A rare gas is another name for a noble gas, like helium, neon, argon, krypton, or xenon. These group 18 elements are very unreactive, with full outer shells, and rarely form compounds.
Being a rare gas, it usually migrates freely through faults and fragmented soils, and may accumulate in caves or water. Source: Internet
Larochelle et al. citation have compared the theoretically predicted ion versus intensity curves of rare gas atoms interacting with a Ti:Sapphire laser with experimental measurement. Source: Internet