Optical absorption coefficients of water

Abstract

THE absorption spectrum of water in the visible region has been widely investigated1–9 because of the basic, technological and environmental importance of liquid water. Despite these efforts, there are significant disagreements between experimental results: discrepancies of factors of ∼2 in the absorption coefficients are not uncommon between various data. Possible reasons for the disagreements among the various studies are: (1) a lack of a reliable, sensitive technique for measuring small absorpton coefficients in liquids; (2) the presence of a significant amount of light scattering by particles (note that the amount of Rayleigh scattering by pure water is quite small and predictable in the visible8; and (3) measurements are often not done for pure distilled water stored in a noncontaminating vessel. We present here the first accurate measurement of the absorption coefficient of pure water at 21 °C in the 450–700-nm region. We have utilised a recently developed opto–acoustic (OA) technique10, using pulsed dye lasers and immersed piezoelectric transducers. This technique is ideally suitable for measuring weak linear or nonlinear absorptions in nonfluorescing neat liquids or solutions11,12. Our present absorption spectra for water, having typical accuracies of ±5%, should be useful not only for a basic understanding of the properties of water, but also for practical applications like underwater light propagation or intracavity dye laser measurement of dissolved materials in aqueous solutions.