For example, in a gas discharge tube, an atom at the low energy level E1 may get energy from the outside and be excited to the high energy level E2 by colliding with other atoms or free electrons. Conversely, atoms at high energy level E2 may also transfer energy to the outside world and return to low energy level E1 by colliding with other atoms or tube walls. In this kind of process, the atom only changes its internal energy value by colliding with the outside world, which has nothing to do with the absorption or emission of photons.
Vibration relaxation (VR); Internal conversion (IC); Inter system transition (ISC); External conversion (EC)
