B.2.9—Greenhouse models

Resonance model

A greenhouse-gas molecule can absorb infrared radiation when the radiation frequency matches one of the molecule’s natural vibrational frequencies. The molecule is driven into a larger-amplitude vibration: this is the resonance description.

Molecular energy-level model

The same absorption can be described as a photon whose energy matches the gap between molecular energy levels. The molecule is excited, then returns to a lower level and emits infrared radiation.

Complete mechanism

Earth’s surface emits long-wave infrared radiation. Greenhouse gases absorb part of it and re-emit radiation in random directions; some is directed back toward the surface, raising the surface temperature compared with an atmosphere-free model.

Frequency test from local Question Bank row 31337

If a carbon dioxide vibration has period $5\times10^{-14}\,\mathrm{s}$, its natural frequency is

$$f_0=\frac1T=\frac1{5\times10^{-14}}=2\times10^{13}\,\mathrm{Hz}=20\,\mathrm{THz}$$

Earth's outgoing infrared is around $30\,\mathrm{THz}$, much closer to this molecular frequency than the cited solar infrared near $300\,\mathrm{THz}$. The resonance model therefore predicts stronger absorption of part of Earth's outgoing infrared; the energy-level model describes the same selectivity as matched photon-energy gaps.

Exam boundary

Do not write only “greenhouse gases trap heat”. Name infrared absorption, molecular resonance or energy-level matching, and re-emission in all directions.