Comparative studies on the excitation mechanism of Fe II lines in low-pressure laser-induced plasma of argon and neon

Emission characteristics of Fe II lines excited from low-pressure laser-induced plasma were investigated when Ar or Ne was employed as a plasma gas. Emission intensity measured in time-resolved mode was strongly dependent both on the kind of plasma gas and on the excitation energy of Fe ionic lines. In the initial plasma, just after the breakdown, fast electrons and gas species were major energy donors for excitations of ablated Fe atoms. With an expansion of the plasma plume, particularly intense emission lines of Fe ion appeared in Ar and Ne plasmas. Especially, different emission lines of Fe ion were selectively excited when Ar or Ne were employed as the plasma gas: for example, the Fe II 242.415-nm and the Fe II 248.424-nm lines in Ar plasma, and the Fe II 250.388-nm and the Fe II 257.686-nm lines in Ne plasma. The excitation mechanism for these ionic lines are different in both plasmas: a charge-transfer collision with Ar ion principally caused their excitations in Ar plasma, while a Penning-type collision with the metastable state of Ne atom could populate the corresponding excited levels of Fe ion in Ne plasma.