Bad vibrations

Bad vibrations? A brief explanation of the heat trapping properties of CO₂

I'm pickin' up good vibrations
She's giving me the excitations
Brian Wilson / Mike Love / The Beach Boys

Basic mechanism for the absorption of radiation

When radiation (visible light, infra red, ultra violet …) impacts an atom or a molecule, if the frequency of that radiation matches the natural resonant frequency of the molecule energy it is momentarily absorbed, 'exciting' the molecule into a higher energy state. After a short time the same amount of energy is re-radiated and the molecule returns to its normal or 'ground' state. Although the radiation is only 'borrowed' this capture/release is nevertheless an absorption mechanism, because the re-radiation occurs in a random direction so in the simplified 1-dimensional case there would be a 50% probability the radiation were sent back in the direction it came from. It can be readily understood that for a sufficiently deep and concentrated layer of molecules, this scattering and attenuation could amount to absorption.

The three main component gases of earth's atmosphere (diatomic Nitrogen and Oxygen, monatomic Argon) are transparent to heat (infrared) radiation. Or, in terms of the explanation in the previous paragraph, these molecules' bonds are too rigid, too 'highly strung' to respond or resonate to frequencies in the infra-red.

Molecules with three (or more) atoms are different. Additional bonds provide more ways to twist and bend the molecule and in the same way it takes much less energy to bend a ruler than to stretch it, these vibrational modes match specific infra red wavelengths / frequencies. All the 'greenhouse gases' (Carbon dioxide CO_{2, Water vapour H2O, Methane CH4, CFCs etc. are molecules with three or more atoms.}