Excitation and ionization of iron in argon and neon glow discharges: towards the true picture

Abstract

A systematic detailed analysis of calibrated Fe I, Fe II emission spectra from an argon- and a neon glow discharge was performed, involving a large number of Fe I and Fe II levels. The relevant excitation and ionization processes of iron in those discharges are described and discussed, based on experimental population functions of Fe I and transition rate diagrams of Fe II in both discharges. The Fe I spectrum reflects electron impact excitation occurring at conditions far from local thermodynamic equilibrium. Some processes involving heavy particle collisions were considered also as potentially responsible for the features observed in the Fe I population function. Charge transfer between neutral iron atoms and the ions of the discharge gas is the prevailing ionization mechanism of iron in both discharges and the decay of the charge transfer-excited ionic levels dominates the Fe II spectrum. In a neon discharge, the decay proceeds step-wise, causing a massive cascade excitation of lower Fe II levels on the way. Emission associated with electron impact excitation of ground state iron ions was also observed. A simple semi-quantitative empirical collisional-radiative model is presented for iron atoms and ions in an argon glow discharge.