional view of this reaction is that the acid and the base have to diffuse into direct contact to enable proton transfer. However, we found that proton transfer occurs primarily via Grotthuss conduction, through a hydrogen-bonded 'water wire' of 2-4 water molecules which connects the photoacid with the base.

We also found that the excitation of the second excited state of the OH stretch vibration leads to a strong delocalization of the hydrogen atom along the O-H...O hydrogen bond between two water molecules. An important implication of this finding is that this second excited vibrational state forms, energetically, the most favourable transition state for the autodissociation of water, i.e. the process in which two water molecules split spontaneously into H3O+ and OH-.

Future directions

Water near surfaces and biomolecules
We intend to investigate the translational and orientational dynamics of
water-solvating (bio)molecules and near surfaces. This study will clarify the role of water in determining the spatial structure of other molecular systems.

Aqueous proton transfer reactions
We will investigate the mechanism of proton transfer in water both for
equilibrium systems, such as protons dissolved in nanodroplets of water and in ice lattices, and for non-equilibrium systems, in which the release of a proton is triggered by light.

Direct detection of low-frequency dynamics
In the past the dynamics of low-frequency vibrations such as hydrogen bonds and conformational motions have mostly been studied by probing the response of high-frequency vibrational and/or electronic excitations. We intend to probe the low-frequency dynamics of aqueous (bio)molecular systems directly in the far-infrared (THz) region of the spectrum. For this purpose we will develop a sub-picosecond mid-infrared/far-infrared pump-probe technique that allows the direct probing of hydrogen-bond and conformational vibrations.

Time-resolved single-molecule spectroscopy
We propose to identify the dynamics of O-H and C=O vibrations at the
single-molecule level using a new visible/mid-infrared double-excitation scheme. The technique will provide information on the vibrational frequency fluctuations of (bio)molecules, thus enabling the study of molecular dynamics at the single-molecule level.