Role of the triplet metastable levels in ionization/excitation of palladium atom in glow discharge plasmas using several plasma gases

Abstract

The major excitation mechanism of palladium ionic lines (Pd II) in glow discharge plasma was investigated when argon, neon, krypton, and argon‑helium mixed gas were employed as the plasma gas. Different types of Pd II lines were intensively excited depending on the kind of the plasma gases. Correspondences in the excitation energy were found for each plasma gas, the excited levels of 8.1–8.6 eV for argon, 13.1–14.4 eV for neon, and more than 15 eV for helium, while no intense Pd II lines were found in krypton. These phenomena are principally attributed to an asymmetric charge transfer collision with the plasma gas ion, as previous studies have pointed out for other metallic elements. However, the excitation scheme of the Pd II lines is somewhat different from the normal scheme for the other metallic elements. In particular, it is a unique feature that the triplet metastable levels, 4d95s 3D3,2,1, rather than the ground state level, 4d10 1S0, of the palladium atom play a critical role in excitations of the Pd II lines. This effect is principally because the total excitation energy follows the energy resonance condition, as well as the spin multiplicity follows the spin conservation rule between the colliding partners.