Absolute cross sections for the electron-impact single ionization of Mo4+ and Mo5+ ions

Absolute total cross sections for the electron-impact single ionization of Mo4+ and Mo5+ ions have been measured using a crossed-beams technique from below the ground-state ionization threshold to 500 eV with typical total uncertainties of 9% near the peak of the cross sections. Molybdenum ion production in the source was facilitated by a mini-oven sublimating MoO3. The measured cross sections are in good agreement with distorted-wave calculations and are dominated by contributions from excitation autoionization. Nonzero cross sections below the threshold for ionization of Mo5+(4p64d) ground-state ions indicate that metastable ions were present in the beam extracted from an electron-cyclotron-resonance ion source. No evidence of metastable ions was found in the case of the Mo4+ measurements. Ionization rate coefficients and fitting parameters are presented for the experimental data.