How does radiation heat a gas?

[billyharris72]

Hi all.

I'm trying to get my head around this, in the context of stellar physics, and how stars emit both thermal and line radiation. Most of the books I've read seem to cover electron energy level transitions and then talk about thermal energy and black body radiation as if there is no issue to explain as to how one turns into the other.

I get the idea of electron energy level transitions, and (to some extent) of collisional line broadening, but I'm struggling to get my head round how these quantum energy state transitions relate to the translational (kinetic) energy necessary to raise the temperature of a gas. Presumably this has to happen or the gas in stars would not get hot.

I've seen an explanation on physics stack exchange that suggests that, with the electrons in a higher state, interactions with other molecules become more likely and that kinetic energy can be passed on that way. But what happens in this process - does the electron de-excite without emitting a photon? If not, where does the energy that is passed on come from? And are photons emitted by atoms in other ways (e.g. does the translational acceleration of the atoms release photons, in a way similar to braking radiation)?

Also, are there other ways a photon can heat an atom or a molecule? The same stack exchange thread has a post that suggests that radiation cannot directly heat a gas molecule. I could see (from the above discussion of energy levels) that such a mechanism might not be necessary, but what about cases like co2, where infrared imparts energy to an oscillating dipole? This doesn't seem to involve energy level transitions at all. Presumably, for that matter, neither does transmittion of photons through the at least the inner part of the radiative zones of stars, as it's presumably all plasma at those temperatures.

Struggling a bit with this one. Any help (or suggestion for not-to-advanced reading) much appreciated.

Thanks,

Billy.

[andrew s]

Both atoms and molecules can emit and absorb radiation.  These can be electronic transitions and in addition for molecules rotational  and vibrational  modes. An example is a  microwave oven where microwave excite water molecules. 

In the interior of a star radiation scatters off ions and electrons exhanging energy and momentum. What starts as gamma rays from nuclear reactions emerge after a long random walk due to scattering to emerge from our sun as visible light with roughly thermal distribution (modified by the photosphere).

Regards Andrew

[JRWASTRO]

Hi all.

I'm trying to get my head around this, in the context of stellar physics, and how stars emit both thermal and line radiation. Most of the books I've read seem to cover electron energy level transitions and then talk about thermal energy and black body radiation as if there is no issue to explain as to how one turns into the other.

I get the idea of electron energy level transitions, and (to some extent) of collisional line broadening, but I'm struggling to get my head round how these quantum energy state transitions relate to the translational (kinetic) energy necessary to raise the temperature of a gas. Presumably this has to happen or the gas in stars would not get hot.

I've seen an explanation on physics stack exchange that suggests that, with the electrons in a higher state, interactions with other molecules become more likely and that kinetic energy can be passed on that way. But what happens in this process - does the electron de-excite without emitting a photon? If not, where does the energy that is passed on come from? And are photons emitted by atoms in other ways (e.g. does the translational acceleration of the atoms release photons, in a way similar to braking radiation)?

Also, are there other ways a photon can heat an atom or a molecule? The same stack exchange thread has a post that suggests that radiation cannot directly heat a gas molecule. I could see (from the above discussion of energy levels) that such a mechanism might not be necessary, but what about cases like co2, where infrared imparts energy to an oscillating dipole? This doesn't seem to involve energy level transitions at all. Presumably, for that matter, neither does transmittion of photons through the at least the inner part of the radiative zones of stars, as it's presumably all plasma at those temperatures.

Struggling a bit with this one. Any help (or suggestion for not-to-advanced reading) much appreciated.

Thanks,

Billy.

Greetings Billy,

You have raised many questions that only a course in physics will help answer.  I could offer a few buzz words to aid in your research.

1. Re: thermal energy and black body radiation.  Thermal radiation is exactly that - radiation one can feel as heat, from a bar radiator for example. This energy will be in the vicinity of 8 to 14 micrometers (human body temperatures) and 3 to 5 micrometers in the case of Jet engine exhausts.

Buzz words : See Stephan-Boltzmann law and look up Ultra violet catastrophe - See Wein's displacement law.

2. A Black Body can and will emit "Thermal Radiation" depending on its temperature.  BTW. Black bodies are not black as such they can be dull red (hot) or white hot or extremely cold (above absolute zero).

3. About Buzzword : Look up Entropy and the work of Ludwig Boltzmann

4. You should study about the dual nature of radiation - waves and particles. The waves will be electro-magnetic waves - orthogonal electric and magnetic fields - look up "Poynting Vector"

Good luck in your research,

Jeremy

[robin_astro]

There are a number of ways photons can transfer momentum eg to electrons, atoms and molecules. For example

Directly through scattering 

By exciting vibration modes in a molecule

Through radiationless transitions where electron excitation levels generated by photons are translated into vibration modes within the molecule

The atoms/molecules then interact with each other throughout the medium (solid, liquid or gas) raising its temperature

Edited October 16, 2019 by robin_astro
clarification

[robin_astro]

Also presumably the absorption of a photon imparts some small velocity to an atom to conserve momentum and again in recoil when a photon is emitted?  Dont ask me to calculate the dominant mechanisms in a particular scenario though 😉

Robin