Glow discharge excitation and ionization of chromium, comparison with iron

Abstract

Cr I and Cr II emission spectra from a Grimm-type glow discharge (GD) in argon and neon were analyzed. Intensities of strong resonance Cr I lines were corrected for self-absorption and population functions (experimental Boltzmann plots) of the Cr I system were established. A prevailing excitation mechanism in Cr I is the electron impact (EI) excitation. Cr II spectra in both discharges were described by transition rate (TR) diagrams, under the model of radiatively-dominated deexcitation. The contribution of cascade excitation was subtracted and the resulting net-TR diagrams of Cr II were analyzed. Unlike ionic spectra of some other transition elements, asymmetric charge transfer (ACT) between the ions of the discharge gas and ground state Cr atoms does not play a role in the excitation of the Cr II spectrum, because of the absence of suitable states in Cr II that would allow simultaneous conservation of energy and total angular momentum. Cr II states below ≈17 eV are populated largely by EI excitation from long-lived low Cr II states. In a neon discharge, ACT reactions between neutral neon metastables and doubly charged Cr ions and between neutral chromium and neon molecular ions were identified. The findings reported here were compared with the excitation of Fe I and Fe II systems at similar conditions. A well-known ‘hydrogen effect’, an enhancement of the Cr II line at 267.7165 nm in the presence of hydrogen in a GD plasma, was explained by charge transfer reaction between H+ ions and neutral chromium.