Assume a very large volume of a gas of a specific molecule. This molecule can absorb a photon and spontaneously quickly re-emit a photon of the same energy before the molecule can physically interact with another molecule. Therefore the energy of these photons never participate in the equilibrium temperature of the gas. Given the specific molecule, its numeric density (number of molecules per m3), and the energy of the photon, how does one calculate the average distance such a photon will travel between emission and absorption by another molecule? I am not sure if this is the right forum to pursue this question. It seems like physical chemistry, but I may be mistaken. I have been unable to find any reference to explain this calculation. I have found references to absorption coefficient, cross section, and mean free path, but nothing that puts all the pieces together. I would appreciate any help.