Experimental evidence of a liquid-liquid critical point in supercooled water

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.