Gas radiation question - He, H2 vs O2, N2

[Tandem78]

A question has been bothering me for some time - when we look at the sun, what are we looking at?
The standard answer is - the photosphere. The sun consists mostly of hydrogen and helium, and that the radiation from the photosphere is approximately a black body continuum spectrum at 6000K with some absorption lines in the visible light range.

However, in atmospheric physics, we learn that diatomic molecules like O2 and N2 as well as noble gases like Argon neither absorb nor emit infrared nor visible radiation, because they are symmetric and have no dipole moment. Their energy levels are so far apart that they only interact in the UV.
This the reason why the atmosphere is transparent to visible light, and why N2 and O2 cannot act as greenhouse gases, whereas H2O and CO2 do.

I have a problem trying to reconcile these two pieces of information, which seem to contradict each other. H2 and He are just like O2, N2 and Argon, diatomic or noble gases. If visible light from the photosphere is radiated by He and H2 at 6000K, why do N2 and O2 on Earth at 300K not radiate or absorb at infrared wavelengths?

I haven't seen any explanation of this contradiction. I suppose either a) the sun's light is not coming from the photosphere but a lower layer or b) O2 and N2 do indeed absorb visible and IR, but the interaction is so minimal that it can be ignored.

Is one of these correct, or is there another explanation?

T.

 

[Archosaur]

...However, in atmospheric physics, we learn that diatomic molecules like O2 and N2 as well as noble gases like Argon neither absorb nor emit infrared nor visible radiation, because they are symmetric and have no dipole moment...

Hmm... that part is fishy to me...

...N2 and O2 cannot act as greenhouse gases, whereas H2O and CO2 do.

CO2 is symmetric and has no dipole moment. Same with CH4, which is one of the worst greenhouse gasses. I think maybe the information you received was "simplified".

I don't know much on the subject, but I'm going to spend some time on google and see what I find.

 

[Borek]

You may assume there are no diatomic molecules in photosphere, only ionized gas.

That makes things even worse, as ions are perfectly symmetrical as well

 

[Tandem78]

You may assume there are no diatomic molecules in photosphere, only ionized gas.

Hey, that might be the explanation - at least ions have an electric charge. So an ion moving with thermal energy can interact with the electromagnetic field and convert the thermal (kinetic) energy into radiation. Not a very good formulation, but does that makes sense?

T.

PS thinking about this more, it means that only ionized hydrogen can radiate this way - does that mean a star consisting only of helium would be invisible?

 

[Borek]

Helium is ionized as well.

 

[Tandem78]

Hmm... that part is fishy to me...

CO2 is symmetric and has no dipole moment. Same with CH4, which is one of the worst greenhouse gasses.

The explanation I saw in a lecture was that when one of the vibrational modes of a complex molecule (H2O, CO2, CH4 etc) is excited, this creates an asymmetry in the structure, and this creates an asymmetry in electric charge - in other words a dipole. This is not the case for diatomic molecules.

T.

 

[Tandem78]

Helium is ionized as well.

You're right - I'm thinking of ions like in chemistry, duh...

So is the ionization the key to the answer?

 

[Borek]

Part of the answer, but honestly I am not sure about the radiation mechanisms, or it is too early for me to think clearly. There are two parts of it - one is emission when electrons go from excited state to the ground one, that means we can observe well defined discrete spectrum. Second part is a thermal radiation, which is continuous and observed for each object (and approximated by black body radiation). As far as I remember emission of thermal radiation is universal and doesn't require any assumptions about the body (well, perhaps one - its temperature must be above 0 K) - so whether it is made of ions, atoms, molecules, doesn't matter.

 

[Tandem78]

emission of thermal radiation is universal and doesn't require any assumptions about the body

Yes, this is not the same as emission of photons due to changes in electron orbits or vibrational states, it's about how an atom or molecule can convert its thermal (ie kinetic) energy into electromagnetic energy, even when it carries no charge. You say that thermal radiation is universal, however it's true to say that not all substances are black bodies, correct?
T.

 

[Borek]

Yes. In reality black body doesn't exist - it is just an approximation. As it happens with some approximations it turns out to be good enough for many applications and good enough to understand basic properties of thermal emission.