Probing water with X-ray lasers

Abstract

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.

XFELs for structure and dynamics in biology

“…Apart from reduced radiation damage, the XFEL offers the advantage of improved time resolution. As a result, we have seen remarkable studies of the phase transitions in water at low temperature (Nilsson et al, 2016) and of photosensitive protein molecules by timeresolved pump-probe XFEL solution scattering (Arnlund et al, 2014;Kim et al, 2015). In the study by Arnlund and coworkers of the Blastochloris viridis reaction center, 500 fs time resolution and about 0.4 nm spatial resolution were obtained in the difference maps between the optically pumped and dark states, allowing a molecular movie to be obtained following photon excitation.…”

Ultrafast nuclear dynamics in the doubly-core-ionized water molecule observed via Auger spectroscopy

“…Because the derivative along the antisymmetric mode is zero by symmetry, the broadening due to antisymmetric stretching is not yet taken into account via Eq. (11). To also implement the effects of dissociation along the antisymmetric stretching mode, we have expanded the potentialenergy-surface difference up to the second derivative along the antisymmetric stretch mode, d 2…”

“…One of the possibilities to reduce the effect of nuclear dynamics, proposed in Ref. [11], refers to double-core-hole (DCH) spectroscopy. Since the lifetimes of the DCH states can be significantly shorter than the lifetimes of the corresponding single-core-hole states (SCHs) [12][13][14][15], one can expect a considerable reduction of the nuclear dynamics in DCH states.…”

Evaporating laminar microjets for studies of rapidly evolving structural transformations in supercooled liquids

“…time), while the high photon flux allows one to obtain a statistically significant diffraction image in one shot. While ultrashort X-ray laser pulses were recently employed to probe liquid water jets [25,81], their time structure and photon flux, accompanied by the possibility to focus such x-rays down to a nanometre-sized spot, can be further exploited to access the earliest stages of crystal nucleation and growth in a large variety of supercooled atomic and molecular liquids. The XFEL properties would also allow performing experiments based on the use of liquid jets that go beyond simple scattering.…”

“…Recently, a liquid jet in vacuum was successfully employed to investigate the structure and crystallization of deeply supercooled water [25,[78][79][80][81]. In particular, Sellberg et al [25] obtained the scattering structure factor S(q) by probing supercooled water droplets with ultrashort pulses from an X-ray free-electron laser (XFEL).…”