Temperature dependence of the optical constants for liquid H₂O and D₂O in the far IR region

Calibrated thin films of ordinary and heavy liquid water have been measured over the temperature range -5.6-81.4° in the spectral region extending from 25 to 450 cm^-1 by classical absorption techniques with an FTIR interferometer. From these experimental spectr, the optical constants n and k were calculated by iteration using the Kramers-Kronig transformation which has been especially adapted to the problem of fringe correction for a flat absorbing sample in contact with highly refractive silicon substrates. As the principal result, we show that this method yields new quantitative data for the optical constants n and k of liquid H₂O and D₂O in the cited spectral region and temperature range. A comparison with earlier data for H₂O at 19°, measured by dispersive FT spectrometry, shows very good agreement. Further results are given concerning the parameters of the FIR bands, namely the evolution of band positions and band widths with temperature. Analysis of the shape of the librational band led us to suppose the existence of a second IR active component of this band which has hitherto only been reported in Raman and inelastic neutron scattering (INS) spectra. Finally, we confirm the different relaxation behavior of H₂O and D₂O in the high temperature range found in a recent Raman study.

H. R. Zelsmann, "Temperature dependence of the optical constants for liquid H₂O and D₂O in the far IR region," J. Mol. Struct.,350, 95-114 (1995).