Blueshifting the Onset of Optical UV Absorption for Water under Pressure

First-principles calculations show that the optical UV absorption onset of solid water is blueshifted with increasing pressure. Across several crystal structures and a wide pressure range, the optical gap increases almost linearly with external pressure, making solid water more transparent. The origin of this unusual effect can be traced back to an increased Stark shift caused by water’s electrostatic environment at smaller volumes.

Figure 1

Energy-volume relations for all ice phases studied here. Small symbols indicate data points, solid lines indicate equation of state fits, and black circles indicate some of the points where optical spectra were calculated.

Figure 2

Many-body optical absorption spectra of various ice crystal structures under pressure. Spectra are offset vertically for clarity.

Figure 3

Upper panel: Optical band gaps of spectra from Fig. 2 (filled symbols, solid lines) and from electric field calculations with fixed (hashed symbols, dotted lines) and scaled (open symbols, dashed lines) point charges, all plotted against external pressure. Lower panel: Occupied (black lines) and unoccupied (red lines) Hartree-Fock energy levels of water in vapor and molecular crystal structures, with a single-particle gap increase relative to vapor phase.