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Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics
Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.ORCID iD: 0000-0001-9863-9811
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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Number of Authors: 292018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 1917Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2018. Vol. 9, article id 1917
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-156793DOI: 10.1038/s41467-018-04330-5ISI: 000432115300021PubMedID: 29765052OAI: oai:DiVA.org:su-156793DiVA, id: diva2:1211143
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2020-05-05Bibliographically approved
In thesis
1. X-Ray Investigations of the Liquid-Liquid Critical Point Hypothesis in Supercooled Water
Open this publication in new window or tab >>X-Ray Investigations of the Liquid-Liquid Critical Point Hypothesis in Supercooled Water
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental x-ray scattering studies on supercooled liquid water. A liquid-liquid transition between two structurally distinct configurations has been found in deeply supercooled water, indicating the existence of a liquid- liquid critical point. The experiments were performed at large-scale x-ray facilities, mostly using free electron x-ray lasers including PAL-XFEL in Korea, SACLA in Japan, LCLS in the USA, SwissFEL in Switzerland and European XFEL in Germany, as well as using synchrotrons including APS in the USA, PETRA III in Germany and ESRF in France.

Two conceptually different experimental approaches have been used to investigate the metastable phase of supercooled water. The first approach is based on rapid evaporative cooling of μm-sized water droplets that are injected into a vacuum chamber. Using this method, supercooled liquid water samples with temperatures down to approximately 227 K have been obtained, with the lowest temperature limited by homogeneous ice crystallization occurring after just a few milliseconds. In a second approach, structurally arrested high-pressure and therefore high-density amorphous ice samples are heated by an ultrafast infrared laser pulse. The fast heating melts the ice into a corresponding high-density liquid. At short time delays between the heating laser pulse and a subsequent x-ray probe pulse, the supercooled liquefied sample still experiences the high internal pressure of the initial state. At longer pump-probe delay times the supercooled water sample releases its internal pressure through structural relaxation. Hence, varying the pump-probe delay allows to probe the sample at different pressures.

Together, these two approaches have been used to access a region within the metastable phase diagram of supercooled water that has previously been inaccessible. Using elastic x-ray scattering measurements as a structural probe of the liquid, we identified the existence of a liquid-liquid phase transition in deeply supercooled water. The observed phase transition is interpreted as the transition between a high-density and a low-density liquid phase. At high pressure this phase transition is discontinuous or first-order like, featuring a characteristic double-peak feature in the observed x-ray scattering intensity of the first diffraction maxima. At ambient pressure, however, we observe a continuous shift of the first diffraction maxima that is consistent with a continuous or second-order phase transition between the two liquids. Further evidence of a continuous phase transition at ambient pressure is seen in the temperature dependent maxima of the measured correlation length, isothermal compressibility and heat capacity, which indicate the existence of a Widom line.

In summary, the experiments support the existence of a liquid-liquid critical point where the experimentally observed Widom line and phase coexistence line would both meet. The main result, however, is the first experimental observation of a liquid-liquid transition within a pure liquid.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2020. p. 58
Keywords
water, supercooled water, x-ray scattering, free electron x-ray laser, liquid-liquid phase transition, liquid-liquid critical point, x-ray speckle visibility spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-180847 (URN)978-91-7911-092-5 (ISBN)978-91-7911-093-2 (ISBN)
Public defence
2020-06-12, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, digitally via Zoom: https://stockholmuniversity.zoom.us/s/239996391, Stockholm, 13:00 (English)
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Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 7: Manuscript. Paper 8: Manuscript. Paper 9: Manuscript. Paper 10: Manuscript.

Available from: 2020-05-20 Created: 2020-04-23 Last updated: 2020-05-25Bibliographically approved

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