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

Abstract

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${}^6$3d${}^9$4s was proposed to explain the presence of the Mo${}^{15+}$ ions with a lower energy of the incident electron. Finally, the significance of the metastable levels in ionizing Mo${}^{14+}$ ions is highlighted.

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