Electron-impact ionization of the SO free radical

Abstract

We studied the electron-impact ionization of the SO free radical using the fast-neutral-beam technique. Absolute cross sections were measured for the formation of the SO+ parent ion as well as for the S+ and O+ fragment ions from threshold to 200 eV. At 70 eV, cross section values of 3.25±0.50×10−20 m2 (SO+), 1.32±0.23×10−20 m2 (S+), and 0.45±0.08×10−20 m2 (O+) were obtained. We found little evidence of the presence of vibrationally excited SO radicals in the incident target beams and only negligible contributions to the measured ion signal from excited SO radicals in Rydberg and/or metastable states were observed. The only significant pathway contributing to the formation of the observed SO+, S+, and O+ ions is the single positive ion formation. The measured partial ionization cross sections were combined to yield the absolute total single ionization cross section of SO which was compared to calculations using a recently introduced modified additivity rule. The agreement between measured and calculated cross sections was found to be very good.