Liquid-liquid transitions under pressure Theoretical simulations suggest that deeply supercooled water undergoes a transition between high- and low-density forms, but this transition is difficult to study experimentally because it occurs under conditions in which ice crystallization is extremely rapid. Kim et al. combined x-ray lasers for rapid structure determination with infrared femtosecond pulses for rapid heating of amorphous ice layers formed at about 200 kelvin. The heating process created high-density liquid water at increased pressures. As the layer expanded and decompressed, low-density liquid domains appeared and grew on time scales between 20 nanoseconds and 3 microseconds, which was much faster than competing ice crystallization. Science, this issue p. 978 The transition of rapidly-formed compressed high-density liquid water layers into low-density water domains was followed. We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

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加压下散装过冷水中液-液转变的实验观察

压力下的液-液转变 理论模拟表明，深度过冷水会经历高密度和低密度形式之间的转变，但这种转变很难通过实验进行研究，因为它发生在冰结晶极快的条件下。金等人。用于快速确定结构的组合 X 射线激光器，使用红外飞秒脉冲快速加热约 200 开尔文形成的非晶冰层。加热过程在增加的压力下产生了高密度液态水。随着层的膨胀和减压，低密度液体域出现并在 20 纳秒到 3 微秒之间的时间尺度上增长，这比竞争性冰结晶快得多。科学，这个问题 p。978 紧随其后的是快速形成的压缩高密度液态水层向低密度水域的转变。我们使用红外飞秒激光器将高密度无定形冰等容加热至 205 ± 10 开尔文，在压力下制备了大量过冷液态水样品。由于样品密度在超快加热过程中保持不变，我们可以估计高密度液相中的初始内部压力为 2.5 至 3.5 千巴。加热后，样品迅速膨胀，我们用飞秒 X 射线激光脉冲在不同的泵浦探测延迟时间捕获了由此产生的减压过程。发生了不连续的结构变化，其中低密度液体域出现并在 20 纳秒到 3 微秒之间的时间尺度上增长，而结晶发生在 3 到 50 微秒的时间范围内。这两个过程的动力学相隔一个数量级以上，这为散装过冷水中的液-液转变提供了支持。