Excimer comes from the term excited dimer, which refers to a diatomic molecule, usually of an inert gas atom and a halide atom, which are bound in excited states only. The gases that JPSA excimer laser systems use include helium, neon, krypton and fluorine (for 157nm, 193nm an 248nm systems) and xenon (for 308nm and 351nm systems). These gases must be ultra high-purity grade (UHP). A reactive gas, such as fluorine, is mixed with inert gases such as helium or krypton. When electrically stimulated, a dimer molecule is produced that, when lased, produces light in the ultraviolet range.