Stepwise ionization of Mo¹⁴⁺ ions in EBIT: The importance of the metastable level

The visible spectrum of Mo¹⁵⁺ ions was measured using a high-temperature superconducting electron-beam ion trap at the Shanghai EBIT Laboratory, with an electron beam energy Eₑ=400 eV, significantly lower than the ionization potential (IP=544.0 eV) of Mo¹⁴⁺ ions in the ground state. To expound on the experiment, the energy level structure, radiative transition properties, electron-impact excitation, and electron-impact ionization cross section for both the ground state and low-lying excited state of the Mo¹⁴⁺ ions were calculated using Dirac-Fock-Slater method with a local central potential and distorted wave approximation. The results demonstrated reasonable agreement with both available experimental and theoretical data. Through an analysis of the related atomic processes of Mo¹⁴⁺ ion, a scenario involving the stepwise ionization of the metastable state 3p⁶3d⁹4s was proposed to explain the presence of the Mo¹⁵⁺ ions with a lower energy of the incident electron. Finally, the significance of the metastable levels in ionizing Mo¹⁴⁺ ions is highlighted.