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When a ball bounces on the ground, each bounce is smaller than the previous one because of friction in the system, i.e. the collision between the ball and the ground is not completely elastic. We are taught that the kinetic energy lost in an inelastic collision is turned into heat.

What about when two gas molecules collide in the air? Is their collision always elastic, or does part of their kinetic energy sometimes turn into heat? And if so, does this heat leave the collision in the form of an IR photon?

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Yes, it is common for colliding molecules to emit IR and microwaves.

When molecules collide some of the kinetic energy may go into rotational and vibrational excitations, so that after the collision one or both of the molecules are in an excited state. Then the molecules decay by emitting a photon. Typically the decay of rotational excited states emits microwave photons and the decay of vibrationally excited states emits infra-red photons.

At room temperature the kinetic energy is usually too low to excite vibrational states, so the gas would emit only microwave radiation from the decay of the rotational modes. Typically you need to heat the gas to a few hundred degrees C to see much infra-red emission.

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  • $\begingroup$ Awesome, thanks! But a follow-up qn: Surely only a certain type of gases, namely the so-called greenhouse gases, can emit IR photons? So that wouldn't apply to e.g. the nitrogen in the air? But perhaps the nitrogen can emit microwaves, as you mentioned? And could you maybe point me to a source where I could read more about this? 🙂 $\endgroup$ Dec 27, 2022 at 12:27
  • $\begingroup$ Oh yeah... And does your answer mean that no photon is emitted in the collision itself, but the collision causes an excitation of the molecules which later causes the emission of a photon? $\endgroup$ Dec 27, 2022 at 12:30
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    $\begingroup$ @KarolinaHagegård As a first approximation the molecules have to have an electric dipole moment to radiate photons, and that's true for both rotational and vibrational transitions. So homogenous diatomics like N₂ and O₂ would not radiate as their dipole moment is zero. You mention CO₂, and that can undergo vibrational transitions because a dipole moment is created as the molecule bends. There will be a non-zero probability to radiate a photon during the collision but I don't know how large that probability is. I would guess most photons are radiated after the collision. $\endgroup$ Dec 27, 2022 at 12:51
  • $\begingroup$ @KarolinaHagegård Just to be clear about the main problem with CO2/CH4 ... they are transparent to visible light .. all the energy in the visible spectrum hits the ground/water and warms it during the day ..... all that visible energy is turned into IR energy (photons) by the ground/water and these photons can not escape as easily because increased CO2/CH4 which absorbs IR. $\endgroup$ Dec 27, 2022 at 13:41
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    $\begingroup$ @OrangeDog gases do not black body radiate. BBR needs a high electron density and gases simply don't have enough electrons per cubic metre. The gases will be radiating at specific frequencies instead of a continuous BBR spectrum. $\endgroup$ Dec 27, 2022 at 19:27

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