Water absorption in different phases

This is about the interaction of light with water in its different phases: vapour, liquid, solid and in the special enviroment within an emerald crystal structure.

As we saw in a previous posting: Water, ice (and fire), vibrations within the water molecule produce emission and absorption bands throughout the red and infrared spectrum. We also noticed that the bands shift in wavelength depending on the surroundings on the water molecule, appearing at longer wavelengths in liquid water and ice when compared with the vapour.

A particularly interesting case of this is the gemstone, emerald (Be_3 Al_2 (SiO_3)_6), where water molecules can be trapped, along with alkali metal ions, in 'channels' within the crystal structure (D.L. Wood & K. Nassau, The American Mineralogist, Vol. 53, May-June, 1968, p777). The orientation of the water molecules with respect to the channel axis determines both the wavelengths and the polarisation properties of the absorption bands.

This plot shows a wide wavelength range, from 400 to 2500nm (obtained with two spectrometers). The y-axis shows the absorption strength (absorbance) - except in the case of the butane flame (red line) where it represents the emitted flux per unit wavelength. Most, but not all, of the peaks in this plot represent the effects of water: absorption in the case of the Earth's atmosphere (grey line), liquid water (dark blue line), ice (light blue line) and emerald (green line); but emission in the case of the flame.

Examination of the regions marked with 'notes' on the plot shows that the wavelengths of the bands increase in the sequence: vapour (atmosphere and flame)' emerald, liquid, ice - representing increasing effects of the environment on the vibrations of the water molecule.

The strength of the water bands in this emerald pebble (about 6mm across) represents the equivalent of about a tenth of a mm depth of liquid water - similar to the depth of water across a typical green leaf.

The notes (boxes) on the plot give the identification of the water molecule's vibrational bands within this wavelength range with the three numbers within the brackets representing the harmonic (overtone) of the symmetric stretch (n--), the bending mode (-n-) and the asymmetric stretch (--n) respectively.

The atmospheric absorption is taken from an observation of a total eclipse of the Moon (E. Pallé et al., "Earth's transmission spectrum from lunar eclipse observations", Nature, Volume 459, Issue 7248, pp. 814-816 (2009)) while the water and ice absorbance are taken from D. J. Segelstein, "The complex refractive index of water," University of Missouri-Kansas City, (1981) and S. G. Warren, "Optical constants of ice from the ultraviolet to the microwave," Appl. Opt., 23, 1026-1225, (1984) respectively.

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