# Thermal radiation between monatomic gases

1. Jul 24, 2012

### QuasiParticle

Suppose we have two separate containers of helium gas in thermal equilibrium and completely isolated from the rest of the universe. Let's assume the containers are ideal in the sense that they are completely tranparent to all electromagnetic radiation.

Since helium is monatomic, the only degree of freedom (at low enough temperatures) is traslational motion of the atoms. I can understand how collisions of helium atoms create thermal radiation. But how about absorption of the radiation by the other gas. Are the thermal photons absorbed in three-particle collisions (which seem quite infrequent) or do they lose energy in successive two-particle collisions with the atoms?

2. Jan 1, 2013

### QuasiParticle

After some thought the "problem" with the asymmetry between the emission and absorption probabilities is resolved by increasing the density of the photon gas. That is, in thermal equilibrium the photon gas surrounding the two helium containers has higher particle density than the helium.

But still I wonder if it's possible for the photons to transfer energy to helium atoms in simple photon-atom collisions.

Also the intensity of thermal radiation by helium gas (or other noble gases) would be nice to know. Anybody have any references?

3. Jan 1, 2013

### Drakkith

Staff Emeritus
I believe gases act as normal blackbody emitters if they are at a high enough pressure. Otherwise they will normally radiate only specific frequencies that depend on their atomic orbitals. The intensity and frequency spread of the radiation depends directly on the temperature of the gas.