Upper secondary · 15 min

Why greenhouse gases block infrared

Scan the infrared spectrum and locate the absorption bands that let $\ce{CO2}$ , $\ce{H2O}$ , $\ce{CH4}$ and $\ce{O3}$ trap Earth’s heat.

Goal

Connect each IR absorption band to a molecular vibration, and explain the atmospheric window.

Apparatus and reagents

A simulated IR transmission chart from 200 to 4000 cm $^{-1}$ with a gas selector.

Procedure

Open the combined view and find the deep dips near 667 and 2349 cm $^{-1}$ .
Switch to the $\ce{CO2}$ -only dataset and check which two vibrations absorb.
Switch to the $\ce{H2O}/\ce{CH4}/\ce{O3}$ view and find the transparent region — the atmospheric window near 8–12 µm.

What to observe

$\ce{CO2}$ absorbs strongly at 667 cm $^{-1}$ (bending, 15 µm — right at the peak of Earth’s thermal emission) and at 2349 cm $^{-1}$ (asymmetric stretch).
Near 1000 cm $^{-1}$ (8–12 µm) the atmosphere stays transparent, except for the $\ce{O3}$ band — this window lets Earth radiate to space and is used by weather satellites.

Explanation

A molecule absorbs IR only if a vibration changes its dipole moment. Symmetric $\ce{N2}$ and $\ce{O2}$ do not, which is why air is mostly transparent; bent $\ce{H2O}$ , linear $\ce{CO2}$ , tetrahedral $\ce{CH4}$ and bent $\ce{O3}$ all have IR-active modes. The 15 µm $\ce{CO2}$ band coincides with the peak of Earth’s outgoing radiation ( $T \approx 288$ K, $\lambda_{max} \approx 10$ µm by Wien’s law), so increasing $\ce{CO2}$ thickens the blanket exactly where it hurts. The 8–12 µm window is the escape route that keeps the surface from overheating — clouds and ozone partly cover it.

Chemists behind it

Svante Arrhenius