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?