Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics

Abstract

The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.

Double-pulse speckle contrast correlations with near Fourier transform limited free-electron laser light using hard X-ray split-and-delay

“…In our previous studies 42,50 , we measured a typical photon flux of 0.03 photons/pixel/pulse from gold nanopowder at q = 2.6 Å −1 . Subsequent studies have shown that it is still possible to obtain meaningful speckle contrast from such low intensity speckle, as it was demonstrated for scattering signals from amorphous solids and liquids using coherent x-rays after Si(111) monochromator 49,52 . While the narrower energy bandwidth of Si(422) reflection used in the split-and-delay helps to improve the longitudinal coherence, it decreases the photon flux considerably.…”

“…However, thanks to high energy resolution provided by delay line optics the speckle contrast will increase at high scattering angles. For instance at q = 1.95 Å −1 the expected contrast is 2.2 times higher compared www.nature.com/scientificreports www.nature.com/scientificreports/ to Si(111) optics used in 52 . Blue triangle in Fig.…”

“…where n px and n pt are number of pixels on the 2D detector and number of speckle patterns measured in an experiment, respectively. Figure 4 shows how the SNR in the speckle contrast varies as a function of mean photon intensity for various experimental conditions (β,⟨ ⟩ I , n px , n pt ) reported in high q static contrast measurements and XPCS studies at FEL 14,42,49,52 . The minimum signal-to-noise ratio required for XPCS is 5 53 which corresponds to 20% uncertainty of measured contrast β (see horizontal dashed line in Fig.…”

“…The blue square in Fig. 4 shows the SNR from a water sample achieved in 52 . Extending these studies to ps-ns time scales,e.g., a critical time scale to measure ISF of liquid water 54 , can be achieved using the split-and-delay line.…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…The scope of XPCS will be significantly enlarged with the availability of extremely brilliant synchrotron sources and even faster pixel array detectors. The feasibility of studying ultrafast dynamics using an alternative method-speckle visibility spectroscopy-has already been demonstrated at the X-ray free-electron laser (XFEL) [117]. However, this method has limitations when studying complex relaxation processes involving non-exponential g 2 (q, t) and non-monotonic q dependence.…”