Interaction of light with matter is of paramount importance in nature. The most fundamental property of a material in relation to light is its oscillator strength distribution, i.e., how strongly it absorbs light as a function of wavelength. Once the oscillator strength distribution is determined precisely for a wide enough energy range, the optical constants such as absorbance and reflectance as well as a number of other properties of the material, some of which are seemingly unrelated to photoabsorption, can be deduced. Most important of all is the fact that the interaction of matter with fast charged particles can be described by its complete optical spectra [Inokuti, M. (1986) Photochem. Photobiol. 44, 279 -285]. Despite their importance, however, the complete optical spectra of volatile liquids including water have never been obtained accurately because of experimental difficulties inherent in vacuum UV spectroscopy. Inelastic x-ray scattering spectroscopy can provide quantitative data equivalent to those from vacuum UV absorption spectra. Herein, we show the complete optical spectrum of liquid water determined by making use of intense monochromatic x-rays supplied by the wiggler line X21 of the National Synchrotron Light Source.

By means of inelastic X-ray scattering spectroscopy, the generalized oscillator strength (GOS) of liquid water has been experimentally determined over a range 0.69 ≤ q ≤ 3.59 a.u., where q is the momentum transfer. The Bethe surface was constructed from the GOS, and the loss function, the dielectric response function, and the static structure factor were deduced. It was found that the GOS of liquid water is dominated by individual molecular properties over the q range studied, and that the static structure factor agrees quite well with a calculation including electron-electron correlation effects.

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