Water near infrared absorption spectrum

Fig. 15. The near-infrared absorption spectrum of trapped water molecules in pollucite. Peak assignments refer to the three principal HjO vibration modes. ...

Chamberlain and co-workers (17) have noted that no distinct peak occurs in the infrared absorption spectrum of water below 193 cm.-1. The Raman scattering does give rise to a peak near 60 cm."1 while slow neutron scattering give rise to a number of peaks, including a notable one at 56 cm.-1. This discrepancy implies that a significant number of low frequency oscillations may exist in liquid water with little if any infrared activity. 

Another consequence of the strong absorption properties of water is the spectral impact of the displacement of water by dissolved solutes. Generally, in absorption spectroscopy, the solvent is selected not to absorb over the wavelength range of interest. When the absorption properties of the solvent are negligible, any displacement of solvent molecules from the optical path by the dissolution of solute molecules has a negligible effect on the measured spectrum. For near-infrared spectra of aqueous solutions, however, the absorption spectrum depends heavily on the degree of water displacement by solutes in the sample. 

Amerov AK, Chen J, Arnold MA. Molar absorptivities of glucose, water and other biological molecules over the first overtone and combination regions of the near infrared spectrum. Applied Spectroscopy 2004, 58, 1195-1204. 

W.S. Pegau, J.R.V. Zaneveld (1993). Temperature-dependent absorption of water in the red and near-infrared portions of the spectrum. Limnol. Oceanogr., 38,188-192. 

Fig. 1.6. General form of the absorption spectrum of water vapour in the near-infrared region (from data given by Hackforth, i960).

The near-infrared spectrum of fully deuterated water is very similar to that of H2O, except that the peaks are all shifted because of the mass difference of the hydrogen atom. Table 6.2 summarizes the absorptions. Figure 6.4 shows a portion of the near-infrared spectral region, illustrating the isotope shift of the + V3 combination band (indicated by stars). 

Liquid water as well as some of the organic salts have several strong near infrared (NIR) absorption features that are not fully masked by sihca s. The spectrum in Figure 2-17 a reflection spectrum taken in the NIR from the surface of a body very near its SCP. The surface is low in water content and combination bands from organic salts are evident. The NIR reflection (NIRR) spectra following the full water removal path are shown in Figure 2-18. The CP and SCP are marked and easily distinguished. 

---