A powerful modern technique for studying ionization or dissociation processes is velocity imaging . Charged particles originating from such processes are guided by electric fields to a detection screen. Recently we studied ionization of metastable xenon atoms that produced extremely slow electrons. This is what happens in the orange lighting tubes along the highways, when sodium atoms are excited by collisions with electrons and produce an orange glow. Some excited states cannot emit light for reasons of symmetry.
Light is by nature an asymmetric thing.
So only a change in the atom that is sufficiently asymmetric can produce light. In this case it cannot find any state of lower energy and different symmetry, and light emission is impossible. Only one out of every 20 million atoms in air is xenon. Xenon is so rare, because it is the heaviest noble gas. Other noble gases are krypton, and argon.