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Q & A: radiative heating

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Most recent answer: 10/22/2007
Q:
Please help me with this question.

What happens at the molecular level when infrared radiation shines on a gas? I have two answers and I donít know which is correct:

1. The radiation excites the vibrational levels of the molecules. So, they vibrate faster.
2. The radiation warms the gas. So, its molecules move through space faster.

Motion increases in either case. But is it vibrational motion or translational motion?

Thanks for your answer,
- Rich
Chicago State University
A:
In a way, both answers are right. The main initial way the radiation will interact is by exciting vibrational states in the molecules, assuming that some of the radiation is at the right energies for those particular molecules. However, that energy can be transferred from internal vibrational modes to other modes, including the translational kinetic energy, when molecules collide. So the gas will in fact warm up. The time for the energy in the different modes to equilibrate, i.e. come to the same temperature, is typically very short, so that unless you are attempting to make measurements on a time scale much less than a second, or paying attention to the particular pattern of which frequencies are absorbed, you wonít notice that itís really the vibrations which initially get the energy.

Mike W.

(published on 10/22/2007)

Follow-Up #1: radiative heating and transparent gas

Q:
This is all right, but what about the radiative heating of a monatomic gas? If you have something like argon gas and expose it to an infrared radiation field. Will it heat up? And what if you have a neutral hydrogen gas where all the atoms are in the ground state? Will an infrared or even a visible radiation field have any effect on the gas? Will it heat the gas?
- Johan
Potchefstroom, South Africa
A:
You're concerned about the case where the frequencies of the light are too low for a single photon to excite the gas atom even to its lowest excited state. In other words, the gas is nearly transparent for this type of light, so the light just passes through it.

It can take a very long time for a gas of atoms to exchange energy with low-frequency light. The most dramatic example is provided by the universe as a whole. In the early days, before it had cooled enough for atoms to form, the ions easily exchanged energy with light, so the light and the ions stayed in equilibrium (same temperature). When things cooled enough to form transparent atoms (mostly H and He), the light and the atoms quit easily equilibrating. That happened 13.7 billion years ago, and they're still out of equilibrium!

So you're absolutely right: transparent atomic gases don't trade energy at all well with low-frequency light.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.