Supplement 7.3: Raman Scattering (6/6)

Raman scattering of water (cont.)

A closer look at the spectra of Raman scattering shown on the previous page reveals a certain structure in the Raman bands. The diagram below shows this in a higher spectral resolution. The structure is the result of the permanent dipole moment of the water molecule: as explained in previous pages, liquid water is made up not only of free individual molecules but also of molecules that are bound together as clusters by hydrogen bonds. The hydrogen bonds weaken the bonding energy of the oxygen atom to either of the two hydrogen atoms in a molecule, thereby causing the wave number to decrease and the wavelength of the Raman shift to increase.

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Raman bands of monomer and polymer water
The Raman bands of free individual molecules (monomer) and of molecules bound by hydrogen bonds (polymer) differ in their spectral position and interfere with each other to result in a broader spectral structure.


The relationship of monomer and polymer water molecules depends on temperature (see page 2). The sum spectrum of the two interfering Raman bands is likewise dependent on temperature, as can be observed in the diagram of experimental data shown below.

Temperature 24.92°C Temperature 25.31°C
Temperature 24.92°C
Dots: Experimental data of water Raman scattering measured at the given temperature, plotted versus the wavenumber (for comparison with the graph in the left column: smaller wavenumbers correspond to longer wavelengths and vice versa). Linies: Fitted curves of the bands of monomer and polymer molecules and of the total signal. The broken line denotes the Raman band of polymer water.
Source: EARSeL Advances in Remote Sensing

This brought up the idea to determine the temperature of liquid water out of the spectral signature of the Raman scattering. Technically, this method is much more complicated and difficult than measuring temperature with conventional thermometers. But it has one great advantage and that is, the possibility of determining water temperature from a great distance without physical contact - through remote sensing.