Temperature and density dependence of the light and heavy water ultraviolet absorption edge.

Characteristics of the ultraviolet absorption band edge of high-pressure light and heavy water are reported over the temperature range of 25-400 degrees C, extending into the supercritical regime. A gradual redshift in the absorption band edge of approximately 0.6 eV is observed with increasing temperature. This shift cannot be explained by vibrational hot band growth or changes in the degree of Rayleigh scattering with increasing temperature, and is ascribed to a shift of the electronic transition energy. The density dependence for the absorption edge in 400 degrees C supercritical water was also examined, and showed a surprising approximately 0.1 eV blueshift over a factor of 3.5 decrease in density. This shift may be due to a narrowing of the absorption spectrum with decreasing density. It is proposed that the previously reported "red tail" of the water absorption extending into the near ultraviolet and visible could be attributed to preresonant Rayleigh scattering, and that the true onset of liquid water absorption is approximately 5.8 eV at 25 degrees C.