IR AbsorptionThe homonuclear diatomic molecules that make up the bulk of the atmosphere (N2 and O2) do not absorb IR radiation because they have no IR active vibrations. What makes a vibrational mode IR-active?
An electromagnetic field interacts with a molecular bond by affecting the dipole moment of that bond. A bond with a dipole moment can be visualized as a positive charge and a negative charge separated by a spring. An electric field will cause this bond to stretch or be compressed. Stretching the bond increases the dipole moment of the bond while compressing the bond decreases the dipole moment of the bond.
A rapidly alternating electric field is one of the components of an electromagnetic wave and will cause a bond with a dipole moment (polar bond) to alternately stretch and compress. When this stretching and compressing occurs at the frequency of the molecule's natural rate of vibration, energy is absorbed.
A bond must have a dipole moment in order to interact with the electromagnetic field of infrared spectroscopy and that dipole moment must be changed by the vibration of the bond in order for the bond to be IR-active.
Examine the simplest situation. Infrared radiation is absorbed by the stretching vibration of a simple diatomic molecule (CO, HCl, NO, for example). This stretch is the only vibrational mode for a diatomic molecule.
Non-linear molecules have more vibrational modes. Water has 3 of these.
Carbon DioxideIR radiation excites some of the possible vibrational modes in carbon dioxide. The vibrational absorption spectrum of CO2 is shown below.
Other Greenhouse GasesOther gases can absorb even more solar energy than carbon dioxide. Early in the history of our planet, the high methane concentration in the atmosphere kept the Earth's surface warm and water in a liquid form.