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Carbon dioxide molecules CO2 comprise one carbon atom covalently joined to two oxygen atoms. The three atoms lie in a line (it is a linear molecule). The central carbon atom is shown in black and the two oxygen atoms in red. Absorption of Infrared Radiation by carbon dioxide happens because the carbon-oxygen bonds can stretch or flex at a frequency that allows them to absorb an infrared photon. There are a number of ways in which the molecule can oscillate. One type involves the bonds stretching and contracting like springs. The molecule can absorb an IR photon when this stretching is asymmetric. The alternative is for the bonds to flex as shown in this animation. The flexing movement allows absorption at a longer wavelength. Because the CO2 molecule can vibrate in different ways it can absorb different frequencies of IR. This means that the infrared absorption spectrum of carbon dioxide shows two main peaks. Emission of Infrared Radiation by carbon dioxide the oscillating molecule can then re-emit the infrared energy as a photon travelling in a random direction. This will happen repeatedly over time as the particular molecule absorbs and then re-emits IR photons. Consequently, roughly half of the emitted photons from a given molecule will travel back towards the surface. GREENHOUSE EFFECT this repeated absorption and re-emission by carbon dioxide effectively scatters the infrared radiation and so causes the heating known as the greenhouse effect. Without an atmosphere containing greenhouse gases this heat would be radiated back into space. However, carbon dioxide and other greenhouse gases absorb and re-emit this infrared in various directions. Some IR will ultimately escape back into space after taking a tortuous route from molecule to molecule but the overall effect is to heat the atmosphere. |