Water absorption

  Water absorption is a phenomenon in the transmission of electromagnetic radiation through a medium containing water molecules. Water molecules are excited by radiation at certain wavelengths and tend to selectively absorb portions of the spectrum while allowing the balance of the spectrum to be transmitted with minimal effect.

Strong water vapor absorption bands occur at wavelengths around 2500, 1950 and 1450 nanometers (nm),^[1][2] with weaker absorption around 1200 and 970 nm,^[3] and three additional sets of water-vapor absorption lines near 930, 820, and 730 nm,^[4] all in the infrared spectrum. Water has a complex absorption spectum — the 2007 HITRAN spectroscopy database update lists more than 64,000 spectral lines corresponding to significant transitions of water vapor ranging from the microwave region to the visible spectrum.^[5]

The liquid water absorption features are offset to longer wavelengths from the water vapor absorption features by about 60 nm.^[6] In hexagonal ice, the features are shifted even further. In liquid water and ice the infrared and Raman spectra are far more complex than in the vapor.^[7]

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Atmospheric effects

Water vapor is a greenhouse gas in the Earth's atmosphere, responsible for 70% of the known absorption of incoming sunlight, particularly in the infrared region, and about 60% of the atmospheric absorption of thermal radiation by the Earth known as the greenhouse effect.^[8] It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing^[5] because water vapor absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimeter wave bands. The South Pole Telescope was constructed in Antarctica in part because the elevation and low temperatures there mean there is very little water vapor in the atmosphere.^[9]

Similarly, carbon dioxide absorption bands occur around 1400, 1600 and 2000 nm,^[10] but its presence in the Earth's atmosphere accounts for just 26% of the greenhouse effect.^[8] Carbon dioxide gas absorbs energy in some small segments of the thermal infrared spectrum that water vapor misses. This extra absorption within the atmosphere causes the air to warm just a bit more and the warmer the atmosphere the greater its capacity to hold more water vapor. This extra water vapor absorption then further enhances the Earth’s greenhouse effect.^[11]

Conversely, there is an atmospheric window between approximately 800 and 1400 nm, in the near-infrared spectrum where carbon dioxide and water absorption is weak.^[12] This window allows most of the thermal radiation in this band to be radiated out to space, keeping the Earth's atmosphere from going into thermal runaway. This band is also used for remote sensing of the Earth from space, for example with VNIR imaging.

Technical explanation

The water vapor absorption bands are related to molecular vibrations involving different combinations of the water molecule's three fundamental vibrational transitions:

• V1: symmetric stretch mode
• V2: bending mode
• V3: asymmetric stretch mode

The absorption feature centered near 970 nm is attributed to a 2V1 + V3 combination, the one near 1200 nm to a V1 + V2 + V3 combination, the one near 1450 nm to a V1 + V3 combination, and the one near 1950 nm to a V2 + V3 combination.^[3]

In liquid water, rotations tend to be restricted by hydrogen bonds, leading to librations, or rocking motions. Also stretching is shifted to a lower frequency while the bending frequency increased by hydrogen bonding.^[7]

See also

• Hydroxyl ion absorption in optical fiber
• Color of water

References