We are all told at school water vapour and carbon dioxide are the top two greenhouse gases, and that they absorb thermal infra red radiation, trap heat and warm up the Earth. My question is how do they do that? Why can't Oxygen or Nitrogen or any other gas not absorb infra red radiation as well as water vapour or CO2?
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4 Answers
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The absorption of IR radiation is due to vibrations of molecules. When a vibration causes change in charge distribution (or dipole moment to be more specific) the IR radiation is absorbed.
Generally, hetero-polar molecules, like $\rm H_2O$ and $\rm CO_2$, have permanent dipole moment. The external oscillating electric field in this case perturbs the Hamiltonian and causes IR absorption. Hence, they contribute to "Green House effect" by absorption of heat.
$\rm H_2O$, which is non-linear molecule, has three fundamental modes of vibrations. Symmetrical Stretching, asymmetrical Stretching and scissoring (bending). $\rm CO_2$, which is linear molecule, has four fundamental modes of vibrations. Symmetrical stretching, asymmetrical stretching and two degenerate scissoring modes, in planes perpendicular. The symmetric stretching mode in $\rm CO_2$ does not produce or absorb any IR, as it does not cause change in dipole moment, but other modes do change charge distribution causing absorption of IR.
Well, the homo-polar molecules, like $\rm N_2$ and $\rm O_2$, does not have any permanent dipole moment. The external oscillating electric field does not perturb the Hamiltonian for nuclear motion, and does not absorb IR, though it perturbs Hamiltonian for electronic motion. Hence, do not contribute for "Green House effect".
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$\begingroup$ Chutsu was seen here last time 1st of may. And homo-polar is a nonsenseword. This words are used in chemistry for different types of bonds, but for polarity of molecules this words are nonsense. $\endgroup$– GeorgCommented Oct 27, 2011 at 16:50
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3$\begingroup$ Actually, CO2 does not have a permanent dipole moment. What is important is the transition dipole moment, i.e. how the dipole moment changes due to the vibrational mode, as you've explained. $\endgroup$– RobCommented Oct 2, 2015 at 7:55
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$\begingroup$ Can you point to a reference where the exact equations, Hamiltonians, are showing the mechanism of change in dipole moment causing the molecules to absorb IR? $\endgroup$– HansCommented Jun 4, 2017 at 20:12
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As you can see on these absorption spectra for $\textrm{H}_2\textrm{O}$ and $\textrm{C}\textrm{O}_2$, both molecules have moderate to strong absorbtion in the mid-IR wavelengths, with the absorption of $\textrm{C}\textrm{O}_2$ extending out into the longer wavelengths.
Other molecules common in the atmosphere don't have such strong absorption at the wavelengths given off by thermal radiation. If you are asking why that is, I'm afraid I can't give you a very detailed answer, except to say that the absorption spectra of molecules (and atoms) is governed by quantum mechanics. Maybe somebody else can explain how they would be calculated from principles, but that is beyond my education.
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$\begingroup$ OP states in the body of the question that water and carbon dioxide absorb IR... not sure what this answer adds $\endgroup$– pentaneCommented Oct 2, 2015 at 14:08
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$\begingroup$ Wow. This is over 4 years old. Anyway, what it adds is essentially a statement that these molecules absorb the wavelengths they absorb because it is an intrinsic property, dictated by the laws of the universe in which we live. $\endgroup$– Colin KCommented Oct 3, 2015 at 0:12
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$\begingroup$ isn't IR radiation governed by classical mechanics? It has to do with vibrations that can be calculated based on force constants and harmonic/anharmonic oscillator systems. $\endgroup$– gannexCommented Nov 13, 2016 at 19:35
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CO2 and H2O absorb a lot of IR radiation because they have internal vibrations with the same energy as infrared photons.
Every molecule has a different set of internal vibrations, and those match the frequencies that it absorbs and emits most readily. When the photon is emitted, the molecule loses the energy in that vibration, and the energy is transferred to the photon.
Essentially it boils down to conservation of energy.
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Since CO2 re-radiates IR energy, it can't be said that it "absorbs" the energy. It just converts it to another frequency in the electromagnetic spectrum. The end result is transparency.
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$\begingroup$ That's not correct. All atoms, when they interact with light, absorb and release it in either different directions and/or at different frequencies maintaining conservation of energy. Transparency has to do with not interacting with visible light, not absorption and re-release. If we could see the IR light that CO2 absorbs and re-releases, then CO2 gas would not appear transparent. $\endgroup$– userLTKCommented Oct 25, 2017 at 8:12