When a molecule of carbon dioxide gas (or any greenhouse gas molecule) absorbs a photon of IR radiation, the bonds of the molecule begin to stretch and bend. As carbon dioxide molecules move throughout the troposphere, they frequently collide with nitrogen and oxygen molecules. When these collisions occur, the vibrational energy (stretching and bending of carbon dioxide molecules) is transferred to nitrogen and oxygen molecules. The nitrogen and oxygen molecules gain kinetic energy and move faster resulting in an increase in temperature. This process, which transfers energy from greenhouse gas molecules to molecules of nitrogen or oxygen gas through collisions, is called collisional de-excitation.
The process of collisional de-excitation is grounded in kinetic molecular theory, a model that has proven to be very useful in explaining the nature of matter, including the behavior of gases. Collisional de-excitation provides a logical explanation for how the energy in a photon of IR radiation can be translated into an increase in temperature, as temperature is understood in terms of the kinetic molecular theory.
Why Should We Care?
Collisional de-excitation is an important application of kinetic molecular theory and demonstrates the explanatory power of this theory. Understanding atmospheric warming in terms of collisional de-excitation applies the ideas of continuous motion of particles, constant collisions and the relationship between temperature and the kinetic energy of particles—all of which are based upon kinetic molecular theory.
Your Turn
How does the absorption of a photon of IR radiation by a greenhouse gas molecule lead to an increase in atmospheric temperature?
The absorption of a photon of IR radiation results in an increase in atmospheric temperature through the process of collisional de-excitation. If greenhouse gas molecules absorb photons of IR radiation, the bonds in the molecules bend and stretch more intensely. When greenhouse gas molecules collide with other atmospheric gases, the energy contained in these bending and stretching vibrations is transferred to other gas molecules. The other gas molecules gain kinetic energy, which is measured as an increase in atmospheric temperature.