I have a narrow gas jet, with a laser pointed at it. I want to figure out whether the gas will absorb any thermal energy from the laser photons, if they are at a wavelength that the gas can absorb.

For example, I have a 532nm laser, and a Ne gas jet. As far as I can tell from the (oddly contradictory?) sources I have seen, Ne has an absorption peak at 532nm. When it absorbs light from the laser, will it heat the gas up? Or will it simply excite an electron that later de-excites and produces another photon?

And if it does heat it up, is there any way I can get a figure on the added energy given the cross sections and laser power?

The only literature I can find on this topic just talks about gas-assisted heating of solid surfaces. Am I misunderstanding something here?

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    $\begingroup$ This is quite a good question! If the emission excites an electron that is later emitted at the same wavelength, no net energy should be transferred to the gas. But of course the momentum of the photon was absorbed by the atom, and when the photon is re-emitted there will be some transfer of momentum to the atom (recoil). I suspect these things would cancel. So it probably depends on the absorption mechanism. A good answer would have to properly account for that... sorry I can't dig into it right now. $\endgroup$ – Floris Mar 22 at 14:13
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    $\begingroup$ Since there are non-radiative energy transfer mechanisms in real materials (including gases), after absorption there are other channels than just re-radiating the energy. (And, those are a reason that you need a He-Ne mixture, not pure Ne, to make a He-Ne laser work). $\endgroup$ – Jon Custer Mar 22 at 14:24

The answer to the (general) title question is definitely 'yes,' and the answer to the more specific text is probably yes as well.

In the limit in which the only emission is stimulated emission by the 532 laser, then the effect of absorption is precisely counteracted by that of emission. This will only happen for light that has a strong intensity relative to the characteristic saturation intensity of the atomic transition. For light that is weaker than this, there will be both spontaneous emission and stimulated emission. Because the spontaneous emission is isotropic, it leads to random momentum kicks in every direction, and thus heating. If there are any alternate decay channels besides the driven transition, this can also lead to similar heating from random emission.

Just for fun, I should also point out that it's even possible to have a gas be cooled by laser light, by arranging things so that emission is preferentially in the opposite direction of the atom's motion. This relies on a careful engineering of the light detuning and balancing of non-cooling forces, and it's easy to get a little away from these conditions and cause runaway heating instead.

A good reference for light-atom interactions is Laser Cooling and Trapping by Metcalf and van der Straten. They mostly develop the theory for the simple case of an alkali atom with a hydrogen-like structure; I'm not sure exactly how this changes for an atom like neon but I suspect it allows more inelastic decay channels.


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