The proposed existence of a liquid–liquid transition (LLT) in supercooled water has sparked decades of debate. Recent pump–probe experiments interpret two peaks in the structure factor S(q) during and after decompression of high-density liquid (HDL) as evidence of coexistence with low-density liquid (LDL). However, this interpretation presents a fundamental puzzle: such coexistence is implausible at ambient pressure, below the estimated location of the liquid–liquid critical point (LLCP). Here we use decompression simulations with ML-BOP to reconcile this contradiction. Even when water decompresses along the LLT, S(q) retains a single peak because HDL and LDL domains remain nanoscopic. We explain the two-peak S(q) observed experimentally as a single evolving liquid peak superimposed to a static high-density amorphous (HDA) background. The simulations reveal that the decisive LLT signature is a transient growth and decay of the apparent correlation length at low q, which emerges only when decompression proceeds along the LLT and then Widom line, with maximum near the LLCP. Importantly, remains low when decompressing from Tc or above, or too rapidly. The experimental signatures could be explained by an exponential pressure drop to the LLT in ~10 ns, growth of as LDL domains develop, peaking near the LLCP at ~50 ns, and subsequent entry into the single-phase regime, from which crystallization proceeds. Our findings resolve the contradiction between the LLCP location and structural signatures, identifying low q region of S(q) evolution—not peak splitting—as the key structural marker of the LLT in water.
Kumar et al. (Tue,) studied this question.