Stepwise ionization of Mo^14+ ions in EBIT: The importance of the metastable level

The visible spectrum of Mo^15+ ions was measured using a high-temperature superconducting electron-beam ion trap at the Shanghai EBIT Laboratory, with an electron beam energy E_e=400 eV, significantly lower than the ionization potential (IP=544.0 eV) of Mo^14+ 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 Mo^14+ 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 Mo^14+ ion, a scenario involving the stepwise ionization of the metastable state 3p^63d^94s was proposed to explain the presence of the Mo^15+ ions with a lower energy of the incident electron. Finally, the significance of the metastable levels in ionizing Mo^14+ ions is highlighted.