The previously reported nonreproducibility of the intensity of the OH stretching band of liquid water has been explored. It was found that it can be eliminated in measurements with the Circle® multiple ATR cell by ensuring that the ATR rod is coaxial with the glass liquid holder. It was also found that normal laboratory temperature variations of a few degrees change the intensity by ≤~1% of the peak height. A new imaginary refractive index spectrum of water has been determined between 4000 and 700 cm<sup>-1</sup> as the average of spectra calculated from ATR spectra recorded by four workers in our laboratory over the past seven years. It was obtained under experimental and computational conditions superior to those used previously, but is only marginally different from the spectra reported in 1989. In particular, the integrated intensities of the fundamentals are not changed significantly from those reported in 1989. The available imaginary refractive index, <i>k,</i> values between 15,000 and 1 cm<sup>-1</sup> have been compared. The values that are judged to be the most reliable have been combined into a recommended <i>k</i> spectrum of H<sub>2</sub>O(l) at 25°C between 15,000 and 1 cm<sup>-1</sup>, from which the real refractive index spectrum has been calculated by Kramers-Kronig transformation. The recommended values of the real and imaginary refractive indices and molar absorption coefficients of liquid water at 25 ± 1 °C are presented in graphs and tables. The real and imaginary dielectric constants and the real and imaginary molar polarizabilities in this wavenumber range can be calculated from the tables. Conservatively estimated probable errors of the recommended <i>k</i> values are given. The precision with which the values can be measured in one laboratory and the relative errors between regions are, of course, far smaller than these probable errors. The recommended <i>k</i> values should be of considerable value as interim standard intensities of liquid water, which will facilitate the transfer of intensities between laboratories.
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