  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.
