The ultraviolet absorption spectrum of liquid water

The absorptivities of carefully purified water have been measured at 1 nm intervals in the wavelength range 196 to 320 nm. The measured absorptivities fell monotonically from a value of (1.26±0.03) m−1 at 196 nm, to a value of (0.0100±0.0006) m−1 at 320 nm. The water was purified by ion–exchange, followed by four subsequent distillations, the first being carried out under mild oxidizing conditions, and the last being performed in all-silica apparatus, the water vapor being heated to a temperature of 870 K in the presence of oxygen to remove residual organic impurities. The absorptivities were measured using a differential pathlength method and a correction for the effect of double reflections in the absorption cells was derived and applied to the data. The measured absorptivities were much lower than all previously reported values in the 200–300 nm region, but gave considerably better agreement with theory and aligned well with the edge of a set of recent vacuum UV measurements on high purity water. It appears that previously reported absorptivities of liquid water in the 200–300 nm region are too large by up to two orders of magnitude, due to the presence of oxygen and of persistent absorbing impurities. The small absorption peak reported around 275 nm by Larzul et al. [C. R. Acad. Sci. Paris 261, 4701 (1965)] and which has often been cited to corroborate electron impact evidence for the existence of a low-lying 3B1 state of the water molecule, was not observed, despite adequate sensitivity and stability. The broad and unstructured absorption observed across the 200–300 nm region was only slightly in excess of that attributable to the aggregate of Rayleigh scattering and the tail of the well known ? 1A1→1B1 absorption of water.