Optical Spectroscopic Signatures of Hydrogen-Bond Ordering in Two-Dimensional Water

The phase diagram of nanoconfined monolayer water, as predicted by first-principles calculations, reveals fascinating states such as a hexatic phase and a superionic phase under high pressure. However, the strong coupling between the water layer and its confining substrate (e.g., graphene), combined with the challenges of maintaining ultrahigh pressures, obscures direct thermodynamic and structural measurements, making experimental identification of these phases exceptionally challenging. In this work, we overcome this obstacle by computing the optical absorption spectra for the solid, hexatic, and superionic phases of monolayer water using advanced ab-initio many-body perturbation theory (G₀W₀ and Bethe-Salpeter equation). We demonstrate that the reorganization of the hydrogen-bond network across these phases leaves distinct imprints on the spectroscopic features, including exciton peak positions, oscillator strengths, and linewidths. Our results provide crucial, quantitative spectroscopic benchmarks to guide the experimental identification of high-pressure phases in two-dimensional water using optical spectroscopy.