F.Perakis, TJ.Lane, T.McQueen, J.Sellberg, M.Sikorski, S.Song, H.Ogasawara, D.Nordlund, F.Lehmkühler, W.Roseker, G.Gruebel, A.Nilsson

SLAC National Accelerator Laboratory

Water features a unique three-dimensional hydrogen bond network topology, which is responsible for many of the anomalous thermodynamic properties of the liquid. One of the open questions which we address in the current contribution relates to establishing experimentally the size and lifetime of the hydrogen bonded molecular arrangements, which have been proposed to be vastly heterogeneous. The recent development of femtosecond x-ray pulses in free electron lasers (LCLS, SACLA) allow to probe molecular structures with nearly atomic resolution. Furthermore, it has been proposed that one can go beyond high resolution static images and additionally extract the underlying dynamics by using X-ray Photon Correlation Spectroscopy (XPCS). Here we utilize this novel experimental capability to probe directly the equilibrium intermolecular dynamics of the water molecules.

We present the first ultrafast XPCS realization, applied on liquid water from ambient conditions to the supercooled regime. With XPCS one can go beyond pump-probe experiments and probe the true ground state dynamics of the oxygen atoms, which reflect directly the intermolecular motion of the water molecules. In the presented study we actually resolve the speckle pattern originating from the water molecules at a wide angle scattering, which arises from the rearrangement between first neighboring molecules. From the pronounced temperature dependence of the speckle pattern contrast we probe experimentally a very fast component within the pulse duration, which is in the order of 50 fs.