let me clarify that Argon and Krypton need a electron stripped off the atom to lase, and cadmium and neon in HeCad and HeNe just need the electron excited to a higher level orbit. Neon and Cadmium lase when the high orbit electron falls back down. The helium is ionized, and it collides with neutral neon or neutral cadmium, and transfers the energy during the collision between the atoms.
Argon and Krypton lase just before recombination of the ion and the electron.
Both types depend a lot on collisions of the atoms in the tube with the walls of the tube to release energy and encourage lasing, in fact Hene wont lase without it.
Steve


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stanwax

Um... Steve.
