I am surprised that Lawrence Crum says that "no one knows" why noble gases help luminescence ("Bubbles hotter than the Sun", 29 April). As an honours student in 1936 and 1937 I learnt that quantum mechanics predicts a metastable state for a helium atom. This only radiates very weakly to the ground state but can transfer its energy to another atom by a "collision of the second kind", thus exciting it.
Since the helium atom is not used up, it can go through the sequence again and again, explaining why only a small amount is needed.
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