title: Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature.
abstract: Water has a number of anomalous physical properties, and some of these become drastically enhanced on supercooling below the freezing point. Particular interest has focused on thermodynamic response functions that can be described using a normal component and an anomalous component that seems to diverge at about 228 kelvin (refs 1-3). This has prompted debate about conflicting theories that aim to explain many of the anomalous thermodynamic properties of water. One popular theory attributes the divergence to a phase transition between two forms of liquid water occurring in the 'no man's land' that lies below the homogeneous ice nucleation temperature (TH) at approximately 232 kelvin and above about 160 kelvin, and where rapid ice crystallization has prevented any measurements of the bulk liquid phase. In fact, the reliable determination of the structure of liquid water typically requires temperatures above about 250 kelvin. Water crystallization has been inhibited by using nanoconfinement, nanodroplets and association with biomolecules to give liquid samples at temperatures below TH, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear. Here we demonstrate that femtosecond X-ray laser pulses can be used to probe the structure of liquid water in micrometre-sized droplets that have been evaporatively cooled below TH. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of 227(-1)(+2) kelvin in the previously largely unexplored no man's land. We observe a continuous and accelerating increase in structural ordering on supercooling to approximately 229 kelvin, where the number of droplets containing ice crystals increases rapidly. But a few droplets remain liquid for about a millisecond even at this temperature. The hope now is that these observations and our detailed structural data will help identify those theories that best describe and explain the behaviour of water.
authors.affiliation: 1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA [2] Department of Physics, AlbaNova University Center, Stockholm University, S-106 91 Stockholm, Sweden.
authors.forename: J A
authors.initials: JA
authors.lastname: Sellberg
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: C
authors.initials: C
authors.lastname: Huang
authors.affiliation: 1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA [2] Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: T A
authors.initials: TA
authors.lastname: McQueen
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: N D
authors.initials: ND
authors.lastname: Loh
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: H
authors.initials: H
authors.lastname: Laksmono
authors.affiliation: Department of Physics, AlbaNova University Center, Stockholm University, S-106 91 Stockholm, Sweden.
authors.forename: D
authors.initials: D
authors.lastname: Schlesinger
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: R G
authors.initials: RG
authors.lastname: Sierra
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: D
authors.initials: D
authors.lastname: Nordlund
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: C Y
authors.initials: CY
authors.lastname: Hampton
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: D
authors.initials: D
authors.lastname: Starodub
authors.affiliation: 1] Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany [2] Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: D P
authors.initials: DP
authors.lastname: DePonte
authors.affiliation: 1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA [2] Institute for Methods and Instrumentation in Synchrotron Radiation Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Wilhelm-Conrad-Röntgen Campus, Albert-Einstein-Strasse 15, 12489 Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Beye
authors.affiliation: 1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA [2] Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: C
authors.initials: C
authors.lastname: Chen
authors.affiliation: Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: A V
authors.initials: AV
authors.lastname: Martin
authors.affiliation: Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Barty
authors.affiliation: Department of Physics, AlbaNova University Center, Stockholm University, S-106 91 Stockholm, Sweden.
authors.forename: K T
authors.initials: KT
authors.lastname: Wikfeldt
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: T M
authors.initials: TM
authors.lastname: Weiss
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: C
authors.initials: C
authors.lastname: Caronna
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: J
authors.initials: J
authors.lastname: Feldkamp
authors.affiliation: Mineral Physics Institute, Stony Brook University, Stony Brook, New York, New York 11794-2100, USA.
authors.forename: L B
authors.initials: LB
authors.lastname: Skinner
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: M M
authors.initials: MM
authors.lastname: Seibert
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: M
authors.initials: M
authors.lastname: Messerschmidt
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: G J
authors.initials: GJ
authors.lastname: Williams
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: S
authors.initials: S
authors.lastname: Boutet
authors.affiliation: Department of Physics, AlbaNova University Center, Stockholm University, S-106 91 Stockholm, Sweden.
authors.forename: L G M
authors.initials: LG
authors.lastname: Pettersson
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.
authors.forename: M J
authors.initials: MJ
authors.lastname: Bogan
authors.affiliation: 1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA [2] Department of Physics, AlbaNova University Center, Stockholm University, S-106 91 Stockholm, Sweden [3] Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, PO Box 20450, Stanford, California 94309, USA.
authors.forename: A
authors.initials: A
authors.lastname: Nilsson