Excitation and transition rate diagrams of singly ionized iron in analytical glow discharges in argon, neon and an argon–hydrogen mixture
The emission spectra of iron observed using a Grimm-type glow discharge in argon, argon with 0.3% v/v hydrogen and neon were studied to identify major excitation and ionization processes of iron ions in the plasma. Transition rate diagrams are given for iron ions in those discharges. These are particularly useful for describing situations in which the rate of radiative decay of excited states is comparable to or exceeds the rate of their collisional de-excitation. Relationships between transition rate diagrams and Boltzmann plots are discussed. The following processes were identified to be responsible for excitation of Fe II glow discharge spectra: charge transfer from argon/neon ions to ground state- and metastable iron atoms, Penning excitation of ground state- and metastable iron ions by argon metastables and collisional excitation by electrons of metastable iron ions. No evidence was found that local thermodynamic equilibrium exists for iron ions in a glow discharge plasma in any interval of excitation energies. Open questions remain concerning charge transfer excitation of iron, in particular about the range of energies in which the Fe II levels can be directly populated by charge transfer and about the conservation of spin in such reactions.