Greenhouse Gases and Resonance

[Peter G.]

Hi,

I am now learning about climate change and have come across resonance. When I previously studied SHM however, I learned about resonance, and I believe I have a good understand of what it means. If the frequency of a driving force matches the natural frequency of the system, this leads to a better absorption of energy by the system, resulting in large amplitude oscillations. But I am confused regarding how resonance is related to how atoms absorb energy. How is infrared radiation a driving force? The energy transferred leads to an increase in temperature which forces the molecules to vibrate at their natural frequency?

Any ideas?

Thanks!

 

[ehild]

The molecules are something like tiny oscillators, in equilibrium at the bottom of a potential well. This potential well is formed by interaction of the ions and the electron cloud surrounding them.
The electric field interacts with the ions and electrons of the atoms, and displaces them in opposite directions. You get an oscillating dipole in the oscillating electric field.
When the frequency of the electric field is near to the natural frequency of an oscillation of the molecule, the amplitude will be high: The molecule absorbed energy from the field.
Infrared radiation drives the vibration of the atoms; visible and UV radiation excites the electron cloud.
The greenhouse gases like CO2 or several organic gases have natural vibration frequency in the infrared domain. They absorb it very strongly. On the other hand, they are transparent for visible and UV light. So the visible and UV light arrives to the surface of the Earth, and is absorbed by metals, rocks, soil, and the leaves of the vegetation and also by waters. The molecules can re-radiate the absorbed energy, but it is more probable, that they collide with other molecules and excite vibrations. These vibrations emit infrared photons which can not go through the atmosphere, as the greenhouse gases absorb them.

ehild