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JNickson
Post  Post subject: What Causes IR Emittance  |  Posted: Wed Feb 15, 2017 6:21 pm

Joined: Wed Feb 15, 2017 6:13 pm
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Hey

Something caught my mind lately
What mechanism is responsible for Infra Red emittance?

The rate of IR is proportional to temperature, which is proportional to kinetic energy, so how is this kinetic energy converted into IR?

I was thinking it might be similar to gamma radiation which is caused by nucleus energy states, but that's too high energy for IR

Thanks for any help :)


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PhDemon
Post  Post subject: Re: What Causes IR Emittance  |  Posted: Wed Feb 15, 2017 6:29 pm

Joined: Wed Jun 26, 2013 10:44 am
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Location: Newcastle-upon-Tyne

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It is usually due to vibrations of bonds in molecules. The kinetic energy of the molecules does go up with temperature, but so do the vibrational and rotational energy of the molecules. It is the vibrational energy that causes IR emission, not the kinetic energy... When the molecule is heated (or otherwise excited) the bonds vibrate with higher energy, when they relax to a lower energy state the excess energy is emitted as an infrared photon. This is used in infrared spectroscopy among other things.

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JNickson
Post  Post subject: Re: What Causes IR Emittance  |  Posted: Wed Feb 15, 2017 6:51 pm

Joined: Wed Feb 15, 2017 6:13 pm
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Right, so does that mean it's caused by shell orbitals changing energy states? (since it's inter molecular bonds)

So, if there were only 2 atoms in the universe (or deep space), one at a much higher temp than the other, at a range such that forces between them is negligible, would IR still be emitted?
Or would the the rate of emission be proportionally as small as the IMF between the atoms, taking an extremely long time to reach an equilibrium?
Guess I'm asking if a lone atom has a way to emit IR to another distant atom of a lower energy

Cheers for the reply


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PhDemon
Post  Post subject: Re: What Causes IR Emittance  |  Posted: Wed Feb 15, 2017 6:59 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 491
Location: Newcastle-upon-Tyne

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You misunderstand, it is vibrations of intramolecular bonds. Bonds within the molecule not intermolecular bonds... A lone atom will not emit IR, it has no bonds to vibrate, it is molecules which have atoms bonded together that will emit IR. You can think of the bond between two atoms in a molecule as a spring (this is an oversimplification but may help you see the point). When the molecule is "hot" (I.e. in a vibrationally excited state) it is stretching and contracting very quickly, when it loses energy the rate of stretching and compression is slower and the excess energy is emitted as an IR photon. A single molecule can emit IR, it does not need another molecule to absorb it (although this is what happens in reality).

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"The big trouble with dumb bastards is that they are too dumb to believe there is such a thing as being smart"

- Kurt Vonnegut, The Sirens of Titan


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