I wonder how inelastic a collision can be? I am thinking about a gas at room temperature; for the sake of argument let it be helium, where the first excited state is well above the available collision energies. Even in a low-velocity collision, it seems there ought to be some distortion of the orbitals, which to me suggests that the atoms are at least partially driven into a higher state. The question is: must this energy of distortion be completely recovered in the kinetic recoil of the atoms, or is there some residual distortion which is left over to be radiated away electromagnetically? I can put the question another way. Even at room temperature, according to Planck's Law of radiation, there is a very small amount of equilibrium radiation in the ultraviolet band corresponding to the first excititation level of helium. We can explain this by saying that the velocity distribution of helium atoms contains a very tiny percentage of atoms with enough kinetic energy to drive this transition, which explains where the radiation comes from. Or we can say that every single collision drives this transition to a very small extent, and the equilibrium of radiation results from the relaxation of these very small distortions. Are these two explanations equivalent or do the facts require us to choose between them?