DOI: 10.5061/dryad.pc866t23j
title: Experimental evidence of a liquid-liquid critical point in supercooled water

description: The experimental search for the liquid-liquid critical point in water is
 challenging due to rapid ice crystallization. We studied supercooled water
 at time scales prior to ice formation by isochoric heating of high- and
 low-density amorphous ices using infrared ultrafast laser pulses, followed
 by x-ray scattering. By varying the pump laser fluence, we accessed liquid
 states spanning the region of the predicted critical point. Our results
 revealed a crossover from a discontinuous to a continuous transition at
 which broad and slow structural variations occurred, consistent with
 critical fluctuations and slowing down.  We also observed a rapid
 increase in the heat capacity consistent with a critical divergence at
 210±10 K coincident with enhanced density fluctuations.  These
 results suggest that our experiments have directly probed states in the
 vicinity of a critical point in supercooled water.
description: # Experimental evidence of a liquid-liquid critical point in supercooled
 water Dataset DOI:
 [10.5061/dryad.pc866t23j](https://doi.org/10.5061/dryad.pc866t23j) ##
 Description of the data and file structure Pump-probe measurements using
 infrared and x-ray laser beams were performed on high-density amorphous
 (HDA) ice and low-density amorphous (LDA) ice samples. The samples were
 isochorically heated by the IR laser and then decompressed. The X-ray
 scattering patterns were measured along the decompression process.  ###
 Files and variables #### File: main_figure_data_fig2.xlsx
 **Description:** X-ray scattering during the decompression process of HDA
 at different laser fluences. Wide angle x-ray scattering (WAXS)
 intensities around the first peak and small angle x-ray scattering (SAXS)
 intensities at different delays after laser pumping.   (A) 3 J/cm^2^ WAXS
 (B) 3 J/cm^2^ SAXS (C) 11 J/cm^2^ WAXS (D) 11 J/cm^2^ SAXS (E) 15
 J/cm^2^ WAXS (F) 15 J/cm^2^ SAXS (G) 20 J/cm^2^ WAXS (H) 20 J/cm^2^ SAXS
 ##### Variables * *q* in Å^-1^ unit * *I* (*q*) in arbitrary unit (a. u.)
 - After the pump pulse, the scattering intensity changes as a function of
 time. - The changes are q-dependent, meaning they reflect structural
 rearrangements at specific length scales. - Early times (13–50 ns) already
 differ from the “before pump” curve represents the system responds very
 fast. - Later times (200 ns–1 µs) show partial relaxation or evolution
 toward a new state. * The pump excites the system (laser, thermal,
 electronic, etc.). * This causes short-range structural changes that
 modify how the sample scatters. * The persistence of changes up to 1 µs
 suggests non-instantaneous recovery. #### File: main_figure_data_fig3.xlsx
 **Description:** Determination of temperature reached after laser heating
 of LDA. (A) The shift of the ice Bragg peaks after crystallization as a
 function of laser fluence. Mean values and standard deviations from
 approximately 10 measurements. (B_data) Laser energy versus the average of
 estimated temperatures.  (B_fit) The power-law fit results for the average
 temperature data shown in Fig. 3B. ##### Variables * Pump laser power in
 J/cm^2^ unit * Difference of peak position in Å^-1^ unit * Estimated
 temperature in K unit - As pump fluence increases, the peak position
 shifts. - The shift increases rapidly at low power and then saturates at
 higher power. - The decreasing standard deviation at higher fluence
 suggests more reproducible structural response. * The peak shift indicates
 a change in characteristic spacing (e.g., lattice spacing, intermolecular
 distance). * Saturation implies: * A maximum achievable structural
 distortion, or * A phase/state that cannot be further driven by higher
 fluence. This figure supports the idea that the response is nonlinear but
 bounded. #### File: main_figure_data_fig4.xlsx **Description:** X-ray
 scattering during the densification and decompression processes of LDA at
 different laser fluences. (A) 4 J/cm^2^ WAXS (B) 11 J/cm^2^ WAXS (C) 15
 J/cm^2^ WAXS (D) selected scattering shots from 15 J/cm^2^ (E) LDA 15
 J/cm^2^ 25 ns WAXS, HDA 15 J/cm^2^ 200 ns WAXS  ##### Variables * *q* in
 Å^-1^ unit * *I* (*q*) in arbitrary unit (a. u.) - Overall intensities are
 higher than Fig. 2 (note the larger baseline values). - Time-dependent
 changes are still present, but: * The relative magnitude * The relaxation
 behavior\ differ from Fig. 2. * This corresponds to: * A different sample
 * A different pump fluence * Or a different experimental condition
 (temperature, environment, composition) * Comparing Fig. 2 and Fig. 4
 shows how initial structure or excitation strength affects dynamics.