Although nitrogen and oxygen cannot absorb IR, they can interact with greenhouse gas molecules such as carbon dioxide and water. N₂ and O₂ molecules gain energy through conduction when they collide with excited greenhouse gas molecules. This process is called collisional de-excitation. To visualize this process open the Collisional Heating learning tool.

Question: What happens to infrared photons when they interact with atmospheric CO₂ molecules?

Notice that there is a CO₂ molecule in the center of the learning tool. At first, infrared photons pass through the molecule without interacting with it.

Move the bar on the IR spectrum to one of the peaks of absorbance and watch until a photon is absorbed by the molecule. This causes bending or stretching of the molecule's bonds.

If you select 'Display' from the Options drop-down menu and click on the 'Atmosphere' option, you will see N₂ and O₂ molecules interacting with the CO₂ molecule. There are two ways that CO₂ can lose the energy it has absorbed:

1. it can re-emit a photon in a random direction, or
2. it can collide with another molecule, such as oxygen or nitrogen, causing the other molecule to gain kinetic energy (move faster). This will lead to atmospheric heating.