Probing water with X-ray lasers

Here, we discuss three cases where the X-ray free-electron laser, the Linac Coherent Light Source, has been used to probe water. The ability to cool water very rapidly down to 227 K and to probe it with ultrashort (50 fs) X-ray pulses before freezing has allowed for investigating water structure below the previous limit of homogeneous ice nucleation. It was found that at the temperature where the thermodynamic response functions, such as heat capacity and isothermal compressibility, seem to diverge there is no discontinuous change in structure, but instead an accelerated transformation from a disordered state to a strongly tetrahedral. The ice nucleation rate in bulk-like micron-sized water droplets could be determined for temperatures down to 227 K and was shown to be ~8 orders of magnitude lower than previous data for nanodroplets. The application to X-ray spectroscopy is discussed based on measurements at high X-ray fluence where multiple valence-hole states can be generated. Finally, a perspective to the future is presented regarding X-ray photon correlation spectroscopy with which true equilibrium dynamical properties can be studied.