Do greenhouse gases need to be polar?

[Maartenc]

Hi,

I have been wondering if greenhouse gases always need to be polar molecules. Can a gas also absorb energy, and heat up, without being polar?

 

[DrClaude]

Obviously no, since CO₂ and CH₄ are not a polar molecules

What is important is that the molecule be active in the infra-red, which corresponds to vibrational excitation. This precludes non-polar diatomic molecules, for which pure vibrational transitions are dipole-forbidden, but for molecules of 3 and more atoms, there are asymmetric vibrational modes that are active in the infra-red.

 

[Maartenc]

Ah, thanks, and apologies for my ignorance. I guess what I wanted to know was can you heat up a gas through an absorption line?

 

[sophiecentaur]

I don't know the definitive answer to this but I suspect that there could be insufficient power in the limited bandwidth of a well single absorption line (even starting with 1kW/m² from the Sun) to give a measurable temperature rise from that mechanism.
Someone may well tell me different.